Old CH 231 Exams

The questions on these old exams are typical of the kinds of questionsyou can expect on the exams in CH 231 this semester.

For the midterm exam, you should know all of the material inchapters 1-5, with one exception: you will not be tested on Fischerprojections (section 5-4).

The absence of questions about any particular topics on the old examsdoes not imply that questions on those topics will also be absent on themidterm exam this fall.

The overall degree of difficulty of the midterm exam this fall will becomparable to that on the old exams, although there will undoubtedly besome variations in the degree of difficulty of individual questions onspecific topics.

You should be able to answer all of the questions on old exam 1 andall of the questions except 9, 12, 13, and the bonus on old exam 2.

L. T. Scott10/12/2004

BOSTON COLLEGEChem 231 - Organic Chemistry I

Exam I - September 27, 2000

name

Please write your answers on the exam.Do not spend too much time on any one question.

question points1. (06 points)2. (06 points)3. (04 points)4. (08 points)5. (05 points)6. (06 points)7. (04 points)8. (06 points)9. (06 points)

10. (06 points)11. (06 points)12. (10 points)13. (10 points)14. (06 points)15. (05 points)16. (06 points)

total

Chem 231 Exam I - September 27, 2000

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1. (6 points) Draw a molecular structure for each of the following compounds:

(a) t-butylcyclopentane (b) 3-(1-chloroethyl)heptane

2. (6 points) Write the IUPAC name for each of the compounds shown below.

3. (4 points) In the drawing to the right, identifyeach of the labeled carbon atoms as primary,secondary, tertiary, or quaternary.

4. (8 points) For each of the organic species below, (a) indicate the hybridization of theatoms marked with an arrow and (b) indicate the best description of the geometry ateach of the atoms marked with an arrow, choosing from the following list ofdescriptors: linear, bent, planar, pyramidal, or tetrahedral.

5. (5 points) Circle the most acidic proton of those shown explicitlyin the molecular structure to the right.

FHF H

(CH3)2CCH2CH3BrH3C N C O

N

H

HH

O H

HH

Chem 231 Exam I - September 27, 2000

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6. (6 points) Every covalent bond in the molecule shown to the right can be describedas resulting from the overlap of an atomic orbital onone atom with an atomic orbital on the attachedatom. For the bonds marked by arrows, indicatewhat atomic orbitals are overlapping to make thebond.

7. (4 points) In the group of five compounds shown below, (a) circle the compoundthat should have the lowest boiling point and (b) draw a box around the compoundthat should have the highest boiling point.

8. (6 points) The C2H4Br cation is known to have acyclic structure (3-membered ring). Draw the mostreasonable Lewis structure for this cation. Show allthe valence electrons around every atom, and be sureto indicate which atoms bear formal charges.

9. (6 points) The heat of combustion ofmethylcyclopentane is –942.1 kcal/mol, and the heatof combustion of cyclohexane is –937.7 kcal/mol.Which of these two C6H1 2 hydrocarbons is thethermodynamically more stable isomer? Draw apotential-energy diagram that shows the potential-energies of each of these two hydrocarbons plusoxygen relative to the potential-energies of theircombustion products.

10. (6 points) The halogenation of methane under the influence of heat or light isactually a reversible reaction (equation shown below). When X=Br, the equilibriumconstant is greater than 1.0 (Keq > 1.0). When X=I, however, Keq < 1.0. Is the change inGibbs standard free energy (ΔG°) for the bromination of methane positive or negative?What is the equation that relates Keq to ΔG°?

CH4 X2 CH3-X H-X

CN

Chem 231 Exam I - September 27, 2000

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11. (6 points) Which of the following four reactions will be the FASTEST?(a) ΔG° = +4.9 kcal/mol and Ea = +2.3 kcal/mol.(b) ΔG° = +4.1 kcal/mol and Ea = +8.7 kcal/mol.(c) ΔG° = -6.8 kcal/mol and Ea = +4.2 kcal/mol.(d) ΔG° = 0.0 kcal/mol and Ea = +5.7 kcal/mol.

12. (10 points) Free radical chlorination of 2-methylpropane (“isobutane”) gives twoproducts with the molecular formula C4H9Cl in a ratio of about 2:1. If that mixture ofproducts is then irradiated (hυ) in the presence of elemental bromine, those twocompounds are converted to a mixture of four products with the molecular formulaC4H8BrCl.(a) Draw bond-line formulas for the two compounds formed with molecular formula

C4H9Cl.

(b) Draw bond-line formulas for the four compounds formed with molecular formulaC4H8BrCl.

(c) Two of the four C4H8BrCl compounds make up more than 99% of the productmixture formed by this sequential halogenation procedure. Circle them.

(d) When the order of the halogenation steps is reversed, i.e., the bromination isperformed before the chlorination, a single compound of formula C4H8BrCl makesup more than 99% of the product mixture. Draw a box around it.[Hint: It might or might not be the same as one of the compounds you have alreadycircled.]

Chem 231 Exam I - September 27, 2000

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13. (10 points) Draw a Newman projection for the lowest energy conformation of2–bromobutane (looking down the C2-C3 bond). Then draw a plot of potential energyvs torsional angle from 0° to 360° around the C2-C3 bond, with the torsional angledefined so that the lowest energy conformation corresponds to 0°. You do not have todraw Newman projections for any of the other conformations, but be careful to showwhich energy maxima are higher than others, which energy minima are lower thanothers, and which conformations have the same energy.

14. (6 points) A Lewis structure for the cyanate ion is shown below. (a) Draw a secondreasonable Lewis structure for the cyanate ion, being sure to indicate which atoms bearformal charges. (b) Circle whichever Lewis structure contributes more to the actualstructure of the cyanate ion. (c) Briefly justify your answer to part (b).

15. (5 points) Which of the compounds below will have the largest dipole moment?Briefly show why.

16. (6 points) The bond-dissociation energy of H2O is 119 kcal/mol, whereas the bond-dissociation energy of HCl is 103 kcal/mol. When the •Cl radical abstracts a hydrogenatom from an alkane, will it exhibit higher or lower selectivity than the •OH radical inchoosing tertiary vs secondary vs primary hydrogen atoms? Briefly tell why.

NCO

Cl-C C-ClH H

HHCS2CH2Br2

BOSTON COLLEGEChem 231 - Organic Chemistry I

Exam 2 – October 18, 2000

name

discussion section (time or TA)

Please write your answers on the exam.Do not spend too much time on any one question.

question points1. (04 points)2. (04 points)3. (04 points)4. (06 points)5. (08 points)6. (06 points)7. (06 points)8. (06 points)9. (04 points)

10. (12 points)11. (06 points)12. (12 points)13. (14 points)14. (08 points)

bonus (10 points)total

Chem 231 Exam 2 – October 18, 2000

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1. (4 points) Draw a molecular structure for each of the following compounds:

(a) trans-1-chloro-3-isopropylcyclobutane (b) 4,4-bis(fluoromethyl)heptane

2. (4 points) Write the IUPAC name for each of the compounds shown below.

3. (4 points) In the group of five compounds shown below, (a) circle the compoundthat should have the highest boiling point and (b) draw a box around the compoundthat should have the lowest boiling point.

4. (6 points) The heat of combustion for methylspiropentane (compound 1 below) is–782.0 kcal/mol, and the heat of combustion for cyclopropylcyclopropane (compound 2below) is –778.3 kcal/mol. (a) Which of these two C6H1 0 hydrocarbons is the morestable one? (b) What aspect of the structure of the less stable isomer would youconclude is causing that compound to be less stable than the other isomer?

Br Cl

ICl

Cl Cl BrCF4

1

2

Chem 231 Exam 2 – October 18, 2000

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5. (8 points) For each of the cyclohexane derivatives below,(a) Tell whether the groups are cis or trans,

(b) flip the ring and draw the other chair conformation, and

(c) circle the more stable conformation for each compound (i.e., the original drawing vsyours).

6. (6 points) A colorless crystalline compound isolated from guaiac wood has beengiven the name “guaiol.” Its molecular formula (C1 5H2 6O) and structure stronglysuggest that guaiol is a sesquiterpene. Given this information, which of the followingstructures is most likely correct for guaiol? Show your reasoning.

7. (6 points) The groups on each stereogenic carbon atom below are labeled in order ofdecreasing priority according to the Cahn-Ingold-Prelog system (i.e., highest priority =a ... lowest priority = d). Indicate for each center whether it has the R or the Sconfiguration.

Cl CH3OH

HO OH

HO

bd c

ab

a dc

ab c

d

Chem 231 Exam 2 – October 18, 2000

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8. (6 points) Circle the “substituent” in each set below that has the highest priority inthe Cahn-Ingold-Prelog system for assigning stereochemistry (R vs. S), and draw asquare box around the “substituent” in each set that has the lowest priority.

9. (4 points) Fischer projections are shown below for four of the stereoisomers of 2,3,4-trichloropentane.

Some questions below may have more than one correct answer; circle ALL the correctanswers to each question.

(a) Which isomers are chiral? A B C D

(b) Which isomers are meso? A B C D

CH2CH2CF3

CH2OH

CN

COOHOCH2CH3

N(CH3)2

O

COOH

S

N

NCH3

CH3H ClH ClH ClCH3

A

CH3Cl HH ClH ClCH3

B

CH3H ClCl HH ClCH3

C

CH3H ClH ClCl H

CH3

D

Chem 231 Exam 2 – October 18, 2000

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10. (12 points) For each pair of compounds in the list below, circle the word that bestdescribes their structural relationship. Assume that the rings are planar.

A & B identical enantiomers diastereomersA & C identical enantiomers diastereomersA & D identical enantiomers diastereomersB & C identical enantiomers diastereomersB & D identical enantiomers diastereomersC & D identical enantiomers diastereomers

11. (6 points) Refer to the compounds above in question 10. Which of the followingsamples would be optically active? (Circle all the correct answers).

pure A pure D 1:1 mixture of A and B

1:1 mixture of A and D 1:1 mixture of C and D 3:1 mixture of C and D

12. (12 points) Write the major organic product expected from each of the followingproposed reactions. If no reaction will occur, write NO REACTION.

(a)

(b)

(c)

(CH3)2CHCH2-Br Na CNDMSO

Cl-CH2CH2CH3 K SCNCH3OH

CH3-I Na OCH(CH3)2DMF

mirror

O

O

CH3H

HCH3

O

O

CH3H

HCH3

A Bmirror

O

O

HCH3

HCH3

O

O

HCH3

HCH3

C D

Chem 231 Exam 2 – October 18, 2000

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12. (continued)

13. (14 points) Predict the effect of the changes given below on the rate of the reaction:

Circle one answer only for each question.(a) Change substrate from CH3Cl to CH3I. faster slower no change(b) Change nucleophile from CH3O– to CH3S–. faster slower no change(c) Change nucleophile from CH3O– to CH3OH. faster slower no change(d) Change nucleophile from CH3O– to (CH3)2CHO–. faster slower no change(e) Change substrate from CH3Cl to (CH3)2CHCl. faster slower no change(f) Change solvent from CH3OH to (CH3)2SO. faster slower no change(g) Double the concentration of CH3O–. faster slower no change

14. (8 points) Bromination of pure (3R,4S)-3,4-dimethylhexane (shown below) with onemolar equivalent of elemental bromine under the influence of light produces 3-bromo-3,4-dimethylhexane in good yield. Consider carefully how this reaction would occurand then circle all of the products in the list below that you would expect to be formed.

(3R,4R)-3-bromo-3,4-dimethylhexane(3R,4S)-3-bromo-3,4-dimethylhexane(3S,4S)-3-bromo-3,4-dimethylhexane(3S,4R)-3-bromo-3,4-dimethylhexane

(d)

(e)

(f)

CH3CH2CH2CH3 Na ClCH3OH

H3CHCOH

CH3 Na CNDMSO

CH3-I NH3DMF

CH3-Cl OCH3CH3OH

CH3-O-CH3 Cl

H3C

H

H

CH3

Chem 231 Exam 2 – October 18, 2000

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Bonus. (10 points) When pure cis-1-t-butyl-4-iodocyclohexane is added to a solution ofsodium iodide in acetone (propanone) at room temperature, some trans-1-t-butyl-4-iodocyclohexane is formed. Likewise, when pure trans-1-t-butyl-4-iodocyclohexane isadded to a solution of sodium iodide in acetone (propanone), some cis-1-t-butyl-4-iodocyclohexane is formed.

(a) Draw structural representations for the two stereoisomers of 1-t-butyl-4-iodocyclohexane in their most stable conformations, indicating clearly the 3-dimensional arrangement of atoms in each, and then use the curved arrow notation toshow how sodium iodide could convert each isomer into the other.

(b) When starting with the pure cis isomer, the reaction “stops” after only 70% of thestarting material has been used up. Even in the presence of excess sodium iodide, about30% of the cis isomer always remains unconverted to trans. When starting with thepure trans isomer, the reaction “stops” after only 30% of the starting material has beenused up. Explain (briefly) why these reactions do not convert the starting materials100% into the product.

(c) Can you explain why one reaction gives 70% conversion to the product, while theother goes only 30% of the way?