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Unformatted text preview: CEM251/Fall 2007/First Exam.....(page 1) (20 points total) I. Draw the structure of the following molecules. Show stereochemistry (where indicated) clearly:
1) ethylene 2) 3-t-butylcyclopentene 3) cis-5-methyl-2-hexene 4) trans-1-ethyl-4-methylcyclohexane 5) 3-chloro-2-methylpentane 6) 1,4-hexadiene 7) (Z)-3,4-dimethyl-3-hexene 8) (E)-2-bromo-3-methyl-2-hexene 9) isopropyl chloride 10) styrene CEM251/Fall 2007/First Exam.....(page 2) II. (20 points total) A) Give the formal charge on the indicated atoms in the following resonance structures: O O NHCH3 CH3CH2 C CH3CH2 C NHCH3 B) Compound X has 12 carbons, one chlorine, two oxygens, two p bonds and one ring. How many hydrogens does X have? Answer
C H3 C) N C H2 C C H Y
For the above molecule Y:
1) The number of Sp3 carbons is? 2) The number of Sp2 carbons is? 3) The number of Sp carbons is? 4) The number of s bonds is ? 5) The number of p bonds is? CEM251/Fall 2007/First Exam.....(page 3) III. (8 points total) 1. Complete the following Newman projections of the two chair conformations of molecule B, viewed simultaneously down the C3-C4 and C1-C6 bonds. C H3 C H3 and B
Answer Answer 2. How many of the methyl groups in molecule A will occupy an equatorial position in the more stable chair conformation?
C H3 C H3 C H3 C H3 C H3 C H3 Answer A
3. Draw a Newman projection of any staggered conformation of 3-chloro-4-methyl-hexane, viewed down the C3-C4 bond. Answer CEM251/Fall 2007/First Exam.....(page 4) IV. 1. (6 points total) Based on chair-form stability, draw a wedge-dotted line structure of the most stable configuration of 1,3,5-trimethylcyclohexane: Answer 2. Draw a wedge-dotted line structure for molecule Y. Br H Br H C H3 C H3 H H Y
Answer 3. Draw a wedge-dotted line structure for molecule X. C H3 H H Br H Br H C H3 X
Answer CEM251/Fall 2007/First Exam.....(page 5) V. (16 points total) 1. Label (in the boxes) the acids and bases in the following acid-base reaction:
O O C H3C O H (+) O H C H3C O H (-) O + HOS OH O + O S O OH 2. Given that nitro substituents (-NO2) make acids stronger and alkyl substituents make acids weaker, encircle the side favored at equilibrium in the following acid-base reaction:
C O2H C O2(-) C O2H C O2(-) +
C H3 N O2 N O2 +
C H3 3. Indicate the isomeric relationship (constitutional, configurational or conformational) of the following molecules: a)
H C H3 H H H C H3 C H3 H H and H H C H3 Answer
Br H b) Br and
Br H C H3 H Br H Answer C H3 c)
C H3 and Answer
C H3 CEM251/Fall 2007/First Exam.....(page 6) VI. (16 points total) 1. Label the following carbocations as primary (1), secondary (2) or tertiary (3):
C H3 C H3 C H3 (+) (+) C H3 C H3 C H3 C H2(+) C H3 C H3 Answer Answer Answer 2. How many configurations are possible for alkenes which have the following constitution?
C H3C H CHCH2C H2C H3
Answer b) C H2 CHCH2C H2C H2C H3
Answer c) C H3C H2C H CHCH2C H3
Answer 3. Label the following configurations as (E) or (Z):
Br a) C H3C H2C H2 C (CH3)2C H C C H2C H3 C H3 b) Br C C H3 C CHCH3 C H2CH(Br)2 Answer Answer CEM251/Fall 2007/First Exam.....(page 7) VII.
a) (12 points total)
Draw the structure of the more stable carbocation intermediate and the major product for the following reactions: C H3 C C H3 C H2 + HBr more stable carbocation b)
C H3 C H3 + C H3 HCl major product more stable carbocation major product 2. Carbocation A rearranges to a more stable carbocation, B. Draw the structure of B: Do not draw a condensed structure for your answer. Instead, draw a complete structure, clearly showing all C-C and C-H bonds and the precise location of the (+) charge.
H H For example: H C C (+) for the ethyl carbocation. H H C H3 C H3C H2C HC C H3 (+) C H3 A Carbocation B 3. Which of the following (A, B or C) is most stable? Answer A B C CEM251/Fall 2007/First Exam.....(page 8) VIII. (2 points total) 1. Which energy diagram (A or B) illustrates the modern version of Markovnikov's Rule? carbocation intermediate carbocation intermediate carbocation intermediate carbocation intermediate alkene + HCl alkene + HCl A B answer ...
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- Fall '07