1. (25 points) Multiple Choice: Circle the best possible answer.

(i) Select the correct IUPAC name for the compound incorrectly labeled as 2-ethyl-2-pentene.

(A) 3-Methyl-3-pentene (B) 4-Methyl-3-pentene
(C) 3-Methyl-3-hexene (D) 3-Heptene

(ii) What is the slow, rate-determining step in the acid-catalyzed dehydration of 2-methyl-2- propanol

(A) The simultaneous loss of a b-hydrogen and water from the oxonium ion.
(B) Protonation of the alcohol to form an oxonium ion.
(C) Loss of water from the oxonium ion to form a carbocation.
(D) Loss of a beta-hydrogen from the carbocation to form an alkene.

(iii) Select the alkene that is thermodynamically the most stable.

(A) trans-3-hexene (B) cis-3-hexene
(C) 1-hexene (D) 2-methyl-2-pentene

In general, alkenes with more highly substituted double bonds are more stable than isomers with less substituted double bonds.
Correct Answer: D

(iv) The reaction of 1-butene with bromine, Br₂, in aqueous solution gives primarily 1-bromo-2-butanol. The nucleophilic species that leads to the above product is:

(A) Br₂ (B) Br^-
(C) H₂O (D) OH^-

(v) Compound X, C₅H₁₀O, is optically active. The compound consumes one equivalent of H₂ to form C₅H₁₂O. The hydrogenation product is also optically active. The structure of compound X is most likely to be:

(A) H₂C=CHCH₂CH₂CH₂OH (B) trans-CH₃CH=CHCH₂CH₂OH
(C) CH₃CH₂CH(OH)CH=CH₂ (D) CH₃CH(OH)CH₂CH=CH₂

Neither (A) nor (B) represents an optically active compound, while both (C) and (D) represent compounds with chiral centers. However, on hydrogenation, (C) is converted to CH₃CH₂CH(OH)CH₂CH₃ which is optically inactive. Hydrogenation of (D) produces an optically active product. Correct Answer: D

2. (20 points) Label each of the following pairs of structures as conformers, constitutional (or structural) isomers, diastereomers, enantiomers, or the same.

3. (20 points) Write an arrow pushing mechanism for ANY TWO of the following transformations. Be sure to show all intermediates in the pathway from starting material to product, but do not show transition states. All structures should have proper valence bond structures with correct formal charges and lone pairs as necessary. If you attempt all three, be sure to CROSS OUT the one that SHOULD NOT BE GRADED. If you do not follow the guidelines, only the first two mechanisms will be graded.

4. (10 points) 5-Bromononane on treatment with potassium ethoxide in ethanol produces a mixture of cis-4-nonene (23%) and trans-4-nonene (77%). Draw Newman projections of 5-bromononane, looking down the C5-C6 bond, showing the conformations that lead to cis-4-nonene and trans-4-nonene, respectively. Assume that C5 is closest to you and C6 is pointing away from you.

Based on your conformations, explain in one or two sentences, why the trans-alkene is the major product.

The staggered conformation leading to the trans-alkene is the anti conformation where the alkyl substitutents are separated by a dihedral angle of 180º, while the gauche conformation (dihedral angle of 60º) leads to the relatively less stable cis-alkene. More molecules are present in the anti conformation and the reaction is faster; hence, the trans-alkene is the major product.

5. (25 points) Write the structure of the single major product of each of the following reactions. If two or more major products are formed, write the structures of both. Show the stereochemistry, where appropriate, using wedges and dashes.