CHEM 237 Lab Final - J&D
Which of the following statements describe why moisture must be excluded from any sample injected into the G.C. Select all answers that apply.
-Water signals are very broad and make interpretation of other peaks difficult. -Water contains dissolved salts that can degrade the column.
Infrared Spectra are recorded using an ATR detector. What does ATR stand for?
Attenuated Total Reflectance
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.
Why is TLC a poor technique for the analysis of low boiling liquids? HINT: low boiling means high vapor pressure at room temperature Experiment 7/8
Low boiling point means it will be able to evaporate at room temperature, off of the plate and produce no spots to view.
Staggered Conformation
Lower energy Dihedral Angel = 60
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. - To prevent evaporation from the TLC plate during development.
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. -First, add a small quantity of water. If that does not work, then add solvent.
Which of the following statements are true concerning azeotropes?
-An unknown liquid that boils at a constant temperature could be an azeotrope. -The vapor of an azeotrope has the same composition as the unboiled mixture.
If a student spills a chemical on his skin or clothing, what should he do? Select two.
-Remove affected clothing -Use the safety shower for spills that cover more than the student's arms
Which of the following statements about the collection of a GC trace is true? Select all that apply.
-The amount of sample detected is proportional to the peak area -The stationary phase is a very high-boiling liquid adsorbed onto fire brick.
List the seven procedural steps, from start to finish, that are required to perform a thin layer chromatographic 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 the 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 solvent have moved.
Procedural steps of 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 trhough 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.
List the seven procedural steps, from start to finish, that are required to perform an accurate melting point determination.
1. Press the open end of the capillary tube into the solid sample. 2. Invert the tube and tap gently to move the solid to the bottom of the capillary tube. 3. Place the tube with the 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 point 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.
List the procedural steps 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 afte rthe 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 cut 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 sample. Cap the vials
List the procedural steps 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 disolce 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
What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?
1400-400 cm-1
What region is the following functional group found on the IR spectrum? C=C alkene
1600-1700 cm-1
What region is the following functional group found on the IR spectrum? C=O ketone
1800-1640 cm-1
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? Experiment 10
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.
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 18.5 g in 100 mL of methanol at 0 °C, and 59.2 g in 100 mL of methanol at 60 °C. What is the maximum percent recovery that can be achieved for the recrystallization of acetanilide from methanol?
68.75%
A student was given a sample of crude acetanilide to recrystallize. The initial mass of the the crude acetanilide was 151 mg. The mass after recrystallization was 121 mg. Calculate the percent recovery from recrystallization.
80.13%
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 g mixture of A and B in a 9:1 ration 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? Hint: calculate how much of A and how much of B that could remain dissolved in this solvent after cooling to room temperature. (This calculation assumes an ideal recrystallization that probably wouldn't be totally true in a real experiment.) This allows you to determine the amounts of A and of B in the recrystallized solid. What is the ratio of A:B in the crystalized material (grams of A: grams of B)? Experiment 2
9 g A : 1 g B 10 mL dissolve: 1 g 1 g crystal 8 g 0g solubility = maximum value cab dissolve in unit solvent 0.1 g/mL X 10 mL = 1 g of B dissolved
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 °C before placing the second capillary tube in, and to set the voltage generate 1-2 °C temperature increase per minute?
A rapid temperature increase will give a melting point range that is too high.
In laboratory, your TA tells you that a substance you are working with is a carcinogen. What does the term carcinogen mean?
A substance or agent capable of causing or producing cancer in mammals.
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
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 what layer the water adds to.
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 acetanide solute?
After the first filtration, place the solution in an ice-water bath for about 10 minutes.
What experimental observation on the initial dissolution of the compound with hot solvent would let you know that a hot filtration was not needed? Experiment 2
After you dissolved the compound and no impurities are seen.
Why would concentrated hydrochloric acid be a poor choice as the acid catalyst for the formation of cyclohexane by dehydration of cyclohexanol? Experiment 12
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 (a rapid, simple reaction) NOT useful as a characterization reaction, but reaction with Br2 is a good characterization reaction? HINT: How does a solution of hydrochloric acid differ in appearance from a solution of bromine? HINT: HBr solutions look like hydrochloric acid solutions that you have used in lab. Experiment 9
Br2 is a good characteristic reaction because once it reacts with a molecule containing a double bond, it loses its brown color. (Anti-addition of Br2 across a double bond) HCl is a colorless substance and does not have visible/physical characteristics (color wise) in a reaction with a double bond.
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 NOT do if they observe an oil forming as the solution cools?
DONT Rapidly cool the solution. DO- Reheat the solution gently until the oil dissolves, Add a small amount of additional water to the flask containing the oil. Once the oil has dissolved, allow the solution to cool slowly.
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.
Dissolve the mixture in dichloromethane, shake with sodium hydroxide, collect the aqueous layer, and reacidify to force precipitation of the eugenol.
What should a student do if there is a fire in the laboratory?
Do not panic, evaluate the situation, and then alert your instructor or teaching assistant for help.
Select the statement about equatorial and axial positions in cyclohexane that is correct.
Each carbon in cylcohexane is attached to one axial and one equatorial hydrogen.
Why is ethanol not a suitable solvent for the solvent extraction of benzoic acid and benzocaine from aqueous solution?
Ethanol is miscible with water.
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 solubility properties of ethanol in water, and the properties required for two solvents to be used in an extraction. Experiment 6
Ethanol mixes with water (not immiscible in water). The two solvents used in extraction need not to mix together. They should be immiscible in one another and form two distinct layers like oil and water.
What solvent had the highest eluting power for the analgesics analyzed in this experiment? Which solvent had the lowest eluting power? Experiment 7/8
Ethyl acetate had the highest eluting power while hexanes had the lowest. 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 and all the samples where polar functional groups, therefore it doesn't move the spots. The mixture of 75% ethyl acetate-25% hexane seem to be just the right mix of polarity, providing significantly higher separation of the standards and well as our three analgesics.
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.)? Experiment 6
Extraction is a good way to separate and purify organic material because mixtures of organic compound may have different solubilities, making it easy to separate and purify materials once you start adding acid and base properties.
True or False: The twist boat conformation of cyclohexane has angle strain but is almost free of steric strain.
FALSE
True or false? 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.
FALSE
(T/F) Polar compounds will have higher Rf values than nonpolar compounds under the same developing conditions.
False
(T/F) Rf is not dependent on polarity.
False
(T/F) The less polar the solvent, the higher the Rf values.
False
(T/F) Use a pencil or pen to mark the position of a spot on the TLC plate.
False
(T/F) You should mark the TLC plate after applying the sample
False
Recrystallization is a common laboratory method to purify solids by dissolving them in hot solvent, filtering while hot, then allowing the filtrate to cool so that crystals form. Use the pop-up menus to label the following statements about the practical aspects of recrystallization as either True or False. (T/F) For hot vacuum filtration, the filter paper should be completely dry when pouring the hot solution into the Buchner funnel to filter.
False
Recrystallization is a common laboratory method to purify solids by dissolving them in hot solvent, filtering while hot, then allowing the filtrate to cool so that crystals form. Use the pop-up menus to label the following statements about the practical aspects of recrystallization as either True or False. (T/F) For recrystallization, rapid cooling gives the best crystals.
False
True or False Infrared spectroscopy can be used to analyze almost any kind of sample.
False
When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample?
For a pure substance, the melting temperature range is narrower than that of an impure sample. For an impure substance, the melting point is lower than that of a pure sample.
Gauche Interactions
Gauche interaction between two methyl groups.
Mixture melting point determination, solubility tests, and thin layer chromatography can be used to assess the purity and identity of a substance. Drag and drop each of the chemical wastes generated in this reaction into the appropriate container.
Glass Waste: used melting point capillaries Solid Waste and Used TLC Plates: contaminated paper towels 10% Aqueous Sodium Bicarbonate Waste: sodium bicarbonate solution Hexanes, Ethyl Acetate, Ethanol, and Acetone Waste: TLC solvent
Eclipsed Conformation
Higher energy Dihedral Angel = 120
To get effective mixing in a separatory funnel, combined level of liquids should not exceed:
Higher than the widest part of the separatory funnel.
Clove oil is produced by a steam distillation of cloves. The basic extraction of clove oil to obtain eugenol leaves behind several other organic products. What types of steam distillation will not steam distill? (Read requirements for a molecule to steam distill) Experiment 9
Hydrocarbons and neutral compounds will not steam distill because they are not soluble in water.
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? Experiment 5
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.
What effect would each of the following changes to the conditions of a GC separation have onthe retention time of the sample? The amount of sample injected is doubled... The carrier gas flow is increased... The column temperature is decreased...
If the amount of sample injected is doubled the retention time increases If the carrier gas flow is increased than the retention time decreases If the column temperature is decreased than the retention time increases
When would a hot filtration step NOT be necessary or useful in a purification by recrystallization? Experiment 2
If the solid impurities were soluble, or if there are no insoluble solids present.
When is it possible to use gas chromatography to separate a pair of compounds 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. Experiment 7/8
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
Organic compounds must exhibit what property in order to be visualized on a TLC plate by the technique (UV light) used in this laboratory? Experiment 7/8
Inert organic material that fluorescence under short wave ultraviolet light. Either the TLC plate or the organic compound need to florescent but not both or neither; usually the TLC plate has a powder that glows which makes the spots appear dark under the UV.
IR spectrum
Infrared radiation that measures the vibrational frequencies of bonds.
Which is NOT true about the chair conformation of cyclohexane
It does NOT contain sex equatorial hydrogens that are perpendicular to the ring. True: Contains sex axial hydrogens that are perpendicular to the ring and parallel to the ring axis. Contains six equatorial hydrogens that are in the approximate plane of the ring and that radiate around the ring's equator.
The retention factor, or Rf, 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. What are the minimum and maximum values of Rf?
Minimum Rf value: 0 Maximum Rf value: 1
Is density directly related to the molecular weight of a substance? (That is, does density always increase as molecular weight increases?) Experiment 6
No, it does not. Ex. water has a smaller molecular weight than cyclohexane but a larger density. It has to do with how molecules interact that determines density.
Does density always increase as molar mass increases?
No. Density is mass/volume.
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. Experiment 12
No. It means no liquid present. If the vial is completely dry then it could crack from heat.
At the end of an experiment, a student has an unused portion of reagent. Where should he or she place it?
See if anyone else doing the experiment needs some of the reagent or dispose of it in the proper waste container.
The reaction of bromine and of permanganate with cyclohexene both belong to which of the four general kinds of reactions? Experiment 12
Oxidation 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. Experiment 13
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.
The carrier gas in 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: hydrogen is cheaper than helium, it can be generated in the GC instead of from gas tanks, and there is a 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. Experiment 11
Rate= k[RCl] The rate is based off the concentration of the alkyl halide.
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 too rapidly?
Small impure crystals will form.
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? Experiment 13
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).
The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60°. 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 (and the C2-C3-C4 bond angle) was increased to more than 109.5° ?
Steric strain will decrease because groups are moving further apart.
Fractionating towers in a refinery can be identified as tall thin towers. Why are they always tall rather than short and fat (with same volume)? Experiment 5
Such towers are used to separate the liquids having boiling points very close (fractional distillation) so the length of the tower is useful for separation of condensing material and evaporating material, higher is the length of the tower more purity of a fraction we get.
The different geometries that a molecule can attain by bond rotations and bends are called conformations.
TRUE
True or False: Each carbon is cyclohexane is bonded to one axial and one equatorial hydrogen.
TRUE
True or False: Substituents larger that hydrogen prefer to adopt equatorial positions due largely to a smaller amount of steric strain.
TRUE
Which of the following statements best describes eutectic composition?
The composition of a mixture of two components that generates the lowest possible melting point. (broad low range)
Which, if any, of the fractions you obtained from each of the two distillations conducted in this experiment had boiling point ranges and gas chromatogram that suggested that these fractions were primarily one component? What was the largest ratio observed for the two components in these fractions? Experiment 5
The first and last fractions had primarily one component. The largest ratio was 2:1 for octane.
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 heater to a higher temperature than the column?
The injection port and detector are heated to a higher temperature because a heated injector creates flash vaporization which allows separation at temperatures lower than the boiling point of the sample and the wire in the thermal conductivity detector needs to be cooled by the eluent in order to register a signal
Why is the reaction of cyclohexene with HBR (a rapid, simple reaction) not useful as a characterization reaction? Experiment 12
The is no visible change during the reaction.
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 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). Experiment 10
The naphthol and the phase transfer catalyst will be held behind in the silica gel mini-column as the product passes through. The silica gel polar, therefor polar compounds remain in the column and non-polar compounds will pass through with the non polar solvent (methylene chloride).
Which of the following statements about melting point determination is not true?
The presence of impurities introduces entropy, which raises the energy required to melt the substance. True: - Pure substances typically have a melting range of less than 2 °C - The lower limit of the melting temperature range occurs when the first crystal starts to melt. - The melting point is a physical measure of the forces holding the material into its crystal lattice - The upper limit of the melting temperature range occurs when the last remaining crystal melts.
Define density as applied to organic molecules. Experiment 6
The ratio of mass of a substances to mass of an equal volume.
What error is introduced if the bulb of the thermometer, or the thermocouple, is too high in the distillation apparatus?
The recorded bp will be lower than actual because the bulb is not exposed to the hot vapor.
1-Phenyl-1-chloroethane is a secondary halide. Most secondary halides undergo Sn1 reaction quite slowly. Explain why 1-Phenyl-1-chloroethane undergoes Sn1 nsolvolysis quite readily. (Consider the mechanism shown above) Experiment 11
The resonance from the phenyl group makes the intermediate secondary carbocation more stable after Cl- is lost. Thus allowing the reaction to go faster.
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? Experiment 5
The temperature range would be lower than the actual boiling point. The vapor is already condensing at that point.
How is it possible to separate two liquids with the exact same boiling point by gas chromatography when it cannot be done by fractional distillation?
The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility
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.
Identify the gauche interactions between methyl groups in the shown conformation of 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?) Experiment 1
There will not be any change in the melting point. No observed changing in temperature but particles will sink to the bottom.
Why do you vent the separatory funnel through the stopcock To avoid a build up of pressure inside the glassware
To avoid a build up of pressure inside the glassware
Why are the vials placed into the centrifuge in pairs, 180 degrees apart?
To avoid dangerous vibration due to imbalance
Why should you never point the spearatory funnel at anyone including yourself?
To avoid spilling chemicals on yourself and others
Why should you not rinse the syringe with acetone, but instead rinse it with the sample?
To avoid the introduction of a spurious peak into the GC trace
Why do you add acid to the three component mixture? To convert the benzocaine into a water soluble salt
To convert the benzocaine into a water soluble salt
The simple distillation set up provides one theoretical plate. Use the data in Table 1 of the Distillation Techniques chapter to estimate the minimum boiling point difference between two compounds that would allow effective separation (98% purity) of two compounds by simple distillation. Experiment 5
To have one plate, you would have to have around a 40 degree celsius boiling point difference (+/-1). The lower the theoretical plates the higher the difference in boiling points between two compounds needed.
Why do you add base to the aqueous acidic layer mixture?
To precipitate the benzocaine
Why must you wait for the second peak to be on the rise before beginning its collection
To preserve the purity of the sample because there is a lag between being detected and being eluted
What is the purpose of a hot filtration in a recrystallization? Experiment 2
To remove all the insoluble solid impurities
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 is the purpose of the initial hot vacuum filtration in the recrystallization process?
To remove insoluble solid impurities.
The gauche conformation of butane occurs when the dihedral angle between the C1-C2 bond and the C3-C4 bond is 60°. 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° to 65° ?
Torsional strain increases. BUT the magnitude of steric stain will decrease because the groups are moving further apart.
(T/F) Only touch the edges of the TLC plate
True
(T/F) Rf is dependent on polarity.
True
(T/F) 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.
True
(T/F) Spot the TLC plate on the dull, not shiny side of the plate.
True
(T/F) Spots should be 1 to 2 mm in diameter on TLC plates
True
(T/F) The Rf value is defined as the ratio between the distance the spot moved and the distance the solvent moved.
True
(T/F) The distance the solvent moved is where the spot started to where the solvent front ended up.
True
(T/F) The distance the spot moved is where the spot started to the center of where it ended up.
True
Recrystallization is a common laboratory method to purify solids by dissolving them in hot solvent, filtering while hot, then allowing the filtrate to cool so that crystals form. Use the pop-up menus to label the following statements about the practical aspects of recrystallization as either True or False. (T/F) 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.
True
Recrystallization is a common laboratory method to purify solids by dissolving them in hot solvent, filtering while hot, then allowing the filtrate to cool so that crystals form. Use the pop-up menus to label the following statements about the practical aspects of recrystallization as either True or False. (T/F) Recrystallization requires the material that is to be purified is a solid.
True
Recrystallization is a common laboratory method to purify solids by dissolving them in hot solvent, filtering while hot, then allowing the filtrate to cool so that crystals form. Use the pop-up menus to label the following statements about the practical aspects of recrystallization as either True or False. (T/F) Use clamps to secure the filter flask to avoid tipping and unnecessary exposure to hot liquids or the solute.
True
True or False Infrared spectroscopy can detect impurities within a sample.
True
True or False Infrared spectroscopy is good for detecting and identifying functional groups of a sample.
True
True or False Infrared spectroscopy is non-destructive.
True
True or false? Each vaporization-condensation is called a theoretical plate.
True
True or false? Increasing the length of the column and packing the column with glass beads or rings are methods that produce more theoretical plates
True
True or false? The rate of heating affects the number of theoretical plates in the column.
True
Use the pop-up menus to label the following statements about cyclohexane as either True or False. After a ring flip, an axial substituent in one chair form becomes an equatorial substituent.
True
Statements describing requirements for suitable solvents for recrystallization are listed below. Sort these requirements as either true or false. - The solvent should not chemically react with the compound. - The solvent should chemically react with the compound. - The solvent should either dissolve the impurities at all temperatures or not dissolve the impurities at all. - The solvent should dissolve the compound the compound while cold. - The solvent should dissolve the compound while it's hot. - The solvent should not dissolve the compound while cold. -The solvent should not dissolve the compound while it's hot.
True: -The solvent should not chemically react with the compound. - The solvent should either dissolve the impurities at all temperatures or not dissolve the impurities at all. - The solvent should dissolve the compound while it's hot. -The solvent should not dissolve the compound while cold. False: - The solvent should dissolve the compound the compound while cold. - The solvent should not dissolve the compound while it's hot. - The solvent should chemically react with the compound.
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?) Experiment 1
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. The data falls in less than +/- 10 degrees of literature values of known standards
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 components even if complete resolution of the peaks is not obtained. Experiment 7/8
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.
Match the type of fire to the method used to safely extinguish the fire. a. Small fire contained completely with a beaker. b. Fire confined to a small portion of clothing. c. The entire room is on fire. d. A large portion of your clothes is on fire.
a. Cover the flame with a watch glass or hard-back book. b. Pat the fire out. c. Sound the fire alarm and leave the building. d. Use the safety shower or the fire blanket.
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. Experiment 9
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 asalt with NaOH.
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. HINT: Think about the effect of the change on the equilibrium of the analyzed compound between the gas phase and the immobile liquid phase. More time spent in immobile phase equals longer retention time. a. Column temperature decreased b. Carrier gas flow rate increased c. Concentration of stationary phase liquid increased d. Detector temperature increased 10 degrees celsius e. Amount of sample injected decreased 20% from a sample that give good separation Experiment 7/8
a. longer b. shorter c. longer d. shorter e. no change
What situation do students not need to report to their instructor immediately? a. if glassware is broken b. if they forgot their laboratory report c. if there is a fire in the laboratory d. if there is a chemical spill
b. if they forgot their laboratory report
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 Experiment 11
b. time-1
Use the melting point phase diagram on the back of this page to help answer this question. If you compare the melting point of a slightly impure mixture with the melting point of a eutectic mixture, the melting point of the slightly impure mixture will be: a. broader and lower than the eutectic mixture b. sharper and lower than the eutectic mixture c. broader and higher than the eutectic mixture d. sharper and higher than the eutectic mixture Explain why. Experiment 1
c. broader and higher than the eutectic mixture An eutectic mixture is a mixture that melts at a lower temperature than a pure mixture.
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? Experiment 13
c=o Carbonyl groups c-c Alkane groups
There are four major methods of purification in the chemistry laboratory. Which of the following techniques is not included in the four most common methods for separation and purification? a. chromatography b. distillation c. recrystallization d. subtraction e. extraction
d. subtraction
Indicate inappropriate attire for a laboratory setting. a. tank top shirt b. sandals c. capris d. bare back dress e. all of the above
e. all of the above
What should you locate when entering lab? a. eyewash stations b. emergency exits c. safety shower d. fire extinguisher e. all of the above
e. all of the above
What safety rule does not apply to the organic chemistry laboratory? a. No eating, drinking, or smoking in the laboratory. b. Wear proper clothing at all times c. Wear enclosed safety goggles at all times while in the laboratory d. Know the location of, and how to use the safety equipment e. none of the above
e. none of the above
True or false? The smaller the boiling point difference, the easier it is to separate two liquids and fewer theoretical plates are required.
false
Why is the detector fitted to the prep GC is a TCD not an FID
to avoid the loss of sample
