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Organic Chemistry 32- Practice Questions for Exam #
Part 1: (Circle only ONE choice, circling more than one will be counted as wrong!) 4 points each
- The correct IUPAC name for the following compound is:
(A) (1R, 3R)-1-chloro-3-methylcyclohexane (B) (1R, 3S)-1-chloro-3-methylcyclohexane (C) (1S, 3S)-1-chloro-3-methylcyclohexane (D) (1S, 3R)-1-chloro-3-methylcyclohexane (E) None of the above.
The Key here is to determine the configuration R or S for each chiral carbon. The answer here is 1S, 2S.
2. Consider the SN1 reaction of tert-butyl chloride with iodide ion:
(CH 3 ) 3 C Cl +^ I (CH 3 ) 3 C I + Cl If the concentration of iodide ion is doubled, the rate of forming tert-butyl iodide will: (hint: consider mechanism, i.e. how is the product formed?)
(A) Double. (B) Increase 4 times. (C) Remain the same. (D) Decrease. (E) None of the above.
In SN1 reaction, the rate is independent of the nucleophile involved since the nucleophile is not involved in the rate determining step. Thus the anwer is C.
3. The relationship between the following two structures is:
(A) enantiomers (B) diastereomers (C) structural isomers (D) identical (E) none of the above
The key here is to know the definition of all terms and then determine R or S configuration for each chiral carbon. The answer is (B) (by definition)
4. The specific rotation of pure (R)-2-butanol is -13.5°. What % of a mixture of the two enantiomeric forms is (S)-2-butanol if the specific rotation of this mixture is -5.4°?
(A) 40% (B) 30% (C) 60% (D) 70% (E) None of the above The answer is B since e.e. is 40% favoring R. (5.4/13.5=40%)
H CH 3
Cl H
CH 3
H OH
H OH
CH 3
CH 3
HO H
H OH
CH 3
5. Which of the following alkyl halides would undergo SN2 reaction most rapidly?
(A) CH 3 CH 2 -Br (B) CH 3 CH 2 - Cl (C) CH 3 CH 2 -I (D) CH 3 CH 2 -F (E) they react at the same rate
In Sn2 reaction, the nucleophile attacks from the back of the leaving group. The better the leaving group, the easier it is to leave (faster rate). The answer is C since iodide ion is the best leaving group.
6. Which of the following molecules is chiral?
CH 3 CH 3
H H
CH 3 H
H CH 3
OH
CH 3
(A) (B) (C)
C C
CH 3
H
CH 3
H
C C
H
CH 3
CH 3
H
(D) (E)
Go by definition or look for plane of symmetry/point of inversion. The answer is B.
7. The relationship between (A) and (B) structures in the previous question is:
A) identical B) structural isomers C) diastereomers D) enantiomers E) None of the above
Again need to know terms and how to apply them. The answer is C.
11. Predict which of the following carbocations has the highest energy:
Higher energy means less stable. Stability is related to how much the charge can be spread out, which is, in this case, related to whether alkyl groups (substitution pattern) behave as electron withdrawing or donating groups. The answer is (A)
12. Bromination of alkanes is a much slower reaction than chlorination. Which of the following is expected to be the major organic product when 2-methylbutane is allowed to react with Br 2 in the presence of light or heat?
The answer here is (B). How is rate of a reaction related to selectivity? Which hydrogen in the reactant is the most reactive towards radical abstraction?
13. Which of the following free radical is the most stable?
CH 2 CH 3 CH 3 CH 3 CH 3
(a) (b) (c) (d) (e)
How is radical stability affected by substitution pattern? (answer is (B))
14. Which of the above drawing is the most stable if the radical carbon is changed to a carbocation? (answer is B)
CH 3 CH 2 CHCH 3
CH 3
CH 3 CH 2 CHCH 2 Br
CH 3
CH 3 CH 2 CCH 3
CH 3
Br
CH 3 CHCHCH 3
CH 3
Br
BrCH 2 CH 2 CHCH 3
CH 3
(A) (B) (C) (D)
CH 2 CH 3 CH 3
(E) All are equally stable
(A) (B) (C)
CH 3
(D)
15. Which is the least stable in these structures if the radical carbon is changed to a carbanion?
(answer is A, pay attention to sign of charge!)
16. The major monobromination product in the following reaction is
(CH 3 ) 3 CCH 2 CH 3 + Br 2 h n products
(CH 3 ) 3 CCH 2 CH 2 Br (CH 3 ) 3 CCHCH 3 Br
(CH 3 ) 2 CCH 2 CH 3
Br
(CH 3 ) 2 CCH 2 CH 3
(a)^ CH^2 Br
(b) (c)
(d) (e) none of the above
The answer is (C) because selectivity is high for bromination which prefers to attach the weaker C-H bond.
17. According to the following energy profile, the rate of reaction from A to B is determined by
E
reaction coordinate
A
B
C
(a) the energy of A only (b) the energy of B only (c) the energy difference between C and A (d) the energy difference between B and A (e) the energy of C only
The answer is C (activation energy)
Part 2:
- (14 pt) Draw the organic product expected from each of the following reactions. Be sure to indicate stereochemistry where appropriate and to include stereoisomers if any. In case two or more stereoisomers are formed, label their relationship as diastereomers, enantiomers, structural isomers, or conformers.
- (10 pt) (A) Draw Fisher projections for (2R, 3S)-2-bromo-3-chlorobutane and (2S, 3R)-2-bromo- 3-chlorobutane, with the carbon chain on the vertical line. Label each structure as (2R, 3S) or (2S, 3R).
(B) Assume that you have a mixture of equal amount of each of the above compounds. Can they be separated into two containers based on physical properties such as b.p., m.p., etc.? If yes, which technique would you use? If no, briefly explain why not.
The two compounds as drawn are enantiomers which have identical b.p. and other physical properties (different only towards plane polarized light). Therefore they can't be separated based on physical properties alone.
I
O
CH 3
SN 2
(A)
CH 3
O
O C
C Cl
H
CH 3
D
SN 2
(B) CH 3 O C
H
CH 3
D
C I
CH 3 CH 2
CH 3
CH 3 CH 2 CH 2
H 2 O +
SN 1
(C) C^ OH
CH 3 CH 2
CH 3
CH 3 CH 2 CH 2
HO C
CH 3 CH 2
CH 3
CH 3 CH 2 CH 2
about 50% of each
CH 3
H Br H Cl CH 3
2S, 3R
CH 3
Br H Cl H CH 3
2R, 3S
3.(14 pt) The following equation shows the bromination of methane:
- Propose a mechanism (initiation and propagation steps) to account for the product formation. Label the steps as A, B, C… and calculate the DH° for each step.
See chlorination of methane discussed in the lecture notes and textbook.
- Draw an energy diagram (using the DH° for each step to set appropriate energy levels) for the two propagation steps, and decide which step is more likely the rate-determining step? (label this step as rds)
see the textbook for chlorination of methane and apply it to this case.
energy
reaction coordinate (progress)
CH 4 + Br 2 CH 3 Br^ +^ HBr light
