CHEM 237 Review
What are four important points to remember for a successful TLC experiment?
1. Ensure the solvent level in the chamber is lower than the starting line where you spot the samples. 2. Use only a plain pencil to draw a starting line and mark the compounds' spots. A pen or marker should never be used. 3. Cover the developing chamber with a watch glass. 4. Do not disturb the developing chamber. Do not move it or carry around.
What is the purpose of a hot filtration when performing a recrystallization? When would a hot filtration not be necessary or useful?
> Purify the material and remove insoluble solid impurities. By heating the material in hot solvent, the insoluble materials can be filtered out due to the solute dissolving. > It was not be necessary if there were no solid insoluble impurities present to be filtered out.
To get effective mixing in a separatory funnel, the combined level of liquids should not exceed...
Higher than the widest part of the separatory funnel
The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60 degrees. There is steric strain in the molecule because of the proximity of the two methyl groups. Would the magnitude of this steric strain increase or decrease if the dihedral angle was increased from 60 degrees to 65 degrees?
The magnitude of steric strain will decrease because the groups are moving futher apart.
What is dipole moment?
The magnitude of the charge Q at either end of the molecular dipole times the distance r between the charges. u = Q * r
How would the melting range of a pure organic material be changed if there was a tiny shard of broken glass inside the melting point capillary?
The melting range would not be affected > The tiny shard of glass does not have any appreciable solubility in the liquid phase of the organic material.
What is a determining factor in the relative rate of an SN1? (In terms of the alkyl halide)
The slow step of an SN1 reaction is the breakage of the carbon-halogen bond and formation of a carbocation. The more stable the carbocation, the faster the reaction. Therefore, the fastest SN1 substitution occurs in tertiary alkyl halides, and the slowest in primary alkyl halides.
How is it possible to separate two liquids with the exact same boiling point by gas chromatography (GC) when it cannot be done by fractional distillation?
The use of a polar GC column will separate the liquids based on difference in polar interactions and volatility.
What was the objective of experiment 11? (Kinetic Study of SN1 Solvolysis)
To determine the kinetics of a solvolysis reaction and observe how a change in solvent polarity affects the rate > A solvolysis is an SN1 reaction in which the solvent is the nucleophile. It follows first order kinetics.
During the developing of a TLC plate, it is common to place a cover on the chromatography chamber and have a piece of moist filter paper line the walls of the TLC chamber. Why is the moist filter paper in the TLC chamber of importance during thin-layer chromatography?
To ensure that the air in the chamber is saturated with solvent vapor.
What was the objective of experiment 10? (Phase transfer catalysis)
To examine the use of a phase transfer catalyst to effect an SN2 reaction in dichloromethane solution. The reaction is conducted without extraction for product isolation.
What was the objective of experiment 3? (Molecular Modeling)
To introduce the concept of modeling the properties of molecules by computer generated algorithms. >Visualizing the three-dimensional properties of molecules. >Gain understanding about steric interactions, bond lengths, bond angles, dipole moments, and bond rotation.
What was the objective of experiment 13? (Oxidation of a Secondary Alcohol with Sodium Hypochlorite)
To investigate the oxidation of a secondary alcohol with an environmentally friendly reagent and to use infrared spectroscopy to identify the starting alcohol and product formed
What was the objective of experiment 9? (Natural Products and Alkenes)
To isolate examples of two types of natural products, terpenes (limonene) and acetogenins (eugenol), and to study their properties > Making use of steam distillation and extraction of a weak organic acid to isolate two natural products, limonene and eugenol. > Classification tests provide evidence for some of the functionality present in these structures.
1-Phenyl-1-chloroethane is a secondary halide. Most secondary halides undergo SN1 reactions quite slowly. Explain why 1-phenyl-1-chloro-ethane undergoes SN1 solvolysis quite rapidly.
The carbocation intermediate is more stable due to the resonance present in the ring.
As the solution becomes acidic, what is the color change associated with the bromothymol blue?
The color of the bromothymol blue indicator changes from blue to yellow as the solution becomes acidic.
Describe eutectic composition
The composition of a mixture of two components that generates the lowest possible melting point.
Why is it necessary to use a pencil when writing on a TLC plate?
The ink from a pen will dissolve in the developing solvent
Why is the reaction of cyclohexene with HBR (a rapid, simple reaction) not useful as a characterization reaction?
The is no visible change during the reaction.
Which of the following would be true of any materials that steam distilled with eugenol, but remained behind when extracted with a base? a. (T/F) They might contain large numbers of -OH functional groups. b. (T/F) They might contain -CO2H groups. c. (T/F) There could be long-chain hydrocarbons (alkanes/alkenes) d. Methyl eugenol could be present.
a. False, large numbers of -OH functional groups steam distill as a sodium salt. b. False, -CO2H will also steam distill as a sodium salt. c. True, long chain hydrocarbons will not form salt with NaOH. They will be left behind. d. True, Methyl eugenol will be present as a sodium salt because of the phenyl group that will form a salt with NaOH.
Products from any of the possible alcohols will show similar absorption bands characteristic of two types of bond-stretching in the functional group region. What are these two bond types?
c=o Carbonyl groups c-c Alkane groups
How does the extent of resonance in a molecule affect the carbonyl stretching frequency in an IR spectrum?
A greater extent of resonance lowers the carbonyl stretching frequency.
What experimental observation on the initial dissolution of compound with hot solvent would let you know that a hot filtration was not needed?
After you dissolved the compound, no impurities are seen.
Acetanilide is an aromatic amide. It contains an aromatic pi system, and an amide group. What is an amide group?
Amides contain a three-coordinate nitrogen with an available lone-pair of electrons directly bonded to a carbonyl.
What is an azeotrope? How do you find the boiling point of an azeotrope?
An azeotrope is a mixture that has a composition that doesn't change during distillation. > The vapor of an azeotrope has the same composition as the unboiled mixture. > The boiling point is indicated by the y-value of the minimum point on the figure.
During a solvent extraction between water and an immiscible solvent, a student has two layers in their separatory funnel and then shakes it. A single, milky liquid develops and no real sign of any layers are visible. What has occurred and what should the student do?
An emulsion has formed. The student should first, add a small quantity of water. If that does not work, then add solvent.
Is Br2 an oxidizing agent or a reducing agent?
An oxidizing agent.
Infrared spectra are recorded using an ATR detector. What does ATR stand for?
Attenuated Total Reflectance > A beam of infrared light is passed through the ATR crystal so that it reflects off the internal surface making an interaction with the sample. This reflection forms an evanescent wave, which extends into the sample. The beam is then processed by the detector as it exits the crystal.
What is the difference between axial and equatorial? What is a ring-flip?
Axial: Bonds or positions in chair cyclohexane that lie along the ring axis, perpendicular to the rough plane of the ring. Equatorial: Bonds or positions in chair cyclohexane that lie along the rough equator of the ring. Ring-flip: Interconvents two chair conformations of cyclohexane. Converts an axial substituent into an equatorial substituent.
Rates of reactions, rate in rate equation, always have units of mole L-1 sec-1. In the above equations, what are the units of the rate constant, k? a. time b. time-1 c. liters/mol d. no units
B.
Why is it good practice to perform a mixed melting point determination at two different ratios of unknown to known materials in the melting point capillary?
Because it is possible that one particular combination of materials generates a eutectic. > It is possible to mistake the sharpness of a eutectic as a pure sample, and thus misjudge the temperature reading.
Why would concentrated hydrochloric acid be a poor choice as the acid catalyst for the formation of cyclohexane by dehydration of cyclohexanol?
Because you can end up with a nucleophilic substitution instead of an elimination reaction. The hydrochloric acid will protonate the hydroxyl group giving water, a good leaving group. The chloride ion can then attack the carbon and expel the water molecule. You will end up with a alkyl halide instead of an alkene.
Why is the reaction of alkenes with HCl not useful as a characterization reaction, but reaction with Br2 is a good characterization reaction?
HCl is colorless and would not change the appearance of a solution. Br2 is a good characterization reaction because the reddish color of bromine disappears when added to an alkene or alkyne.
A separatory funnel contains the two immiscible liquids water and hexane. Use the given densities to determine which layer is on top and which is on the bottom in the binary mixture. Hexane density = 0.66 g/mL Water density = 0.998 g/mL
Hexane is on the top layer and water is on the bottom layer
What solvent had the highest eluting power for the analgesics analyzed in this experiment? Which solvent had the lowest eluting power?
Highest: Ethyl acetate Lowest: Hexane > The mobile phase is the solvent. The more polar the mobile phase and the more polar the compound, the more the spot moves. Hexane is nonpolar, so it did not move the spots.
What types of organic compounds will not steam distill?
Hydrocarbons and neutral compounds will not steam distill because they are not soluble in water. > Organic compounds should have a high boiling point in order to steam distill.
During the recrystallization of a solid, the sample is dissolved in hot solvent then filtered while hot. The filtrate is allowed to cool and the mixture is filtered a second time. What effect will result if the filtrate solution is cooled to rapidly?
Small impure crystals will form.
A TLC plate was run in 1:1 toluene:acetone, but when developed the plate only showed baseline spots. What change could be made to get the spots to move further up the plate?
Spot a new plate and run in 1:3 toluene:acetone > All spots move further as the polarity of the mobile phase increases. Switching to a solvent system that includes more acetone than toluene will allow the spots to separate from each other and from the baseline.
What is the difference between staggered and eclipsed? What is the difference between anti conformation and gauche conformation?
Staggered: Lowest energy, most stable conformation. Eclipsed: Highest energy, least stable conformation Anti conformation: Geometric arrangement around a carbon-carbon single bond in which the two largest substituents are 180 degrees apart as viewed in a Newman projection. Gauche conformation: Conformation of butane in which the two methyl groups lie 60 degrees apart as viewed in a Newman projection.
In the positive triiodide-starch paper test, have the iodide anions in the test paper been oxidized or reduced? A.)What type of reagent (oxidizing or reducing) will cause a positive test? B.) What causes the color change in the test paper?
Starch indicator solution consisting of water, starch and iodine which often used in redox titrations: In the presence of an oxidizing agent the solution turns blue, in the presence of reducing agent the blue color disappears because triiodide(I3−) ions break up into three iodideions, disassembling the starch-iodine complex. A.)The presence of Oxidizing agent gives the positive test. B) The oxidizing agent oxidizes the iodide to free iodine. The solution of the free iodine then reacts with the starch to give the blue black color (notice that it is only the solution of the free iodine which gives the blue black color with the starch).
What are the four most common methods or purification?
1. Distillation 2. Extraction 3. Recrystallization 4. Chromatography
Even if two compounds have identical solubility profiles in a solvent, it is possible to effect some purification by a single recrystallization. Assume you have a 10 gram mixture of A and B in a 9:1 ratio of A to B and that both have a solubility at room temperature of 0.1 g/mL. If you used 10 mL of solvent for recrystallization, how much B would be expected to be in the crystalline material if material was cooled to room temperature before filtration?
0.1 grams. The ratio of A:B in the dissolved material is 9:1. Ratio in solid is 8.1:0.9.
Which of the following is NOT true about the chair conformation of cyclohexane? 1. It contains six equatorial hydrogens that are perpendicular to the ring. 2. It contains six axial hydrogens that are parallel to the ring axis. 3. It contains six axial hydrogens that are perpendicular to the ring. 4. It contains six equatorial hydrogens that radiate around the ring's equator. 5. It contains six equatorial hydrogens that are in the approximate plane of the ring.
1 is false. The equatorial hydrogens are in the approximate plane of the ring.
List the steps that are required to convert 1-chloro-1-phenylethane into 1-phenylethanol via SN1 solvolysis.
1. Add a few drops of bromothymol blue to a stirred solution of ethanol and water. 2. Add sodium hydroxide solution from a burette. Note the volume. 3. Add alkyl halide and start the timer. 4. Record the time it takes for the solution to turn green. 5. Repeat the process of adding aliquots of measured base, and recording the time take for a color change. 6. Heat on a steam bath above 50 degrees Celsius for 20 minutes. 7. Titrate the contents of the flask against sodium hydroxide, and record the volume.
What are the steps required to perform a thin layer chromatography separation?
1. Add just enough developing solvent to cover the bottom 3 mm of the TLC chamber. Cover with a watch glass. 2. Make a pencil mark on the TLC plate 10 mm from the bottom. 3. Use a capillary to deliver a 2 mm diameter spot onto the pencil mark. 4. Stand the TLC plate upright in the chamber, and replace the lid. 5. Remove the TLC plate when solvent is near the top of the plate and mark the solvent front with a pencil before the TLC plate dries. 6. Mark the outline of the spots on the plate with a pencil. Unless the spots are colored, some visualization method will be needed. 7. Measure the distance that the spot and the solvent have moved.
What are the conformations of cyclohexane?
1. Chair: Lowest energy, has neither angle strain nor torsional strain 2. Boat: No angle strain but a large number of eclipsing interactions. 3. Twist-boat: Nearly free of angle strain but has both steric strain and torsional strain.
List the steps that are required to separate and collect a two-liquid mixture using preparative gas chromatography.
1. Collect a vial of unknown mixture. Record its number and letter. 2. Inject the mixture into the numbered Prep GC instrument that matches the number on your sample vial. 3. Slip a collection tube over the lower exit port of the Prep GC just after the first fraction begins to elute 4. Remove the collection tube when the first peak is almost finished 5. Add another collection tube, and take a center but for the next fraction 6. Transfer the samples to conical sample vials by connecting the vials to the collecting tubes 7. Centrifuge both vials to collect the samples. Cap the vials
How should the following fires be put out? 1. Small fire contained completely with a beaker. 2. Fire confined to a small portion of clothing. 3. The entire room is on fire. 4. A large portion of your clothes is on fire.
1. Cover the flame with a watch glass or a hard-back book. 2. Pat the fire out. 3. Sound the fire alarm and leave the building. 4. Use the safety shower or the fire blanket.
What is the purpose of the following procedures? 1. Add water to the separatory funnel that contains an acidified three-component mixture in dichloromethane. 2. Vent the separatory funnel through the stopcock. 3. Add aqueous base to the mixture dissolved in dichloromethane. 4. Place a beaker below the separatory funnel.
1. Dissolve the emulsion due to the benzocaine salt exceeding its solubility. 2. To avoid a build up of pressure inside the glassware. 3. Convent the benzoic acid into a water-soluble salt. 4. To recover a mixture in case the stopcock leaks.
List the procedural steps that are required to perform a chemically active solvent extraction of a three-component mixture composed of benzoic acid, benzocaine, and a neutral organic molecule all dissolved in dichloromethane.
1. Dissolve the solid three-component mixture in dichloromethane and place in a separatory funnel. Add 6M HCl to the solution. Shake and allow to settle into layers. 2. Add water to dissolve the brown emulsion between the layers. 3. Separate and collect the aqueous layer in a flask, and add base. 4. Place the dichloromethane layer back in the separatory funnel and extract it with 3M NaOH. Allow layers to settle. 5. Add water to dissolve the white gel-like emulsion. 6. Separate and collect the aqueous layer in a flask, and add acid. 7. Collect and dry the solids in both Erlenmeyer flasks by filtration.
Mark the following statements as true or false. 1. Use a pencil or pen to mark the position of a spot on the TLC plate. 2. Spot the plate on the dull, not shiny side of the plate. 3. Only touch the edges of the plate. 4. Mark the plate after applying the sample. 5. Spots should be 1 to 2 mm in diameter.
1. False 2. True 3. True 4. False 5. True
List the steps that are required to convert 2-naphthol and allyl bromide into allyl 2-naphthyl ether.
1. Fit a conical vial with a stirring vane and add 2-naphthol, allyl bromide, and benzyltriammonium chloride in dichloromethane 2. Add sodium hydroxide to the reaction flask 3. Cover the reaction vial and stir 4. Periodically stop the stirring, allow the layers to settle, and spot the organic layer on a TLC plate. 5. Remove the bottom layer and dry over calcium chloride 6. Run the reaction solution through a silica chromatography column 7. Evaporate the dichloromethane solvent to dryness
Label the following statements as true or false. 1. Silica is toxic 2. Hexane is flammable 3. Ethyl acetate is flammable 4. Acetone is flammable 5. It is acceptable to handle TLC plates with bare hands.
1. Flase 2. True 3. True 4. True 5. False
What is the purpose of the following procedures in the oxidation of cyclohexanol to cyclohexanone? 1. Add bleach dropwise into a flask containg acetic acid. 2. Add sodium bisulfite solution. 3. Touch a drop of the reaction mix onto a piece of KI-starch paper. 4. Place the reaction flask in an ice-water bath. 5. Combine the organic layers over anhydrous potassium carbonate.
1. Generate the oxidizing agent 2. Destroy the oxidizing agent 3. Test for the presence of oxidizing agent 4. Control the rate of reaction 5. Dry the reaction product
Explain the direction in which the retention time would change (longer, shorter, or no change) for a substance in a GC experiment when the experimental parameters are changed as shown below. 1. Column temperature decreased 2. Carrier gas flow rate increased 3. Concentration of stationary phase liquid increased 4. Detector temperature increased 10 degrees 5. Amount of sample injected decreased 20% from a sample that gives a good separation
1. Longer 2. Shorter 3. Longer 4. No change 5. No change
List the steps that are required to convert cyclohexanol to cyclohexanone.
1. Mix cyclohexanol and acetic acid in an Erlenmeyer flask. 2. Add bleach dropwise into the flask. 3. Maintain the reaction tempearature between 40 and 45 C on a steam bath. 4. Remove unused bleach with sodium bisulfite solution. 5. Add drops of 6M NaOH until solution is pH 6-8. 6. Place reaction in separatory funnel, extract with dichloromethane. 7. Dry the dichloromethane layer over potassium carbonate. 8. Decant from the potassium carbonate and collect by distillation.
List the steps that are required to perform a steam distillation of limonene from an orange peel.
1. Place citrus peel into a preweighed round bottom flask. 2. Add water until the level is past the midpoint of the flask. 3. Attach a condenser and receiving flask with greased joints, then distill the mixture. 4. Add ethyl acetate to the receiving flask to extract the droplets of oil. 5. Separate the layers, and dry the organic layer over calcium chloride. 6. Filter off the drying agent, and then remove the ethyl acetate solvent by microdistillation. 7. Collect and weigh the residue of pure limonene.
List the steps that are required to perform an extraction of eugenol from a solution of clove oil in dichloromethane.
1. Place the dichloromethane solution of clove oil into a separatory funnel. 2. Add sodium hydroxide solution. Separate and collect the aqueous layer. 3. Acidify the aqueous layer with hydrochloric acid. 4. Extract the acidic solution into dichloromethane. Collect the organic layer. 5. Dry the dichloromethane solution over calcium chloride. 6. Remove the dichloromethane solvent by microdistillation. 7. Collect and weigh the residue of pure eugenol.
List the procedural steps that are required in the recrystallization of acetanilide.
1. Place the impure substance and a boiling chip into an Erlenmeyer flask on a heating element. 2. Add the minimum amount of hot water to just dissolve the solid. 3. Rapidly filter the hot solution through a pre-warmed funnel. 4. Wash the filter with hot solvent and transfer the filtrate to a clean flask. 5. Allow the filtrate to cool slowly, then cool in an ice bath to force precipitation. 6. Vacuum filter the cold suspension. 7. Air dry the purified solid on the filter.
For the bromine, potassium permanganate, and ferric chloride tests, what results will occur for the following compounds? 1. Cyclohexene 2. Limonene 3. Eugenol
1. Positive for bromine and potassium permanganate 2. Positive for bromine and potassium permanganate 3. Positive for all tests > Bromine: positive for C=C > Potassium permanganate: positive for C=C > Ferric chloride: Positive for phenols
List the steps that are required to perform an accurate melting point determination.
1. Press the open end of a capillary tube into the solid sample. 2. Invert the tube and tap gently to move the solid to the bottom of the capillary. 3. Place the tube with sample into the melting point apparatus, and heat at 10 degrees per minute. 4. Observe the material through the complete melting process to get a rough estimate of the melting range. 5. Turn off and allow to cool by 15 degrees. 6. Replace the existing tube with a new sample. 7. Heat at 2 degrees per minute through the expected range.
What effect would each of the following changes to the conditions of a GC separation have on the retention time for a sample? 1. The column temperature is increased 2. The carrier gas flow is decreased.
1. Retention time decreases 2. Retention time increases
What are four important points to remember about your choice of a recrystallization solvent?
1. The solvent should not chemically react with the compound. 2. The solvent should dissolve the compound while hot. 3. This same solvent should not dissolve the compound when cold. 4. The solvent should either dissolve the impurities at all temperatures or not dissolve the impurities at all.
What is the purpose of the following procedures during a phase transfer catalyst? 1. The reaction was stirred at a moderate, not rapid, pace 2. Solid sodium hydroxide was added to 2-naphthol 3. Allyl bromide was used in excess 4. The contents of the reaction flask were passed through a column of silica 5. Enough calcium chloride to cover half the bottom of the flask was added
1. To avoid making the reaction mix become an emulsion 2. To deprotonate alcohols to become good alkoxide nucleophiles 3. Keep the time for the reaction to go to completion under an hour 4. Separate the non-polar ether product from the polar by-products 5. Remove dissolved water from the reaction mixture
What is the purpose of the following procedural steps during a preparative gas chromatography experiment? 1. Do not rinse the syringe with acetone, rinse it with sample isntead. 2. Vials are placed into the centrifuge in pairs, 180 degrees apart. 3. The detector fitted to the Prep GC is a TCD not a FID 4. Wait for the second peak to be on the rise before beginning its collection.
1. To avoid the introduction of a spurious peak into the GC trace 2. To avoid dangerous vibration due to imbalance 3. To avoid loss of sample 4. To preserve the purity of the sample because there is a lag between being detected and being eluted
Label the following statements as true or false. 1. Sodium hydroxide can cause severe damage to skin and eyes 2. Lachrymators like allyl bromide are specifically dangerous because of their explosion hazard 3. The primary risk associated with dichloromethane is its flammability 4. Unused allyl bromide can be disposed of in the aqueous waste 5. Gloves and goggles are not required when handling sodium hydroxide
1. True 2. False 3. False 4. False 5. False
Label the following statements about simple distillation as either true or false. 1. Distillation is a physical separation, not a chemical reaction. 2. Over time, the amount of the most volatile component in the distillation flask increases. 3. Simple distillation is effective at separating mixtures of liquids with very similar volatilities. 4. Over time, the amount of the most volatile component in the distillate increases.
1. True 2. False 3. False 4. True
Mark the following statements as true or false. 1. Rf is dependent on polarity. 2. Polar compounds will have higher Rf values that nonpolar compounds under the same developing conditions. 3. Rf values should be kept in the range of 0.2 to 0.8 for the most effective separation when more than one compound is present in the sample. 4. The less polar the solvent, the higher the Rf values.
1. True 2. False 3. True 4. False
Label the following statements as true or false concerning the care and use of a separatory funnel. 1. The first step in using a separatory funnel for extraction is to make sure the funnel stopcock is closed and then carefully pour the liquid to be extracted into the funnel. 2. The stopcock can be used to vent excess pressure if either of the liquids are warm to the touch and has a low boiling point. 3. Students should throw away the first layer and acid washes. 4. Hold your thumb firmly over the hole in the top of the funnel during shaking. 5. If one is going to re-extract the upper layer with another volume of liquid, the upper layer can be left in the separatory funnel and fresh solvent added to the funnel.
1. True 2. True 3. False 4. False 5. True
Label the following statements about the practical aspects of recrystallization as true or false. 1. Use clamps to secure the filter flask to avoid tipping and unnecessary exposure to hot liquids or the solute. 2. Recrystallization requires the material that is to be purified to be a solid. 3. For hot vacuum filtration, the filter paper should be completely dry when pouring the hot solution into the Buchner funnel to filter. 4. Recrystallization is the process of dissolving a solid material into a suitable solvent or mixture of solvents and allowing the material to crystallize from this solution. 5. For recrystallization, rapid cooling gives the best crystals.
1. True 2. True 3. False 4. True 5. False
Label the following statements as true or false. 1. Hypochlorous acid is an oxidizing agent 2. Concentrated acetic acid is corrosive 3. Methylene chloride is flammable 4. Methylene chloride is a probable human carcinogen 5. Pipets that contain only water can be disposed of in the trash
1. True 2. True 3. False 4. True 5. False
Label the following statements as true or false. 1. The rate of heating affects the number of theoretical plates in the column. 2. Each vaporization-condensation is called a theoretical plate. 3. The smaller the boiling point difference, the easier it is to separate two liquids and fewer theoretical plates are required. 4. Increasing the length of the column and packing the column with glass beads or rings are methods that produce more theoretical plates. 5. Rapid heating can drive heated vapor up the column, preventing the formation of an equilibrium that allows the lower boiling component to separate. As a result, this will increase the number of theoretical plates and increase the separation.
1. True 2. True 3. False 4. True 5. False
Mark the following statements as true or false. 1. The distance the spot moved is where the spot started to the center of where it ended up. 2. The Rf value is defined as the ratio between the distance the spot moved and the distance the solvent moved. 3. Rf is not dependent on polarity. 4. The distance the solvent moved is where the spot started to where the solvent front ended up. 5. Polar compounds will have higher Rf values than nonpolar compounds under the same developing conditions.
1. True 2. True 3. False 4. True 5. False
Label the following statements about cyclohexane as either true or false. 1. After a ring flip, an axial substituent in one chair form becomes an equatorial substituent. 2. The different geometries that a molecule can attain by bond rotations and bends are called conformations. 3. The twist boat conformation of cyclohexane has angle strain but is almost free of steric strain. 4. Substituents larger than hydrogen prefer to adopt equatorial positions due largely to a smaller amount of steric strain. 5. Each carbon in cyclohexane is bonded to one axial and one equatorial hydrogen.
1. True 2. True 3. False 4. True 5. True
Natural products can be extracted into organic solvent. For example, limonene can be extracted from the largely-aqueous distillate of a steam distillation of orange peel by ethyl acetate, and eugenol can be extracted from aqueous solution into dichloromethane. The diagrams below represent the separatory funnels for the two different solvent systems. 1. Water/ethyl acetate 2. Water/dichloromethane
1. top: ethyl acetate, bottom: water 2. top: water, bottom: dichloromethane
What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?
1400-400 cm^-1. > This is known is the fingerprint region. Each chemical compound produces its own, distinctive pattern in this area. As a result, even the isomers with same functional groups can be distinguished.
What region is the following functional group found on the IR spectrum? C=C alkene
1600 cm^-1
What region is the following functional group found on the IR spectrum? C=O ketone C=O aldehyde
1800-1640 cm^-1 ketone 1800-1640 cm^-1 aldehyde
Which of the following statements about equatorial and axial positions in cyclohexane is correct? 1. Two substituents on the same face of the ring, one of which is axial and one that is equatorial will be trans to each other. 2. Each face of the chair conformation of cyclohexane ring has three axial and three equatorial hydrogens in an alternating pattern. 3. Two substituents on the same face of the ring that are not adjacent to each other will be trans. 4. Each alternating carbon in cyclohexane is attached to either two axial hydrogens or to two equatorial hydrogens.
2 is true. Each carbon in cyclohexane is attached to one axial and one equatorial hydrogen. Substituents on the same face of the ring are cis to another, regardless of whether they are axial or equatorial.
How did you deduce which spot corresponded to 2-naphthol on your TLC plate that analyzed the reaction mixture at 10, 30, and 60 minutes? Did your TLC analysis indicate the presence of any 2-naphthol remaining at the end of the 60-minute reaction time?
2-napthol on a reaction gives respective ether of alcohol which on the TLC plate comes above the 2-naphthol spot. On monitoring reaction mixtures at 10, 30, and 60 minutes we can see spot formation and intensity of the spot. From this, at the end of the 60 minutes reaction time, 2-naphthol can be found out. Also, by staining the TLC plate with Fe(III)Cl solution, 2-napthol spots can be indicated. As this solution only stains phenols which will be consumed in the reaction. > 2-naphthol decreased in intensity as time went on. The allyl-2-naphthyl ether spot had a higher Rf value than the 2-naphthol spot. > Some was present at the end of the 60 minute reaction time.
What region is the following functional group found on the IR spectrum? N-H amine O-H Alcohols
3200-3500 cm-1 amines 3200-3600 cm-1 alcohols
The solubility of acetanilide is 12.8 g in 100 mL of ethanol at 0 degrees Celsius, and 46.4 g in 100 mL of ethanol at 60 degrees Celsius. What is the maximum percent recovery that can be achieved for the recrystallization of acetanilide from ethanol?
72.4% > Mass of recovered crystals if 46.4 g - 12.8 g = 33.6 grams > % recovery = 33.6/46.4 * 100% = 72.4%
A student was given a sample of crude acetanilide to recrystallize. The initial mass of the crude acetanilide was 166 mg. The mass after recrystallization was 125 mg. What is the percent recovery from recrystallization?
75.3%
A 0.7 mL mixture of pentanol and heptanol are separated and collected using preparative gas chromatography on a GC fitted with a polar column and a TCD detector. During the separation, why are the injection port and detector heated to a higher temperature than the column?
> A heated injector creates flash vaporization which allows separation at temperatures lower than the boiling point of the sample. > The wire in the thermal conductivity detector needs to be cooled by the eluent in order to register a signal.
During the recrystallization of an impure solid sample, the sample is dissolved in hot water then filtered while hot. The filtrate is allowed to cool and the mixture is filtered a second time. Which procedure should a student do if they observe an oil forming as the solution cools?
> Reheat the solution gently until the oil dissolves. Once the oil has dissolved, allow the solution to cool rapidly. Add a small amount of additional water to the flask containing the oil.
What are true statements about the collection of a GC trace?
> The amount of the sample detected is proportional to the peak area. > The stationary phase is a very high-boiling liquid adsorbed onto fire brick.
Describe the practice of thin layer chromatography.
> The solvent level should be below the spots. > Do not allow the edges of the plate to touch the filter paper. > Allow the solvent front to get 5 to 10 mm from the top of the plate. > When the solvent front has reached the correct heigh, remove from the chamber and immediately draw a line at the solvent front. > TLC data is interpreted in terms of Rf values.
What is the melting point of a material? How does the presence of impurities affect it?
A physical measure of the forces that are holding the molecules of the compound into its crystal lattice. > Forces can include ionic attraction, dipole moments, hydrogen bonding, packing symmetry, Van der Waals attraction. > Pure substances typically have a melting range of less than 2 degrees. > The lower limit of the range occurs when the first crystal starts to melt. The upper limit of the melting temperature range occurs when the last remaining crystal melts. > The presence of impurities introduces entropy and lowers the energy required (temperature) to melt the compound.
During a melting point determination, it is normal to first take a rapid rough measurement followed by a slower more accurate measurement. Why is it important to allow the Mel-Temp to cool 15 degrees before placing the second capillary tube in, and to set the voltage generate 1-2 degrees temperature increase per minute?
A rapid temperature increase will give a melting point range that is too high.
How does the polarity of the solvent affect the rate of an SN1 reaction?
A solvent with a high dielectric constant (polar) stabilizes the charge-separated transition state, lowers the activation energy, and increases the rate of reaction. > Carbocations are stabilized by protic solvents like water.
During thin layer chromatography, what method is typically used to apply the dilute solution of the sample to a TLC plate?
A thin glass spotter or capillary tube
How is the use of an infrared spectrophotometer fitted with an ATR sample advantageous over a beam-through sample spectrophotometer?
ATR allows for the use of very small samples and the same ATR sample holder may be used for liquid samples and solid samples. > The ZnSe crystal sample holder can accept very small quantities of solids or liquids and is not degraded by the presence of water. Clean up is usually easy. > Beam-through-sample instruments use sample plates that are made from sodium chloride or potassium bromide and are easily damaged by contact with water; they must never be held with bare hands or washed with bromide and are easily damaged by contact with water; they must never be held with bare hands or washed with water. The plates are also fragile and will easily chip or crack. Spectra of polar protic materials recorded on a beam-through-sample instrument may suffer from diminished resolution due to interactions between the sample and the salt plate.
During a solvent extraction, a student lost track of which layer was the aqueous layer and which was the organic layer. What simple, non-destructive test could be used to determine which layer is which?
Add a little distilled water to see which layer the water adds to.
Sodium hypochlorite and glacial acetic acid combine to make hypochlorous acid. Hypochlorous acid oxidizes cyclohexanol to cycloheanone. At the end of the reaction, any excess hypochlorous acid must be neutralized. How is the neutralization of HOCl accomplished?
Add sodium bisulfite, NaHSO3
During the recrystallization of an impure solid sample of acetanilide, the sample is dissolved in hot water then filtered while hot. The filtrate is allowed to cool and the mixture is filtered a second time. Which of the following actions will increase the percent recovery of the acetanilide solute?
Add the first filtration, place the solution in an ice-water bath for about 10 minutes.
Eugenol, the essential oil found in cloves, is insoluble in water. The alcohol group in eugenol is dramatically more acidic than regular straight-chain alcohols. How could eugenol be separated from a mixture of other non-acidic molecules?
Dissolve the mixture in dichloromethane, shake with sodium hydroxide, collect the aqueous layer, and reacidify to force precipitation of the eugenol. > Eugenol will dissolve in dichloromethane. Shaking with sodium hydroxide generates the sodium salt of eugenol, which will migrate into the aqueous layer. The aqueous layer can then be extracted and reacidified until eugenol precipitates out of solution.
Why is it not possible to use ethanol to extract an organic compound from water even though most organics are more soluble in ethanol than in water? Hint: think about the properties required for two solvents to be used in an extraction.
Ethanol is soluble in water, so no organic layer would be formed. > The two solvents used in extraction need to not mix together. They should be immiscible in one another and form two distinct layers.
Why is extraction a good way to separate and purify an organic material from a mixture of the organic material and inorganic salts (NaCl, MgBr2, etc.)?
Extraction can convert an organic acid or base into its corresponding water soluble salt, which allows it to be easily removed from an organic solution. > Mixtures of organic compounds may have different solubilities, making it easy to separate and purify materials once you start adding acid and base properties)
Fractionating towers in a refinery can be identified as the tall thin towers. Why are they always tall and narrow rather than short and wide (with same volume)?
Fractionating towers are tall because an increased length of a column provides more theoretical plates. They are narrow because decreasing the inside diamter of the column decreases the distance vapor particles must travel before striking a condensing surface, thus increasing the number of plates. The more plates present, the better the separation.
Based upon your experiences with this experiment, consider the result if we heated a mixture of dichloromethane (bp 39-40 celsius) and naphthalene (C10H8, mp 80-82 celsius, bp 218 celsius) to 60 degrees celsius until distillation stopped, and then cooled the flask back to room temperature. This type of mixture might be obtained at the end of an extraction experiment starting with a mixture of benzoic acid, benzocaine, and naphthalene. What would be in the distillate and what would be in the distillation flask?
If mixture of dichloromethane and naphthalene heated to 60 C then only dichloromethane will boiled off into its vapour as its boiling point is less than 60 however naphthalene will not distill because it never reaches its boiling point. So the distillate will contain only dichloromethane and distillation flask will have naphthalene.
Why is it important to wash your flask and collected crystals with ice cold solvent after a recrystallization? What would happen if you used room temperature or warm solvent instead?
It increases the recovery of the material by helping the material crystallize with the lower temperature. Room temperature or warm solvent would not allow the material to crystallize and would result in fewer collected crystals.
What is the retention factor? What are the minimum and maximum values of Rf?
It is a number that describes the position of a spot on a chromatography plate relative to the position of the solvent on the chromatography plate. >Rf = distance spot traveled/distance solvent traveled > Minimum value of 0 and maximum value of 1
When is it possible to use gas chromatography to separate a pair of liquids that have identical boiling points and which show no separation by distillation, even fractional distillation? Hint: A column with a polar liquid phase would be required.
It is possible by measuring how fast the liquids travel up the column with a polar phase. The liquid that is more attracted to the column takes longer to travel through it. If you have something with 2 same boiling points, use GC because it will separate values by polar and non polar columns. As the compounds in the mixture having same boiling point, the only principle for separation is their polarity that results in a different retention time in the stationary/mobile phase. The retention time is the total time that a solute in the mixture spends in the stationary and a mobile phase. Therefore, the compounds in the mixture that has a same boiling point are to be separated depends the stationary liquid phase. As the stationary liquid phase is polar then polar compounds in the mixture will be soluble in the polar phase and move with a slower rate than the non polar compounds in the mixture. In case of a stationary liquid phase is non polar then the non polar compounds will be highly soluble in the liquid phase and moves very slowly through the column resulting in a separation
What is a lachrymator? What does it mean for a substance to be corrosive? flammable? What is a flash point?
Lachrymator: A substance with vapors that irritate the eye, causing shedding of tears. Corrosive: Liquid or solid that causes visible destruction or irreversible alterations in skin tissue at the site of contact. Flammable: Defined as a liquid with a flash point below 100 degrees F. Flash point: Temperature at which a liquid will give off enough flammable vapor to ignite with an ignition source such as a spark.
Soaps are sodium or potassium salts of long-chain carboxylic acids. They dissolve grease by forming micelles that are water soluble because the anionic carboxylate groups are on the outside solvated by water while the inside of the micelle contains the long hydrocarbon chains. If you wanted to design a quaternary ammonium salt that would make a god soap, what type of R groups would you want to attach to the nitrogen?
Large alkyl groups as R > Makes organic ammonium salts that are water soluble regardless of the anion nature. > Would also attach an aryl group as R
Will polar molecules have higher or lower Rf values?
Lower > The stationary phase is polar and the mobile phase is nonpolar
In concentrated sulfuric acid is spilled during the experiment, what is the proper procedure for cleaning the spill?
Neutralize the spill with sodium bicarbonate.
Is density directly related to the molecular weight of a substance? (That is, does density always increase when molecular weight increases?)
No. For instance, water has a smaller molecular weight than cyclohexane but a larger density. > Density depends on how molecules interact.
The experiment states that a distillation should never be continued until distilling flask is dry. Does that mean 'no water present' as when using a drying agent on an organic solution? Explain.
No. It means no liquid present. If the vial is completely dry then it could crack from heat.
The reaction of bromine and of permanganate with cyclohexene both belong to which of the four general kinds of reactions?
Oxidation Reactions/ Addition Reactions
Bleach is used to remove colored stains from clothing. What type of reaction would you suspect that bleach is doing to the colored organic compounds that cause the stains? NOTE: most colored organic compounds extended conjugation of pi bonds.
Oxidation. The active ingredient in Bleach, sodium hypochlorite, a strong oxidant that breaks double bonds in body soil, dirt, and various color compounds. During oxidation, the oxidizing agent removes one or more electrons from its reaction partner, changing the chemical structure and making it no longer able to absorb light in the visible range, therefore appearing white.
A student is comparing two samples of crystalline organic solids. A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. Which of the following would provide the most conclusive evidence that the two samples are the same compound?
Perform a mixture melting point.
The carrier gas in a GC separation is usually an inert gas such as nitrogen, helium, or argon, but can also be hydrogen. What are the pros and cons of using hydrogen as a carrier gas in a GC separation?
Pros: > Cheaper than helium > Can be generated at the GC instead of from gas tanks > Greater difference in mass between carrier gas and sample than nitrogen Cons: > Explosion hazard
Write the rate expression for an SN1 reaction of R-Cl.
Rate = k[RCl] > The rate is dependent on the concentration of the alkyl halide. It does not depend on the concentration of the nucleophile.
What is stereochemistry? conformations? What is the difference between a sawhorse representation and a Newman projection?
Stereochemistry: Branch of chemistry concerned with the 3D aspects of molecules. Conformations: The different arrangements of atoms that result from bond rotation. Sawhorse representation: Views carbon-carbon bond from an oblique angle and indicates spatial orientation by showing all C-H bonds. Newman projection: Views the carbon-carbon bond directly end-on and represents the two carbon atoms by a circle.
The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60 degrees. There is steric strain in the molecule because of the proximity of the two methyl groups. Would the steric strain in this molecule increase or decrease if the C1-C2-C3 bond angle was increased to more than 109.5 degrees?
Steric strain will decrease because groups are moving further apart.
The methylene chloride solution at the end of the reaction period may contain a variety of materials (unreacted allyl bromide or naphthol, phase transfer catalyst, and allyl 2-naphthyl ether). What is expected to happen to the naphthol and phase transfer catalyst when the product is passed through the silica gel mini-column with methylene chloride? (Hint: Remember that silica gel contains many Si-OH groups and is very polar).
The naphthol and phase transfer catalyst should stay in the column. > Since silica gel is very polar, these polar compounds are attracted to it and stay in the mini column with it.
What was the objective of experiment 2? (Recrystallization)
The objective was to demonstrate the use of recrystallization to purify an impure sample of a solid compound.
Organic compounds must exhibit what property in order to be visualized on a TLC plate by the technique (UV light) used in this experiment?
The organic compounds must absorb light in order to be visualized and produce a dark spot.
Define density as applied to organic molecules
The ratio of mass of a substance to mass of an equal volume > The ratio of a substance's mass to its volume, measured in g/mL and also equivalent to g/cm^3
What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?
The recorded boiling point will be lower than actual because the bulb is not exposed to the hot vapor.
In order to perform a bromine test on a compound, it must be treated with a dilute solution of bromine. Which of the following results will indicate the presence of an alkene?
The reddish color of the bromine will disappear.
Why is TLC a poor technique for the analysis of liquids with low boiling points?
The spots might not show up on the plate because they will evaporate before they can be analyzed.
In regard to the TLC plate you will be using in lab, is the stationary phase more or less polar than the mobile phase?
The stationary phase is more polar than the mobile phase.
While diluting some concentrated HCl, a student accidentally got some of the acid on his/her bare hands. What is the appropriate response?
The student should tell the TA or instructor immediately and wash his/her hands with copious amounts of water in the sink.
What would be the effect on the temperatures observed during distillation if the thermocouple was placed near the top of the collection bulb rather than near the top of the column portion of the distillation head?
The temperatures observed would be lower because the thermocouple would be farther from the heat source.
What does infrared spectroscopy measure?
The vibrational frequency of a bond. > An IR spectrum is obtained by passing infrared radiation through a sample. When the radiation frequency matches the vibrational frequency of a chemical bond, the molecule absorbs energy, causing a drop in % transmittance. Each bond type has its own characteristic frequency, so from the IR peaks we can determine the types of bonds present in the molecule.
Identify the 1,3-diaxial interactions in the shown conformation of trans-1,2-dimethylcyclohexane. and cis-1,2-dimethylcyclohexane
There are zero 1,3-diaxial interactions in trans with they are equatorial and 4 when they are axial. There are 2 in cis conformation because one is equatorial and one is axial.
How many gauche interactions are present in cis-1,2-dimethylcyclohexane and trans-1,2,dimethylcyclohexane?
There is one gauche interaction between cis methyl substituents and zero gauche interaction between trans substituent.
The contamination of a pure organic with an impurity affects the observed melting point only is the impurity dissolves to some extent in the liquid phase of the organic compound. Therefore, small granules of sand or broken glass (sand and glass have similar compositions, melt only at very high temperatures, and are totally insoluble) will not reduce the melting point of a pure organic compound with which they might be mixed. In a sample containing small grains of sand, what would you observe in the Mel-Temp apparatus when you melt the sample that would indicate to you that your sample contained this type of impurity?(How would the melted sample look different than the melted sample of a sample without the added contaminant?)
There will not be any change in the melting point. No observed changing in temperature but particles will sink to the bottom.
What was the objective of experiment 5? (Simple Distillation and Gas Chromatography)
To demonstrate methods for purification of volatile compounds (compounds that can be converted to gas phase at reasonable temperature without decomposition) and to demonstrate that gas chromatography may be used to quantitatively determine the relative proportions of volatile compounds in each sample.
What was the objective of experiment 1? (Who has my Compound?)
To demonstrate some of the types of physical properties that can be used to determine the identity or non-identity of two organic compounds. > Determine who else in your laboratory section has the same compound as yours. > Methods: Melting point, thin layer chromatography, solubility in a sodium bicarbonate solution, mixture melting point.
What was the objective of experiment 6? (Extraction)
To demonstrate the use of extraction as a method of separating mixtures of compounds. >Liquid-liquid extraction is based upon the difference in solubility of the desired compound between two immiscible liquids. > Converting an organic acid or base into its corresponding water soluble salt allows it to be easily removed from an organic solution.
What was the objective of experiment 4? (Infrared Spectroscopy)
To learn the basics of infrared spectroscopy through reading and practice with a computer-based tutorial. > To use infrared spectroscopy to identify a series of organic unknowns by their spectra. > To learn how to operate an FTIR and become familiar with many of the characteristic IR absorptions of common functional groups.
What was the objective of experiment 12? (The Dehydration of Cyclohexanol)
To understand elimination reactions through observation of the dehydration of cyclohexanol to form cyclohexene. > Classification tests to test for alkenes (Bromine and Permanganate tests) > IR Spectrum showing C=C stretching freqeuncy
What was the objective of experiment 7? (Preparative Gas Chromatography)
To use preparative gas chromatography to separte a mixture of components and then to use infrared spectroscopy to identify the separated components. > GC requires that the compounds to be separated be both volatile (easily transformed to vapor phase) and thermally stable. > Preparative GC can be used if very little sample is available. Compounds that have identical boiling points, but different functional groups, may be separable by GC.
What was the objective of experiment 8? (Thin Layer Chromatography)
To use thin layer chromatography (TLC) to determine the probable identity of the major active ingredients in commercial analgesic preparations.
The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60 degrees. There is steric strain in the molecule because of the proximity of the two methyl groups. When you consider that E(strain) = E(stretch) + E(angle) + E(torsion) + E(steric), would E(torsion) increase or decrease when the dihedral angle of gauche butane is increased from 60 degrees to 65 degrees?
Torsional strain increases. > The steric strain on the molecule decreases because the methyl groups will move further apart from each other. In contrast, the torsional strain will increase because other bonds in the molecule will move closer to being eclipsed.
What is torsional strain? Steric strain? 1-3 diaxial interactions?
Torsional strain: The strain in a molecule caused by electron repulsion between eclipsed bonds. Steric strain: Repulsive interaction that occurs when atoms are forced closer together than their atomic radii allow. 1-3 diaxial interactions: The strain energy caused by a steric interaction between axial groups three carbon atoms apart in chair cyclohexane.
What is toxicity? LD-50? Irritant?
Toxicity: Sum of adverse effects resulting from exposure to a material, generally by the mouth, skin, or respiratory tract. LD-50: The dose found to be lethal in 50% of a group of test animals when administered. Irritant: Substance that, by contact in sufficient concentration for a sufficient period of time, will cause an inflammatory response or reaction of the eye, skin, or respiratory system.
Why must moisture be excluded from any sample injected into the GC?
Water contains dissolved salts that can degrade the column. Water signals are very broad and make interpretation of other peaks difficult.
For very accurate determination of melting points, scientists use thermometers that have been calibrated by measuring the melting points of known standards. You did not calibrate your thermocouple. What data obtained during this experiment allows you to be confident that the thermometer system you were using was not giving readings that were inaccurate by 10 degrees or more? (What material with a known mp did you determine a mp?)
We use mixture melting points with know standards. We know the thermometer was accurate because the measured melting point values were close to the standard melting point values.
When is recrystallization not a suitable method to purify an organic material?
When the organic material is a liquid instead of a solid.
Explain how pure samples of each component could be obtained by preparative gas chromatography from a mixture of two compounds even if complete resolution of the peaks is not obtained.
You collect samples of each before they overlap. Stopping before the end of the first peak and for sample 1 and starting after the beginning of the second peak for sample 2.