Chem 43A Final
Chemical Safety and Disposal: The TOXIC label on a chemical bottle denotes a:
deadly poison
Melting point depression or elevation
wide melting point range impurities
What is the result of boiling the impure product with too much solvent and then cooling on ice?
When too much boiling solvent is used, both the product and the impurities remain in solution. On cooling, the solution will be too dilute to reach saturation, and no crystals will form.
Which of the following statements concerning the principle of separation in gas chromatography is true when using a liquid stationary phase? More than one answer may be correct.
Compounds are separated in the column based on the level of interaction with the stationary phase. The greater the interaction with the stationary phase the slower the compound will travel through the column. When the gaseous sample enters the column, it condenses and dissolves in the liquid stationary phase until its equilibrium vapor pressure has been reached. The lower the vapor pressure of a compound, the greater its interaction with the stationary phase. Therefore, factors that affect vapor pressure, such as size, affect the level of interaction with the stationary phase. The higher the vapor pressure of a compound, the greater the proportion that remains in the mobile gas phase, hence the faster the compound gets carried through the column by the carrier gas.
Which of the following is an appropriate use for distillation apparatus? More than one answer may be correct.
If the objective is to collect a solid from a liquid, use filtration, not distillation. Distillation can be used to remove a liquid from a reaction mixture, and can be used to measure the boiling point of a liquid. Fractional distillation is often used to separate a mixture of solvents.
Why should the solvent level in the elution tank be below the level of the baseline of the TLC plate?
If the spots dissolve into the solvent, they will not move up the plate and be resolved.
Distillation safety
SAFETY: When performing a distillation, the collection end should be in some way open to the atmosphere.
Peak Area
The area (triangle approximation) for symmetrical peaks can be calculated using: The percentage composition can then be calculated for each peak using:
SAFETY: Where should the reflux equipment be clamped?
The equipment should be secured at the neck of the flask. Any extra clamping of the condenser may cause the joint between the flask and the condenser to be compromised, allowing vapor to escape.
Distillation
The height of the vapor can be seen as a ring within the condenser. This should not extend beyond half way up the condenser and preferably lower to ensure no vapor is lost from the system.
When performing a fractional distillation, how can you tell when your liquid is beginning to distill over? More than one answer may be correct.
The temperature will begin to rise when you start heating your sample, but the liquid will only start to distill when it is boiling and vapor is rising up to the still head and thermometer bulb. A tell-tale sign is to see droplets forming on the thermometer bulb. -The temperature remains constant after initially rising.
Which of the following would give you the most accurate mass of your compound?
Top-loading balances give mass to the nearest 0.1-0.001 g, depending on the model, whereas analytical balances give mass to the nearest 0.0001 g. Analytical balances should be used for accurate mass measurements. Weighing by difference removes the error associated with a particular balance, hence increasing the accuracy.
Caffeine
belongs to organic nitrogen bases from alkaloids -moderately polar and can be extracted from water into a polar, non-protic solvent like methylene chloride
Solvents: Which of the following are hazards associated with dichloromethane?
-It can dissolve some of the fatty tissues in the skin with prolonged contact, causing chemical burns -Harmful by inhalation -Limited evidence of a carcinogenic effect Dichloromethane is harmful by inhalation and can dissolve some of the fatty tissues in the skin, causing chemical burns. There is also evidence that suggests it may be carcinogenic (cancer-forming). It is not particularly flammable under ordinary lab conditions; however, you must take care with the vapor, as it is dangerous. Make sure you place the residues into a chlorinated waste container.
Vacuum filtration
1. Isolate 2.0 g of crystals from about 50 mL of solution after performing a crystallization. (Vacuum filtration with a Hirsch or Büchner funnel)
Identification of unknown chemical compounds
1. chemist identify reaction byproducts to improve reaction yield -which component of natural extract is responsible for desired bioactivity (drugs) 2. forensic chemist at DEA needs to identify white powder at crime scene
Gravity Filtration
1. removed powdered decolorizing charcoal from 20 ml of solution (Gravity filtration using a fluted filter) -Dissolved materials cannot be removed by filtration; however, the impurities can be removed on a silica gel or alumina column. Decolorizing carbon (pelletized Norit) may also be used. 2. Remove solid impurities from 5 mL of liquid at room temperature. (Gravity filtration using a filtering pipet)
Chemical Derivatives
BP determination often unreliable -liquid organic compound reacted with specific reagent to generate new compound -can now use Melting point determination purify by recrystallization -determine mp to identify derivative -IR to confirm functional group transformation -no solid means you need to convince derivaitve to crystallize (scratching, removing residual solvent, cooling -recrystallize from methanol, but methanol might not be best (ethanol or water may work better) -water risky bc if it doesnt work, must remove water to try new solvent
What happens if the spots are made too large when preparing a TLC plate for development?
If the spots are made too large they may saturate the slide, show tailing, diffuse radially outward, or run into one another.
What reagent (or compound) causes the observed visual change in a positive Lucas test?
The alkyl halide product is insoluble in water
From the plot of vapor pressures vs temperature above, estimate the boiling point of methanol when the external pressure is 430 mm Hg.
The boiling point is the temperature at which the vapor pressure is equal to the external pressure. From the graph, the vapor pressure of methanol is estimated to be 430 mm Hg at 50 °C.
What observation would you expect to make if a primary or secondary alcohol reacted with acidified potassium dichromate(VI) solution?
The orange solution turns green.
If you separated a mixture of benzene, toluene, and m-xylene by gas chromatography, what would be the expected order of retention times?
The order of retention times is as follows: Shortest retention time → Longest retention time Benzene Toluene m-Xylene
SAFETY: What is the purpose of venting the separatory funnel?
To relieve built up pressure from gases produced, or solvent vapor formed The pressure can build due to the volatility of the organic solvent, or if a reaction occurs between a product and another component of the mixture that releases a gas, such as carbon dioxide. This pressure needs to be relieved by venting, preferably into a fume hood.
When adding drying agent, how do you know when you have added enough?
When the solvent is wet and drying agent is added, it forms clumps as it starts to absorb the water. When clumps no longer form, the water has all been absorbed.
SAFETY: When heating a flammable or volatile solvent for a recrystallization, which of these statements are correct? More than one answer may be correct.
You may be heating potentially flammable solvents, so you need to be aware of the normal associated risks. Keep well away from open flames. You should never stopper a flask that is being heated.
Which structure best fits the above spectra? 3200 (O-H, N-H, C-H), 1700 (C=C, C=O, C=N, N=O, N-H)
aldehyde
Basic organic compounds
amines can be converted to water soluble cations by treatment w inorganic acids -dilute hydrochloric acid used for extraction/removal of basic organic substances from an organic phase
Which key reagents would you need to use to distinguish between following two molecules? secondary alcohol and primary alcohol
chromic acid
Gas Chromatography
gas as mobile phase -compounds vaporized -inert gas (helium or nitrogen) carries vaporized compounds through stationary phase -detector reports time it takes for each component to pass through column (similar to Rf value) -comparison of known and unknown retention times -peak area = content amount in each fraction -clean syringe if there is resistance -detector response factor: set standard to value of one and others are relative to that number (1:1) -multiply fraction by volume
Cellulose
major component of tea leaves - polysaccharide containing of thousands of (1,4)-βD-glucopyranose units -structural element -has many hydrophilic hydroxy groups but is not water soluble bc of high molecular weight -removed in first solid-liquid extraction w hot water
Infrared Spectroscopy
organic molecules absorb infrared light when wavelength corresponds to specific molecular vibrations and rotations -IR spectrum shows peaks at wavelengths on light are absorbed -functional groups recognizable by peaks -IR quickly identifies functional groups and NMR can confirm them -fingerprint region: signature of molecule
Jones Oxidation Test
positive result: red orange Cr(VI) reduced to blue green Cr(III) reagent: chromic acid (CrO3 and H2SO4) functional groups: 1, 2 alcohols and aldehydes -aliphatic and aromatic aldehydes give positive test within 30-90 seconds -1,2 alcohols give very fast rxn -amines can react w chromic acid to give dark brown black ppt with heating
Sublimation
solid --> vapor -caffeine becomes crystal at top of petri dish TLC analysis in exp 4 -identification by melting point is difficult bc caffeine sublimes at 178C before melting (238C)
Good GC Conditions
-symmetrical peaks -sharp peaks -good resolution, fully separated peaks broad peaks from sample taking long time to travel (can increase gas flow rate or temperature to fix) Nonsymmetrical peaks are grouped into 'fronting' and 'tailing' peaks: 1. Fronting (1) occurs when a greater amount (represented by area) of a compound elutes before the maximum detection, and is caused by overloading the column. 2. Tailing (2) occurs when a greater amount (represented by area) of a compound elutes after the maximum detection, and is caused by adsorption to the stationary phase (this occurs if the compound is polar and can be overcome by derivatization). Ideally, the peaks should be Gaussian-shaped (Gaussian-shaped peaks are symmetrical). Peaks broaden due to diffusion of the molecules within the column. The longer it takes for a sample to travel through the column, the greater the extent of diffusion, and the broader the peak. If substantial peak broadening occurs, peak resolution becomes poor. The time that a sample spends in the column can be reduced by increasing the carrier gas flow rate or increasing the rate of temperature increase.
What is a good R f value in order to ensure that a compound will elute over a reasonable timeframe?
0.3 Too high and the compound won't elute. Too low and it will run through the column too quickly.
1. An inexperienced chemist adds too little boiling solvent to an impure sample of product and then leaves this mixture to cool. The crystals: 2. Which of the following statements is true when considering temperature during a recrystallization? 3. In a recrystallization, after the crystals have formed (with cooling on standing), the solution can be cooled with ice. Why is this done? 4. Once pure crystals have formed in the Craig tube, which technique can you employ to isolate the pure solid?
1. As too little solvent is used, the impurities that remain in the solid state act as nucleation centers. Pure product forms on the outside of the impurities, so the crystals that form are still impure. 2. If too much solvent is added, it may not be possible to retrieve the entire sample as it will have a degree of solubility in the cold solvent. If the impurities remain in solution, they can be washed away. If they remain undissolved, they can be filtered off before recrystallization. 3. The solubility is lower at lower temperatures, so the cooler the solution, the more material will crystallize out. 4. After centrifugation, the solvent is present in the bottom of the centrifuge tube, and the solid remains in the Craig tube.
Chemical Tests
1. IR used to identify presence of key functional groups 2. chemical tests: visual change that indicates reaction took place w functional group -color changes, formation of precipitates, formation of bubbles -positive and negative control used to ensure clear analysis
Chromatography
separates compounds, based on intermolecular interactions between the compound, solvent (mobile phase) and stationary phase (silica gel) liquid chromatography: solid stationary phase and a liquid mobile phase 1. thin layer chromatography (TLC): stationary phase is affixed in a thin layer to a plate (glass, plastic, aluminum) - capillary action -spotting: drop containing a sample for separation is placed at one end of the TLC plate -spots analyzed by UV light (if not colored) or exposed to I2 to make compounds turn brown -Rf = distance spot moved / distance of mobile phase - too much sample: streaks 2. column chromatography: stationary phase is packed into a tube and the mobile phase is passed over the stationary phase using gravity or pressure gas chromatography: liquid stationary phase and a gas mobile phase
Which of the following solvents would be the best to separate a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid by TLC?
Acetone (polar)should work well because the mixture is highly polar. An acetone-hexane mixture might work well, too.
Chemical Safety and Disposal: Which of the following statements is a correct description of carcinogens?
Carcinogens are chemicals that are known to cause cancer. This is the main reason why benzene is being replaced by toluene in the lab. Suspected carcinogens that you may encounter are dichloromethane and chloroform. Take care when using these compounds.
A sample is spun using a centrifuge. What name is given to the liquid found above the precipitate?
Centrifuging a sample that consists of a solution and a precipitate causes the precipitate to settle at the bottom of the tube. The liquid above the precipitate is called the supernatant.
3,5-Dinitrobenzoate Ester Derivatives
Chemical reaction: ester formation Reagent: 3,5-dinitrobenzoyl chloride, pyridine Functional groups: alcohols Product: benzoate ester
Chemical Safety and Disposal: If concentrated hydrochloric acid is spilt onto your skin, you should:
If concentrated hydrochloric acid is spilt onto your skin, you should wash it off with plenty of water from the tap. Concentrated acids are corrosive and can do serious damage to living tissue. The water in the lab safety kits is for use in the eye only.
What type(s) of intermolecular forces are expected between CH3CH2CH2CH2CH2CH3 molecules?
Induced dipole forces are always present. Induced dipole forces are also called dispersion forces, or London forces. Hydrocarbons are nonpolar. Therefore, CH3CH2CH2CH2CH2CH3 is a nonpolar molecular compound and the only intermolecular forces are induced dipole forces. Hydrogen bonding is found in situations represented by D-H---A where both the donor atom, D, and the acceptor atom, A, are one of the highly electronegative elements O, N, or F. Hydrogen bonding is not expected to be important in compounds in which all of the H atoms are covalently bonded to C atoms, even when an acceptor atom is present in the molecule.
Once a sample has been separated into precipitate and supernatant, how is the supernatant isolated?
It is important not to disturb the precipitate when removing the supernatant from the sample tube. The best way to isolate the supernatant is using a pipet.
Which of the following solvents would be the best to separate a mixture containing 2-phenylethanol and acetophenone by TLC? (Select all that apply.)
Methylene chloride or hexane or mixtures of the two might be good choices because the compounds are somewhat polar
Solvent Evaporation
Once a reaction is complete, the solvent must be removed so that the yield can be recorded and the melting point determined. Removing the solvent does not purify the product because any solid impurities, or liquids with higher boiling points, are left behind when the solvent is removed. If further purification of the product is required, then recrystallization (for solids), and column chromatography techniques (when appropriate) should be used.
SAFETY: Why is it important to handle silica powder in a fume cupboard? More than one answer may be correct.
Silica powder can be carcinogenic if inhaled. It is a very fine powder that readily forms clouds of dust that are easily inhaled.
A solid substance has a vapor pressure of 100 mm Hg at the melting point (100 °C). Assuming an atmospheric pressure of 760 mm Hg, describe the behavior of this solid as the temperature is raised from room temperature to its melting point.
Solids sublime if their vapor pressures are greater than atmospheric pressure at their melting points. Here, the vapor pressure of the solid is much lower than atmospheric pressure at the melting point, so the substance melts rather than sublimes.
Solvents: Flammable liquids will often have a stated flash point. What is meant by the flash point?
The flash point is the temperature at which the amount of vapor above the liquid is sufficient for it to ignite when an ignition source is present.
Predict the trend in boiling points for the following compounds (1 being the lowest boiling point).
The trend in strength of intermolecular interactions is: Ionic > Hydrogen bonding > Dipole-dipole > London dispersion For organic molecules which exhibit only London dispersion forces, the boiling point will increase with molecular weight and decrease with increasing branching. The greater the molecular weight, the greater the number of electrons there are to produce a temporary dipole, and the greater the distance over which the dipole can be formed. For a given molecular formula, increasing branching will decrease the boiling point, because the surface area available for London dispersion forces decreases.
How long should you run a TLC plate for?
Until the solvent front is just below the top of the plate You should maximize the length of the run in order to get good separation; however, be aware that the longer the plate is run, the more diffuse the spots will become.
Craig tubes
Used to collect a small amount of crystals resulting from crystallizations in which the volume of the solution is less than 2 mL. 1. Collect crystals obtained from crystallizing a substance from about 1 mL of solution.
SAFETY: When injecting a sample solution it is important to
When manually injecting your sample it is important to remember to clean the syringe with an appropriate solvent, and rinse it with a portion of your sample before using it. The sample should be drawn up into the barrel so that no solution remains in the needle; any solution left in the needle would boil upon contact with the hot metal needle guide. The syringe should be held with two hands; one hand holding the barrel and the other positioned behind the plunger. When injecting samples, be careful not to touch the metal needle guide and not to press the plunger until the needle has pierced the septum.
Gravity Filtration
When the filter paper is folded and inserted into the filter funnel, it should not extend past the top of the funnel.
SAFETY: When performing a filtration under vacuum, always make sure that:
When the rubber tubing is attached and the solid/liquid mix is poured in the funnel, the apparatus may become unstable, and there is a risk it might fall over. To avoid accidents, the collection flask should be secured with a clamp and a stand.
2,4-Dinitrophenyl hydrazine (DNP) test
positive result: precipitate formation; red orange precipitate forms from clear orange solution reagent: 2,4 dinitrophenyl hydrazine functional groups: aldehydes, ketones -amines give false positive -used as derivative (DNP test run in ethanol whereas derivatives prepared in DMF)
Fischer Esterification
-small drop of sulfuric acid catalyzes reaction -large excess of acetic acid used and byproduct water is removed -acetic acid neutralized with sodium carbonate and removed in aqueous layer
water soluble compounds
-tea proteins and pigments: very soluble in water, not soluble in methylene chloride Saponins: amphiphiles (both polar water soluble group and large non-polar water insoluble hydrocarbon group) -induce formation of foams and emulsions -emulsions broken up by adding salt to aq layer (increases polarity of water and reduces solubility of org molecules) Tannins: polyphenolic compounds w molecular weights of 50-20,000 -water soluble, extracted from leaves into hot water -bitter taste of tea, coffee and found in wine -D-glucose core with multiple units of gallic acid via ester bonds -gallic acid is a polyphenol which extends the tannin structure through further ester bonds (more gallic acid molecules) -low molec weight tannins soluble in methylene chloride, but esters can be hydrolyzed using water and an acid or base catalyst to give glucose and gallic acid (cleaved w base --> carboxylate anion of gallic acid is formed as salt) very soluble in water -sodium carbonate
Why might you want to heat a reaction? More than one answer may be correct.
-to overcome the activation energy for the rxn -to increase the rate of the reaction In order for some reactions to proceed they require extra energy, which can be supplied in the form of heat, to overcome the activation energy for the reaction. Applying heat also increases the rate of the reaction since the reactive species will have greater kinetic energy, resulting in more successful collisions with the required energy to react. You would not usually want to remove the solvent during the reaction. To make sure this doesn't happen you need to fit the reaction flask with a condenser and check all glass joints are tightly sealed. Adding additional solvent would be preferable to heating a reaction mixture to aid solubility.
Shown below is the phase diagram for a typical liquid mixture of two components, A and B. 1. What is the significance of the horizontal line between points X and Y? 2.Using the phase diagram, determine the molar composition of the vapor in equilibrium with a boiling liquid that has a composition of 80.% A and 20.% B. a. percentage vapor composition A = 98% b. percentage vapor composition B = 2% c. What is the boiling point of this liquid?
1. A boiling liquid with composition X will produce vapor with composition Y. 2. The molar composition of the vapor in equilibrium with a boiling liquid that has a composition of 80.% A and 20.% B can be determined by drawing a vertical line from the x axis to the liquid curve at the point where the molar composition of A is 80. % (such as line WX on the diagram). -A horizontal line is then drawn from the liquid curve to the vapor curve in the direction of the lower boiling point liquid (such as line XY on the diagram). This point corresponds to the molar composition of the vapor and by drawing a vertical line down to the x axis the values for A and B can be read. a. Percentage vapor composition A=98% b. Percentage vapor composition B= 2% c. Boiling point = 95 C
Correlate the microscale procedures needed to accomplish the given steps (1-5) to isolate pure isopentyl acetate (banana oil) from the reaction mixture. (Not all of the steps on the left are required.)
1. Aqueous sodium bicarbonate is added to the reaction mixture. This deprotonates unreacted acetic acid, making a water soluble salt. 2. The mixture is stirred, capped and gently shaken, with frequent venting. This ensures that the evolution of carbon dioxide gas is complete. 3. The lower aqueous layer is removed using a Pasteur pipette and discarded. This removes byproducts. 4. The organic layer is dried over granular anhydrous sodium sulfate. This removes water from the product. 5. The dry ester is transferred using a Pasteur pipette to a clean conical vial. This separates the sodium sulfate from the ester.
Saturated aqueous sodium chloride (d = 1.2 g/mL) is added to the following mixtures in order to dry the organic layer. Which layer is likely to be on the bottom in each case? 1. Sodium chloride layer or a layer containing a high-density organic compound dissolved in methylene chloride (d = 1.4 g/mL)? 2. Sodium chloride layer or a layer containing a low-density organic compound dissolved in methylene chloride (d = 1.1 g/mL)?
1. High-density organic compound dissolved in methylene chloride 2. Sodium chloride layer A saturated aqueous sodium chloride solution is often used to dry an organic layer by shaking the two solutions together. This process helps remove water from the organic layer because the water migrates into the concentrated salt solution due to the greater attraction compared with the organic solution. The bottom layer will contain the liquid with the greater density of the two phases. Note that the density of the saturated aqueous sodium chloride solution has a higher density than water due to the large amount of salt dissolved in it. After removing the organic phase from the aqueous sodium chloride, the organic layer usually requires further drying with sodium sulfate, or with one of the other drying agents.
What happens if the spot line and positions are marked on the plate with a ballpoint pen when preparing a TLC plate for development?
Ballpoint pen ink consists of organic compounds that will move with the development solvent and cause additional spots to appear on the plate. These additional spots could interfere with the analysis of the developed plate. Pencil lead is graphite-based and will not dissolve in any of the development solvents.
After recrystallization, the final weight of biphenyl is found to be 0.02 g. Assume that all steps were carried out correctly, no spillages occurred, and the student lost minimal solid on the glassware or in transfers. Why is the recovery so low?
Biphenyl is highly soluble in both hot and cold benzene. Therefore, a large proportion of the biphenyl remains dissolved in the solvent when the solution is cooled. Solvents for crystallization must be carefully selected so that the compound being purified is soluble in the hot solvent but insoluble in the cold solvent.
If you wish to keep the liquid (the filtrate) but you want a fast filtration, which method would you employ?
Büchner (vacuum) filtration can be used if you need to perform a fast filtration, if, for example, you want to prevent the solid from dissolving into the filtrate.
SAFETY: A risk assessment for a reaction should include the hazards associated with the:
chemical products and by-products. chemical reagents used. procedures involved.
Meting Point
chemical property impurities will lower and broaden the melting point into a melting range -heat quickly until you get close to expected melting point temperature narrow range (0.5 - 2 C) mixed melting point used w standard to see if melting point is sharp
Microscale crystallization
craig tube used w centrifuge to isolate crystal from solution crystals cannot pass through narrow space between the plunger and craig tube tube is inverted and spun in centrifuge drives liquid out, leaving crystals behind crystals collected, washed w cold solvent and dried
Macroscale crystallization
crystals collected on pre-wetted filter paper in Buchner funnel by vacuum filtration (vacuum trap prevents liquids from entering vacuum system) cold solvent used to rinse any remaining crystals from flask collected crystals should be rinsed w cold solvent to wash any remaining soluble impurities crystals dried w desiccator
SAFETY: When using a hot plate or other heating device, you need to be aware of hot surfaces that could cause burns: a hot surface will always glow red to let you know it is hot.
false Whatever heating device is used, you should always be aware of hot surfaces. Avoid contact to prevent burns to yourself and those around you. A hot surface will not necessarily appear hot from a visual inspection. You should place your hand near the heating source to determine whether it is hot.
The functional groups in an organic compound can frequently be deduced from its infrared absorption spectrum. A compound, C5H10O2, exhibits strong, broad absorption across the 2500-3200 cm-1 region and an intense absorption at 1715 cm-1. Relative absorption intensity: (s)=strong, (m)=medium, (w)=weak. What functional class(es) does the compound belong to? List only classes for which evidence is given here. Attach no significance to evidence not cited explicitly. Do not over-interpret exact absorption band positions. None of your inferences should depend on small differences like 10 to 20 cm-1.
j. carboxylic acid The broad, strong 2500-3200 absorption is the signature O-H stretch of a carboxylic acid. The presence of C=O stretch at 1715 cm-1 confirms this conclusion.
Which structure that best fits this image? 3000 (O-H, N-H, C-H) & 1700 (C=C, C=O, C=N, N=O, N-H)
one w double bond O
change in solubility of organic compounds
pH change 1. carboxylic acid sparingly soluble in acid and neutral aq solution but very soluble in basic aqueous solution 2. amine sparingly soluble in basic and neutral aqueous solution but very soluble in acid aqueous solution change in solubility from acid base reactions that change hydrophobic, neutral compounds into highly hydrophilic, water soluble cations and anions
Tollens Test
positive result: precipitate formation (silver mirror forms from clear solution) reagent: AgNO3 and NH4OH functional groups: aldehydes -some sugars, acyloins, hydroxylamines and substituted phenols give false positive
Lucas Test
positive result: solubility change (clear solution becomes cloudy with 2 layers) reagent: anhydrous zinc chloride and HCl functional groups: 2 alcohols (slow) and 3 alcohols (fast) -must be water soluble for test to work -tertiary alcohol: immediate rxn -secondary alcohol: 5-10 minutes -hindered secondary alcohols: gentle warming in hot water bath -primary alcohols very very slow -activated alcohols (allylic alcohol) may test positive -gentle agitation can help give rise to rxn
Distillation
purification technique that relies on differences in boiling points (and vapor pressure) to separate components of a liquid mixture simple distillation -more volatile component will be in higher percentage of vapors -vapor cooled and condensed in separate flask -distillation must be repeated numerous times, each time inceasing the purity of the collected liquid. -can separate liquids with close boiling points fractional distillation: for a large number of simple distillations in a single continuous operation -fractionating column has an extensive surface area that induces the vapors to condense before the collection flask -cooled condensate falls down the fractionating column where it encounters hot vapors rising up -hot vapors transfer heat to condensate to make liquid components return to gas phase according to boiling points -as vapors heat condensate, the vapors cool and condense -vapor that reaches top is high percentage of more volatile component -condenser may have circulating cold water to aid in condensation -insulate fractionating column w cotton wool and aluminum to ensure steady distillation - one drop per second is good -slowly raise temperature to get less volatile component
Water was found to be a good solvent for the recrystallization of organic substance X. 1. Which line correctly shows the solubility vs. temperature profile for this substance? 2. Suppose 0.1 g of X and 1.0 mL of water were mixed and heated to 80 °C. Would all of substance X dissolve? 3. The solution prepared in (b) is cooled. At what temperature will crystals of X appear? 4. Suppose the cooling described in (c) were continued to 0 °C. How many grams of X would come out of solution?
1. The correct line is C. Water is a good solvent choice for the recrystallization of X because X has a low solubility in water at low temperatures, but a high solubility in water at high temperatures. 2. All of the substance X would dissolve at 80 °C. A solubility of 17.0 g in 100 mL of water is equivalent to a solubility of 0.17 g in 1 mL of water. Since there is only 0.1 g of X, this is a greater solubility than is required. 3. Crystals will start to form when the solubility is reduced to 0.1 g/mL (equivalent to 10 g/100 mL on the graph). Therefore, crystals of X should appear around 56 °C. 4. The recovery of X would amount to 0.085 g. A solubility of 1.5 g in 100 mL of water at 0 °C is equivalent to a solubility of 0.015 g in 1 mL of water. Therefore, 0.015 g of X would remain dissolved in the water, with the remainder being formed as crystals.
column chromatography
1. pack w adsorbent stationary phase (SiO2 or Al2O3) -particle size: smaller better to expose greater surface area and slows compound travel and eluent flow -cotton or wool plug at bottom to allow solvent passage but prevent silica from escaping -sand added to give a flat surface at top and bottom a. wet packing: adsorbent is suspended in solvent and slurry is transferred into the column (non-trivial for small scale) compounds dissolved in minimal amount of eluent and transferred using pipette b. dry packing: adsorbent is added directly to the column (compounds not very soluble in eluent. dissolved in diff solvent and silica gel is added. solvent completely evaporated to adsorb compounds onto stationary phase) -cracks, bubbles, gaps ruin separation -air pressure (flash column) used to force eluent down -fractions collected -start with nonpolar solvent (hexane) and increase amount of polar solvent (ethyl acetate)' loading: never add crude solid compound directly to column. solvent may be more polar than eluent so solvent must be removed from silica before loading onto the column running: add more solvent slowly to not disturb top of column. top layer of sand will protect column. Do not let solvent go below the level of silica, it will form cracks pressure: fold back 1/2 inch of latex tube to provide flat end and resistance to gripping pressure to avoid too much pressure -lift tube when solvent gets 3/4 inch above sand level and add more solvent
Correlate the procedures needed to accomplish the given steps (1-6) to separate a mixture containing the following compounds: fluorenone and octanoic acid
A neutral compound can be separated from a mixture containing an acid and/or base by utilizing the change in solubility of the acid/base upon deprotonation/protonation. Step 1: Dissolve the mixture in diethyl ether to get all three compounds in an organic solution. Step 2: Add aqueous NaOH to deprotonate the carboxylic acid, octanoic acid, forming the salt H3C(CH2)6COO-Na+. The salt of the carboxylic acid is water soluble, hence partitions to the aqueous layer, and can be separated. Step 3: Add aqueous HCl to protonate the organic base, dicyclohexylamine, forming the salt (C6H11)2NH2+Cl-. The salt of the amine is water soluble, hence partitions to the aqueous layer, and can be separated. Step 4: Diethyl ether dissolves a relatively large amount of water, therefore, saturated aqueous NaCl solution is added to draw the majority of the water dissolved in the organic layer into the aqueous layer, which can then be removed. Step 5: The small amount of water remaining in the diethyl ether solution is removed by adding anhydrous sodium sulfate. The water is taken up by the anhydrous solid and can be removed by filtration. Step 6: Evaporating off the diethyl ether leaves the neutral compound, fluorenone.
SAFETY: Which of the following are advantages of performing experiments at microscale level? More than one answer may be correct.
A reduced risk of fire or explosion in the laboratory Reduced production of hazardous and polluting waste Reduced contact with hazardous materials that may be harmful to Microscale experiments use small quantities of chemicals. This reduces the risks involved in handling and heating chemicals; however, strict safety procedures should always be followed, even when using small amounts of chemicals in the laboratory.
Which functional group can give a false positive for the Jones test?
Aldehyde
SAFETY: Which of the following can be used to heat volatile chemicals in a microscale experiment? More than one answer may be correct.
Aluminum heat blocks, water baths, and sand baths can be used to heat volatile organic chemicals. Bunsen burners and other heat sources with an open flame should not be used, as there is a greater risk that the chemical will ignite.
2,4-Dintrophenyl hydrazone (DNP) Derivatives
Chemical reaction: hydrazone formation Reagent: 2,4-dinitrophenylhydrazine, HCl Functional groups: aldehydes, ketone Product: hydrazone
Semicarbazone Derivative
Chemical reaction: semicarbazone formation Reagent: semicarbazide hydrochloride, sodium acetate Functional groups: aldehydes, ketones Product: semicarbazone
SAFETY: While setting up a micro-boiling point determination you accidently break a capillary tube. You should:
Dispose of it in the appropriate glass waste container. Broken glass is sharp and could cause harm to you or others. Capillary tubes are small and could easily be lost so you should put them in the appropriate glass waste container immediately. If you are unsure of where to dispose of broken glass ask your instructor.
Which of the following would be expected to form hydrogen bonds with water?
Hydrogen bonding is usually found in situations represented by D-H---A where both the donor atom, D, and the acceptor atom, A, are one of the highly electronegative elements O, N, or F. The D-H bond is a normal covalent bond, and H---A is a hydrogen bond. Hydrogen bonding is not expected to occur between molecules in which all of the H atoms are covalently bonded to C atoms, even when an acceptor atom is present. However, since water is able to act as both a donor and an acceptor, molecules that have an acceptor atom can form H-bonds with water, even if they are not able to act as H-atom donors. Of the molecules shown, both ethylamine and N-methylpropanamide can act as either a donor or an acceptor in an H-bond with water, while acetaldehyde is able to act as an acceptor. Only cyclopentane is missing both a donor atom and an acceptor atom and cannot form an H-bond with water.
Which of the following statements is true when considering temperature during a recrystallization? More than one answer may be correct.
Ideally the hot solvent used will keep the impurities dissolved OR not dissolve them at all. It is important that the sample is dissolved in just enough hot solvent. If too much solvent is added, it may not be possible to retrieve the entire sample as it will have a degree of solubility in the cold solvent. If the impurities remain in solution, they can be washed away. If they remain undissolved, they can be filtered off before recrystallization.
What type(s) of intermolecular forces are expected between CH3CH2CH2CH2CH2CH2OH molecules?
Induced dipole forces are always present. Induced dipole forces are also called dispersion forces, or London forces. CH3CH2CH2CH2CH2CH2OH is a polar molecular compound. Therefore, the intermolecular forces also include dipole forces. Hydrogen bonding is found in situations represented by D-H---A where both the donor atom, D, and the acceptor atom, A, are one of the highly electronegative elements O, N, or F. CH3CH2CH2CH2CH2CH2OH falls into this category.
For each of the following pairs of solutes and solvent, predict whether the solute would be soluble or insoluble. After making your predictions, you can check your answers by looking up the compounds in The Merck Index or the CRC Handbook of Chemistry and Physics. Generally, The Merck Index is the easier reference book to use. If the substance has a solubility greater than 40 mg/mL, you may conclude that it is soluble.
Malic acid, soluble in water Naphthalene, insoluble in water Amphetamine, soluble in alcohol Aspirin, slightly soluble in water; essentially insoluble Succinic acid, insoluble in hexane Ibuprofen, soluble in diethyl ether 1-Decanol, insoluble in water
Filter-tip pipets
May be used to remove a small amount of solid impurities from a small volume (1−2 mL) of liquid; also useful for pipetting volatile liquids, especially in extraction procedures. 1. Remove a very small amount of dirt from 1 mL of liquid.
Observing the formation of a silver mirror on the surface of a test tube when using Tollen's reagent indicates the presence of:
an aldehyde
Buchner funnels
Primarily used to collect a desired solid from a liquid when the volume is greater than about 10 mL; used frequently to collect the crystals obtained from crystallization. 1. Isolate 2.0 g of crystals from about 50 mL of solution after performing a crystallization. (Vacuum filtration with a Hirsch or Büchner funnel)
A student is required to perform a crystallization on an impure sample of biphenyl. The sample weighs 0.5 g, and contains about 5% impurity. Based on their knowledge of solubility, they decide to use benzene as the solvent.
Step 1: A portion of the solvent (in this case benzene) should be heated. Step 2: The minimum amount of hot solvent should be added to the sample to dissolve it. Step 3: The hot solution should be allowed to cool to room temperature and, once no further crystallization is taking place at room temperature, cooled in an ice-water bath. Step 4: The crystals need to be isolated, for example, by centrifugation.
Sublimations are generally performed under reduced pressure because:
Sublimation occurs when the vapor pressure of a solid equals that of the external pressure; therefore, the lower the external pressure, the more readily a solid will sublime, and the less heat that needs to be supplied. Carrying out sublimation under reduced pressure allows it to be performed at lower temperature. Furthermore, some solids may not sublime at atmospheric pressure, as they will melt when heated.
Which of the following would be expected to be the most soluble in water?
Substances dissolve best when solute-solvent interactions are similar to solute-solute interactions. As a result of this, like things tend to dissolve like things: non-polar solutes dissolve best in non-polar solvents, and polar or ionic solutes dissolve best in polar solvents. Since water is a polar solvent that is capable of hydrogen bonding, it will be a better solvent for polar solutes, especially those that can enter into hydrogen bonding, than it will be for nonpolar solutes. The substance acetaldehyde is polar and is capable of accepting a hydrogen bond from water. It would be expected to be more soluble in water than cyclohexane which is nonpolar and not capable of hydrogen bonding.
Dimethylaminopyridine (DMAP) is often used as a catalyst in esterification reactions. A basic solvent such as triethylamine is used to react with the liberated proton from the alcohol group. Draw curved arrows to show the movement of electrons in this step of the mechanism.
The alcohol makes a nucleophilic attack on the carbonyl group, forming a tetrahedral intermediate.
The stationary phase in TLC is polar. Therefore, the less polar component of the mixture will travel a greater distance up the plate, resulting in a higher Rf value for the component. When separating a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid, the benzoic acid spot would be expected to travel the greatest distance up the plate as it is the least polar component. The 2,4,6-trinitrobenzoic acid would travel the least distance as it is the most polar component.
The stationary phase in TLC is polar. Therefore, the less polar component of the mixture will travel a greater distance up the plate, resulting in a higher Rf value for the component. When separating a mixture containing benzoic acid, 2,4-dinitrobenzoic acid, and 2,4,6-trinitrobenzoic acid, the benzoic acid spot would be expected to travel the greatest distance up the plate as it is the least polar component, hence interacts with the stationary phase less. The 2,4,6-trinitrobenzoic acid would travel the least distance as it is the most polar component, hence interacts with the stationary phase more.
Fluted Filters
The volume of liquid to be filtered is greater than about 10 mL, and solid impurities are removed from a solution; often used in crystallization procedures. 1. removed powdered decolorizing charcoal from 20 ml of solution (Gravity filtration using a fluted filter) -Dissolved materials cannot be removed by filtration; however, the impurities can be removed on a silica gel or alumina column. Decolorizing carbon (pelletized Norit) may also be used.
SAFETY: If using plastic joint clips to stabilize your equipment, which clips in the diagram should NOT be used?
These clips are not heat proof and so can melt if they are too close to the heated part of the apparatus.
Hirsch Funnels
Used in the same way as Büchner funnels, except the volume of liquid is usually smaller (1−10 mL). 1. Isolate 2.0 g of crystals from about 10 mL of solution after performing a crystallization. (Vacuum filtration with a Hirsch or Büchner funnel)
Filtering Pipets
Used with volumes less than about 10 mL to remove solid impurities from a liquid. 1. Remove solid impurities from 5 mL of liquid at room temperature. (Gravity filtration using a filtering pipet)
Consider a sample that is a mixture composed of biphenyl, benzoic acid, and benzyl alcohol. Predict the order of elution of the components in this mixture. Assume that the chromatography uses a silica column and the solvent system is based on cyclohexane, with an increasing proportion of methylene chloride added as a function of time.
When using a nonpolar solvent, such as cyclohexane, you should expect the relatively nonpolar substance to elute first (biphenyl), leading to the following order: biphenyl > benzyl alcohol > benzoic acid (elutes first) (elutes last)
SAFETY: When venting a separatory funnel, which of the following statements is correct?
Where possible, vent the separatory funnel in a fume hood. Pressure will build up, so it is important to vent the separatory funnel regularly between shakes. The vapors of organic solvents should be restricted to fume hoods wherever possible.
When using a separatory funnel, which layer should be discarded?
Which layer to keep and which to discard will depend on the experiment you are performing. The top layer will be the least dense solvent, which may or may not be the aqueous layer. The product may have been extracted into or out of the aqueous layer. We can't simply say that the top or the bottom layer should be discarded. Always keep track of your layers and label accordingly.
It is important to pre-wet the filter paper before filtering your sample. Which statements about this step of the procedure are true? More than one answer may be correct.
You should always pre-wet the filter paper with a small amount of the solvent you are using in the experiment, which may or may not be distilled water. Make sure that all parts of the filter paper are wet so that the paper remains firmly in the funnel when the solution is poured onto it.
liquid-liquid extraction
separation method based upon differential solubility of components in a mixture between two immiscible solvents distribution or partition coefficient, K, is ratio of solubility of compound in two solvents (grams/mL in organic solvent dividided by g/mL in water ) 1. dissolution of a mixture in a suitable solvent followed by addition of an immiscible solvent -first organic solvent to dissolve mixture and then immiscible solvent, usually water ) -components of mixture will partition between the layers based upon ratio of their solubilities in each solvent, K) 2. thorough mixing of the immiscible solvent layers -gently shake to speed up mixing. -container must be tightly closed, relieve pressure by venting 3. separation of the layers -vigorous shaking may lead to emulsions -brine often used to help reduce emulsions -important to back extract aqueous layer (add second portion of organic solvent) to fully extract any residual organic compounds) 4. drying of the organic solution -water and organic solvents are immiscible, but still partially soluble (miscible) --> organic solvent still have small amount of water -wash w brine and dried w solid drying agent to adsorb water (magnesium sulfate and sodium sulfate) -sodium sulfate slow 5. isolation of the separated components. (separatory funnel) -filter to remove drying agents and then removing solvent by evaporation -may not be pure still, bc many organic compounds are soluble in organic solutions -separation technique, NOT a purification technique "washing": when water soluble compounds are not desired and the aqueous layer is thrown away
crystallization
solid compound dissolved into hot solvent and slowly reformed in crystalline form cooling --> solution becomes supersaturated and crystals begin to form (if not supersaturated --> no crystals) takes advantage of differences in solubility between the desired compound and impurities colored impurities can be removed by charcoal (activated carbon) -colored molecules have very large number of conjugated double bonds and will adsorb (stick) to surface of charcoal if solution cools too quickly, crystals become unstable and rapidly precipitate (often w trapped impurities) solute should not be too soluble in selected solvent, or it may never crystallize crystal nucleation= induce by addition of previously obtained crystals (seed crystal) scratch side of flask may have to reheat solution to remove excess solvent (need boiling chip)
acidic organic compounds
such as carboxylic acids or phenols treatment of inorganic base (sodium bicarbonate) used to deprotonate carboxylic acids treatment of strong base, sodium hydroxide deprotonate phenols careful selection of bases can be used to separate acids with pKa values differing by 5 pKa units
Boiling point determination
temperature at which vapor pressure equals atmospheric pressure (usually 1.0 atm or 760 torr) -large quantities done using distillation apparatus -small quantities: microscale boiling point determination 1. capillary tube with glass bell (end pointing down) 2. air trapped in top half of glass bell -trapped air expands and exits glass bell -only sample vapors will be in bell as temperature rises -after stream of bubbles, turn off heat and allow temp to drop -last bubble: vapor pressure = atmospheric pressure -sample vapors condense into liquid --> vacuum in glass bell (fast) -if liquid slowly enters, this is not the bp, only shrinking of gas inside bell
Recovery
to recover organic compound, the aqueous solution containing the charged ionic compound is treated w acid or base to reverse the acid/base rxn and extract back into organic solvent, diethyl ether -basic extraction of tea reversed process and converted water soluble caffeine salts back into less polar base form --> more soluble in methylene chloride
In a solvent extraction using a separatory funnel, the densities of the two liquids determine whether the organic solvent is the top layer or the bottom layer. Using the table of densities, determine whether ethyl acetate is the top layer or the bottom layer when used with water. ethyl acetate: 0.90
top layer Solvents with densities greater than that of water (1.00 g/cm3) are the bottom layer in the separatory funnel (when water is the other solvent). Solvents with densities less than that of water (1.00 g/cm3) are the top layer in the separatory funnel (when water is the other solvent).
Pasteur Pipettes
transfer liquids and prevent solid contaminants from entering uses cotton plug
Sublimation can be used to purify your product if:
your solid product has a relatively high vapor pressure but the impurities have a low vapor pressure. Sublimation is the transition from a solid to gas phase, so your product must be a solid. For purification to be effective, your product should have a relatively high vapor pressure, so that it sublimes at a feasible pressure and temperature. Additionally, the impurities should have significantly lower vapor pressures than your product. high vapor pressure= low boiling point
