Chem 237 2
|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?||Would not be affected|
|Which of the following statements best describes a eutectic?||Mixture of two components that generates the lowest possible melting point.|
|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.|
|Which of the following statements about melting point determination are true? Select all that apply.||The upper limit of the melting temp range occurs when the last remaining crystal melts.|
The lower limit of the melting temp range occurs when the first crystal starts to melt.
Pure substances typically have a melting range of less than 2 degrees Celsius.
|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.|
|When comparing melting temperature ranges of pure and impure samples, which of the following statements describes the melting temperature range of an impure sample? Select all answers that apply.||The melting temperature range of a pure sample is higher than that of an impure sample.|
The melting temperature range of an impure sample is broader than that of a pure sample.
|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 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.
|a) Rf is dependent on polarity|
b) Polar compounds will have higher Rf values than nonpolar compounds under the same developing conditions.
c) Rf values should be kept in the range of .2 to .8 for the most effective separation when more than one compound is present in the sample.
d) The less polar the solvent, the higher the Rf values.
e) Rf is not dependent on polarity.
|When is recrystallization not a suitable method to purify an organic material?||When the organic material is a liquid.|
|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.|
|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.|
|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. What should a student do if they observe an oil forming as the solution cools?||-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.
|Which of the following statements are true of the chair conformation of cyclohexane?||-It contains six acial hydrogens that are perpendicular to the ring|
-It contains six equatorial hydrogens that are in the approximate plane of the ring.
|Which statements about equatorial and axial positions in cyclohexane are correct?||-Each carbon in cyclohexane is attached to one axial and one equatorial hydrogen.|
-Each face of the chair conformation of cyclohexane ring has three axial and three equatorial hydrogens in an alternating pattern
|What is measured in [IR Spectroscopy]?||The vibrational frequency of a bond.|
|What does ATR stand for?||Attentuated Total Reflectance|
|In which of the following ways is the use of an infrared spectrophotometer fitted with an ATR sample advantageous over a beam-through-sample spectrophotometer? Select all answers that apply.||-Sample preparation and clean up of an ATR is simple|
-The ATR crystal is not degraded by the presence of water.
|What region of an infrared spectrum is most useful for distinguishing between isomers that contain the same functional group?||1400-400 cm?¹|
|Which of the following statements about infrared spectroscopy is not true?||IR can be used to analyze almost any kind of sample|
|Which of the following statements about infrared spectroscopy is true?||-IR can detect impurities within a sample|
-IR is non-destructive
-IR is good for detecting and identifying functional groups of a sample.
|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.|
|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.
|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.
|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.
|Why is ethanol not a suitable solvent for the solvent extraction of benzoic acid and benzocaine from aqueous solution?||Ethanol is miscible with water.|
|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.|
|How is it possible to separate two liquids with the exact same boiling point by gas chromatography (GC) when it cannot be done by fractional distillation?||The use of a polar GC column will separate the liquids based on differences in polar interactions and volatility.|
|A 0.7 mL mixture of pentanol and heptanol are separated and collected using preparative gas chromatography on a GC fitted with a polar column and a TCD detector. During the separation, why are the injection port and detector heated to a higher temperature than the column? Select all that apply.||-A heated injector creates flash vaporization which allows separation at temperatures lower than the boiling point of the sample.|
-The wire in the thermal conductivity detector needs to be cooled by the eluent in order to register a signal.
|A student is comparing two samples of crystalline organic solids. A TLC analysis shows a single spot for each sample and the co-spot with the same Rf value. Which of the following would provide the most conclusive evidence that the two samples are the same compound?||Perform a mixture melting point.|
|A TLC plate was run in 1:1 toluene and acetone, but when developed the plate only showed baseline spots. What change could be made to get the spot(s) to move further up the plate?||Spot a new plate and run in 1:3 toluene/acetone|
a) Silica is toxic
b) Hexane is flammable
c) Ethyl acetate is flammable
d) Acetone is flammable
|What is the major underlying principle of chromatography? (including TLC)||Separation will be achieved if one component adheres to the stationary phase more than the other component does.|
|What temperature will a mixture of eugenol and water boil at?||Slightly less than 100 °C|
|Both Br2 and HCl undergo electrophilic addition to alkenes. Why is Br2 used as a test for unsaturation instead of HCl?||The reaction with bromine can be followed because of a visible loss of color.|
|In order to perform a bromine test on the compound, it must be treated with a dilute solution of bromine. Which of the following results will indicate the presence of an alkene?||The reddish color of the bromine will disappear.|
|Why will the reaction [of 2-naphthol and allyl bromide] fail to work if the phase transfer catalyst is switched to ammonium chloride while retaining the same reactants, solvent and base?||Because ammonium chloride is not soluble in CH2Cl2 solvent.|
|Why will the reaction [of 2-naphthol and allyl bromide] fail to work if the solvent is switched to ethanol while retaining the same reactants, phase transfer catalyst, and base?||Because the base would deprotonate ethanol and create a second nucleophile.|
a) Lachrymators like allyl bromide are specifically dangerous because of their explosion hazard.
b) The primary risk associated with CH2Cl2 is its flammability.
c) Unused allyl bromide can be disposed of in the aqueous waste.
|For SN1 solvolysis of t-butyl chloride, rank the solvents from fastest reaction to slowest reaction.||FASTEST|
|Rank the following molecules in terms of their rate of reaction, fastest at the top.||MOST REACTIVE|
|In the presence of hydrochloric acid, 3,7-dimethyl-3-octanol is converted to 3-chloro-3,7-dimethyloctane via an SN1 mechanism. SN1 and E1 mechanisms are usually in competition with each other. Why, in this case, is the elimination product not observed?||the alkene formed by any E1 reaction would react with HCl to form the same product as the substitution reaction|
|Sodium hypochlorite and glacial acetic acid combine to make hypochlorous acid. Hypochlorous acid oxidizes cyclohexanol to cyclohexanone. At the end of the reaction, any excess hypochorous acid must be neutralized. How is the neutralization of HOCl accomplished?||Add sodium bisulfate, NaHSO3|