Chemistry 331 - Winter 1996
Elements of Organic Chemistry I

Professor Carl C. Wamser

EXAM 3 - Answer Key
March 14, 1996

1. (15 points) Write complete names for each of the following, including designation of stereochemistry if it is specifically shown.

a)

3,5-dimethoxyphenol

b)

cis-1,3-cyclopentanediol

c)

(S)-3-isobutyl-4-penten-1-ol

2. (15 points) For each of the following groups of compounds, arrange them in order with respect to the particular property indicated.
Write "MOST" and "LEAST" under the most reactive and least reactive in each series.

a) S_N2 reactivity towards NaCN:

MOST , MIDDLE , LEAST

b) S_N1 reactivity in neutral methanol solution:

MIDDLE , LEAST , MOST

c) reactivity as an acid:

MIDDLE , LEAST , MOST

d) reactivity as an acid:

MIDDLE , LEAST , MOST

e) reactivity as a base:

LEAST , MIDDLE , MOST

3. (15 points) Complete each of the following reactions by adding the missing part: either the starting material, the necessary reagents and conditions, or the expected major product. Indicate stereochemistry if it would be specific.

4. (10 points) Write a complete mechanism for the acid-catalyzed dehydration of tert-butyl alcohol.

5. (15 points) Identify each of the lettered compounds in the scheme below. Show all of the reactions that are referred to.

A is an alcohol that can be oxidized by CrO₃ to give a carboxylic acid.
When A is treated with PBr₃, it gives compound B.
When B is treated with strong base, it gives compound C.
When C is hydrated using dilute sulfuric acid in water, it gives D, which is an isomer of A.
Oxidation of D with CrO₃ gives acetone (dimethyl ketone).

A must be a primary alcohol (RCH₂OH) in order to be oxidized to a carboxylic acid.

D must be 2-propanol, since that is the only alcohol that could be oxidized to give acetone.

A must be 1-propanol.

6. (10 points) Develop a sequence of reactions that could be used to synthesize one of the following compounds below. As starting materials, you may use any alcohol, but no other organic compounds.
SELECT JUST ONE.

dibutyl ether OR (R)-2-iodopentane

7. (20 points) 2-Bromobutane undergoes E2 elimination when it is treated with KOH in ethanol as solvent. Three products are obtained:
1-butene, cis-2-butene, and trans-2-butene.
Predict the relative order of abundance of these three products (most, less, least).

trans-2-butene (MOST) > cis-2-butene (LESS) > 1-butene (LEAST)

Write a Newman diagram for (R)-2-bromobutane, looking down the C₂-C₃ bond.
Show all three staggered conformations, and show why E2 elimination from specific conformations may lead to specific products.

E2 elimination requires the H and the Br that are eliminated to be aligned anti to one another. This may occur either while the two methyl groups are also anti (leading to trans-2-butene) or while the two methyl groups are gauche (leading to cis-2-butene).
Trans is the major product because it comes from the more stable conformation.

Write a Newman diagram looking down the C₁-C₂ bond, and illustrate how E2 elimination would give rise to 1-butene.

1-Butene would be a lesser product because it is less stable than the more substituted 2-butenes.

Explain what would be different about your diagrams and explanations if you were working with (S)-2-bromobutane instead of (R).

All the Newman diagrams would be the mirror images of the ones shown. However, the same products would be formed from the enantiomeric structures and in the same amounts.