CHEM 237 Lab Final Practice Questions
What compound will form bubbles and dissolve in aqueous sodium bicarbonate?
Benzoic acid.
The most stable conformation of cis-1,2-dimethylcyclohexane is:
an axial-equatorial chair
During the extraction lab, _____ was extracted using an acid and _____ was extracted using a base.
benzoic acid anion and Na+, aromatic ring with EtO2C and NH3+Cl-. Look at test for structures.
When determining the melting point of a sample, rapid heating causes the observed melting point range to be:
broad and high
In the developing chamber, solvent rises up the TLC plate owing to:
capillary action.
A strong IR absorbance in the 1670-1830 1/cm range is present in:
carboxylic acids, esters, and aldehydes
Most organic compounds are less dense than water and will be on top of the aqueous phase in a separatory funnel. What organic phase breaks this rule and will be on the bottom?
dichloromethane.
In the distillation of hexane/heptane and hexane/octane mixtures, the early distillation fractions contain higher percentages of hexane because:
hexane has a higher vapor pressure.
Three mono-substituted cyclohexanes are given. One is substituted with Sn, Si, or C. The trimethyltin substituent (Sn substituted) has the smallest steric hinderance because:
it has the longer axial bond length.
There is a maximum quantity of sample that should be injected for preparative gas chromatography separations This maximum quantity is determined by:
loss of resolution (separation) with increasing injection size.
Know the anti-conformation of butane.
methyl groups are placed opposite of each other.
You allow a hot recrystallizing solution to cool slowly because:
otherwise small crystals form rapidly, adsorbing impurities
In the preparation of 3-chloro-3,7-dimethyloctane, the organic layer was washed with sodium bicarbonate solution. This washing was for the purpose of:
removing the remaining acid
In the preparation of 3-chloro-3,7-dimethyloctane, the organic layer was washed withs odium bicarbonate solution. This washing was for the purpose of
removing the remaining acid.
The raising of the oven temperature in gas chromatography will _____ the retention times of two compounds and _____ their separation.
shorten, decrease
If you spill sulfuric acid on your hand, after rinsing with water you should rinse the area with a solution of:
sodium bicarbonate
What's the hybridization of a carbon surrounded by two double bonds?
sp
What is the predicted hybridization of the oxygen in ethanol?
sp3
What is the hybridization of the oxygen atom and the approximate bond angles around oxygen in methanol (CH3OH)?
sp3 and 109.5 degrees
What kind of compound will react most rapidly in an SN1 reaction with HCl?
tertiary alcohol
What is the order of increasing reactivity in SN2 reactions?
tertiary<secondary<primary
The peak located at 1741 1/cm wavenumbers corresponds to the vibrational frequency of what functional group?
Carbonyl
In the oxidation reaction with sodium hypochlorite, the oxidant is reduced into
Cl-
Know how to correctly draw atoms on cyclohexane chair conformations.
Up-down-up-down pattern
What should you do if you spill the mixture of nitric acid/sulfuric acid on your skin?
Wash it with cold water and apply 0.6 M sodium bicarbonate solution.
What kind of molecule is expected to be sp3 hybridized at all non-H atoms?
a molecule with only single bonds. (sp2 have double bonds; sp have triple bonds).
At it's melting point, a compound will be:
in both the liquid and solid phases
As the temperature of a liquid increases, the vapor pressure...
increases
You inject a mixture of these three compounds on a non-polar column: acetone, BP 56 degrees C; ethyl acetate, BP 77 degrees C; benzene, BP 80 degrees C. Order the three compounds by increasing retention time.
Acetone, ethyl acetate, benzene.
In reference to thin layer chromatography, how does changing solvent polarity change the Rf value?
Adding a polar solvent causes the Rf of the compound to increase. (Moves farther up)
Recognize the IR spectrum of phenols.
And perhaps benzoic acid.
What is TRUE about E2 reactions?
Atoms being eliminated should be anti-periplanar.
In natural products isolation and alkenes experiment, which of the following tests can detect an alkene?
Bayer's test and Bromine test.
In the Natural Products Isolation experiment, steam distillation was used to isolate limonene from orange peels. What property, of the natural product to be steam distilled, is desired so that rapid distillation will give large amounts of the natural product?
High vapor pressure and high molecular weight
The rate of solvolysis of a benzylic halide will increase when the solvent changes from 50:50 water:ethanol to 60:40 water:ethanol because:
Higher polarity solvents stabilize the intermediate cation and decrease the activation energy of solvolysis.
What observation would be most conclusive in determining that the reactant was successfully oxidized to its carbonyl product?
IR band at 1710 1/cm.
What is TRUE regarding melting points?
It is the temperature at which liquid and solid states exist in equilibrium. It gives an indication of the purity of a compound. One may use a melting point to tentatively identify a known compound. Low purity is usually associated with a broad melting point range.
The dehydrohalogenation of 3-chloro-3,7-dimethyloctane involves an E2 reaction. How much the two atoms being eliminated be oriented for this process to work?
Must be anti, or 180 degrees, with respect to each other.
Tertiary alcohols reacting with sodium hypochlorite yield:
NOT ketones, carboxylic acids, aldehydes, or amides.
You have to decide between simple distillation and other methods for the separation of two liquid compounds. For what kind of case would you choose simple distillation?
One compound is a nonvolatile impurity dissolved in the other liquid.
Know how to calculate percent yield.
Percent Yield = (actual/theoretical)*100
You have performed a recrystallization and want to recover the solid material. What is the BEST method to recover the desired solid?
Pour the contents of the flask into a Buchner funnel and collect the material from the filter paper after washing with cold solvent.
What is TRUE about gas chromatography?
Preparative gas chromatography allows purification and recovery of a compound after passing it through a GC column. AND The moving phase in gas chromatography is an inert gas.
What's TRUE about the formation of 3-chloro-3,7-dimethyloctane from 3,7-dimethyl-3-octanol?
The alcohol is protonated and a water molecule is produced in the reaction.
When analyzing the IR of your product from the Oxidation of a Secondary Alcohol, you were looking for what in the spectra?
The appearance of a peak at approximately 1700 1/cm and disappearance of broad peak at approximately 3300 1/cm.
Know how to rank the conformations of 2-methyl pentane in increasing order to energy strain.
The closer the methyl groups are to each other, the more strain there is.
When performing preparative gas chromatography,
The collection tube must be uncapped to allow free passage of vapor through the tube.
A sample mixture containing naphthalene, naphthol and 2-ethoxynaphthalene was run on a TLC plate. Indicate the order of Rf's from highest (closest to 1) to lowest (closest to 0).
The less oxygen atoms a compound has, the less polar it is. The LESS substituted the oxygen atom is, the MORE polar it is.
When benzoic acid is mixed with inorganic sand, what happens to the melting point of the sample?
The melting point stays the same.
Know how to read a chromatogram.
The more polar a compound is, the smaller the Rf value.
In the kinetic study of SN1 solvolysis, why is the rate constant for the water:ethanol 60:40 mixture larger than for the 50:50 mixture?
The polarity of water stabilizes the carbocation formed in the first step of the mechanism.
The fingerprint region of the IR refers to:
The portion of the spectrum from 1500-400 1/cm that is unique for nearly every compound.
In the phase transfer catalysis lab, what was the purpose of putting calcium chloride in your organic layer after workup?
To remove water from the organic solution
You are performing a solvent extraction of a neutral, basic, and an acidic compound. All are very soluble in CH2Cl2 (dichloromethane) and slightly soluble in water. You have added aqueous HCl to the separation funnel and shaken the funnel. The basic compound is the _____ layer, the acidic compound is in the _____ layer, and the neutral component is in the _____ layer.
top, bottom, bottom. (Basic on top. Acidic and neutral at bottom.)
Steam distillation is often used in natural product isolation to isolate:
water insoluble oils from natural sources
How many hydrogens are removed during the oxidation of 3-heptanol to 3-heptanone?
2
At the end of the phase transfer catalysis experiment, what molecule gave the spot with the highest Rf value when you ran the TLC?
allyl 2-naphthyl ether
The definition of a "solvolysis" reaction is:
an SN1 reaction where the solvent is the nucleophile
In Thin Layer Chromatography, the Rf value is calculated for the purpose of:
comparing with the known Rf values of standards.
A good recrystallization solvent should:
dissolve impurities at all temperatures or not at all
What compounds have characteristic IR absorption at 1640-1820 1/cm?
ester and ketone
What qualitative test can be used to distinguish between limonene and eugenol?
Ferric chloride test. (Limonene and eugenol are both unsaturated, but eugenol has a phenol group.
If you started with 10 grams of 75% pure crystals, and recovered 5 grams of 100% pure crystals after a recrystallization, how much of the product did you lose?
10*0.75=7.5 grams pure crystals (originally) 5*1.00=5 grams pure crystals (finally) 7.5-5=2.5 grams pure crystals (lost)
Use the vapor-liquid composition diagram to estimate the vapor composition upon boiling of a liquid mixture which is 40% B. The initial vapor formed will contain about:
12% B.
The characteristic infrared absorption range for the -O-H stretch ion frequency of an alcohol is:
3200-3600 1/cm
Joe Ag and Joan Mag distill 5 mL of a 1:1 mixture of dichloromethane (BP, 40 degrees C) and 1,1,1-trichloroethane (BP, 74.1 degrees C). Their plot of head temperature versus distillate volume showed:
A continually increasing temperature from 40 degrees C to 74.1 degrees C.
Acetone is NOT suitable as a solvent for extraction of organic materials from water because:
Acetone and water are miscible
Which of the following compounds is a phase transfer catalyst?
Benzyl tri-n-butylammonium chloride
What's accurate about simple distillation and gas chromatography?
Both require that the compounds to be separated be volatile and thermally stable. Neither requires the compounds to be separated be polar. Both can NOT be used to identify the structures of the compounds.
The product from dehydrohalogenation of 3-chloro-3,7-dimethyloctane would give a positive result with what classification tests?
Bromine test and Baeyer's test
In the kinetic study of SN1 solvolysis experiment, how was the disappearance of 1-chloro-1-phenylethane monitored?
By monitoring the formation of HCl.
If a mixture of benzoic acid and aniline in dichloromethane is extracted with aqueous HCl, what structure will be found in the aqueous phase?
C6H5NH3+Cl-
The reaction of naphtholate anion and allyl bromide involved the use of a catalyst, benzyl-tri-n-butyl ammonium chloride. The catalyst:
Carries the water soluble naphtholate anion into the organic phase.
A solution contains three compounds A, B, and C that all have the same boiling points. The solution is injected onto a polar column on a gas chromatogram and three separate peaks are obtained. If the order of elution is A-B-C, which of the following statements is true?
Compound B is more polar than compound A. (The more polar a compound is, the later it will appear in the order of elution.)
What is the similarity with E2 elimination and SN2 substitution?
Concentration of nucleophile affects the rate of reactions
What is a cause of making the retention time longer in a gas chromatography analysis?
Concentration of stationary phase liquid increased.
When compounds with aromatic rings are dissolved in dichloromethane, the resulting solution is extracted with 3M sodium hydroxide. The compounds removed from the dichloromethane:
Contain a carboxylic acid functional group.
Which type of strain energy in a molecule accounts for intramolecular van der Waals interactions and dipole repulsions present in a conformation?
E(steric)
The strain energy of a molecule can be broken down into four terms. What are the four terms?
E(strain) = E(stretch) + E(angle) + E(torsion) + E(steric)
What is correct about the isolation of natural products experiment?
Eugenol should give a positive result in the ferric chloride test (because it contains a phenol).
In the Phase Transfer Catalysis experiment, a general form of quaternary ammonium salt is R4N+X-. The R4N+, where R is an alkyl chain, can be in either the aqueous or organic phase because:
R4N+ is a charged species and charged species can dissolve in the aqueous phase. AND R is a hydrocarbon chain and hydrocarbon chains can dissolve in the organic phase.
What type of a reaction is the formation of 3-chloro-3,7-dimethyloctane from 3,7-dimethyl-3-octanol and conc. Hcl?
SN1
In the phase transfer catalysis experiment, you synthesized a naphthyl ether from allyl bromide and the naphtholate ion. Which type of reaction did this proceed through?
SN2
What is true about thin layer chromatography?
Silica gel is the most often used adsorbent. Small spots should be made by touching the adsorbent with the end of the micropipet. If the spots are below the solvent level, tehyw ill be dissolved off the plate by the solvent. If two substances have the same Rf value, they are likely to be (but not necessarily) the same compound.
A diligent CHEM 237 student noticed that the gas chromatograms from most of the fractions he obtained from the simple distillation of a 1:1 mixture of ethanol (BP, 78 degrees C) and 2-propanol (BP, 82.4 degrees Celsius) showed two peaks. What can you say about separation of this mixture?
Simple distillation cannot be expected to separate ethanol from 2-propanol. Gas chromatography could be used to separate a small amount of the ethanol/2-propanol mixture.
What is a limitation to recrystallization?
Some product will be lost.
In the Natural Products Isolation lab, eugenol was separated from a methylene chloride solution of clove oil by extraction into an aqueous solution. How was this done?
Strong base (aqueous solution) was added and the eugenol formed a water soluble salt
In the oxidation reaction of a tertiary alcohol, the alcohol is converted to:
TRICK: a tertiary alcohol is not oxidized
What is TRUE regarding recrystallization?
The recrystallization solvent should be reasonably volatile. It is desirable to have the boiling point of the solvent below the melting point of the compound being purified. If a solvent pair is used (e.g. ether-acetone), the solvents must be miscible in one another. Small crystals form when a hot solution is cooled too rapidly.
In order to carry out a gas chromatography experiment:
The sample MUST be volatile.
What criteria are essential for a suitable recrystallizing solvent?
The solute should be soluble in hot solvent and less soluble in cold solvent. The solvent should not react chemically with the solute. The boiling point of the solvent is, preferably, below the melting point of the solute. All impurities do NOT have to be soluble in hot solvent.
What is TRUE about an SN1 Solvolysis experiment?
The solvent is the nucleophile. The reaction is a first order reaction. A decrease in polarity of reaction mixture slows down the reaction. The rate constant k was determined without having to know the exact concentration of NaOH. Increasing the concentration of the nucleophile will NOT increase the rate of reaction.
What's TRUE about Kinetic Study of the SN1 Solvolysis experiment?
The water molecule is the nucleophile. The reaction rate is dependent on the polarity of the solvent. The concentration of NaOH does not have effect on the reaction rate. The rate-limiting step is the formation of carbocation. Temperature has an effect on the reaction rate.
The reaction performed in the dehydrohalogenation experiment was an E2 reaction. What is TRUE about E2 reactions?
There are no carbocation intermediates in E2 elimination reactions. E2 reactions are stereospecific. Strong bases encourage elimination reactions. It is easier to perform an E2 reaction on an alkyl chloride than an alkyl fluoride. (Farther down column means easier reaction) Tertiary alkyl halides will undergo faster elimination than secondary alkyl halides.
What type of detector is used in the gas chromatography instruments used in the A&M Organic Labs?
Thermal conductivity detector.
What is the role of the potassium hydroxide in the dehydrohalogenation of 3-chloro-3,7-dimethyloctane experiment?
To produce hydroxide anion which acts as a base in the E2 elimination process.
For a simple distillation to work:
the difference in boiling points has to be at least 40 degrees C AND all compounds have to be thermally stable.
The eutectic point is:
the lowest melting point for a mixture of A and B.
In the phase transfer experiment, a very short liquid chromatography column was sufficient to obtain pure product because:
the naphthyl ether is so much LESS polar than the non-volatile reactants or side products that the ether can move through the HIGHLY polar silica gel more rapidly than reactants, catalysts, or side products.
In an SN1 solvolysis reaction such as the one studied in lab:
the reaction rate is accelerated by the use of a more polar solvent.