Chem2O6 - 1996/97

Assignment #2 ANSWERS November 13, 1996

1. Predict the principal products of the following reactions, showing stereochemistry where appropriate.

Question 1(a)
Good leaving group on 1o carbon, strong weakly basic nucleophile, and polar weakly ionizing solvent favour SN2 substitution.

Question 1(b)
Strongly basic nucleophile, heat, and polar weakly ionizing solvent favour E2 elimination.

Question 1(c)
Strong, weakly basic nucleophile, and polar aprotic solvent favour SN2 substitution. Bromide is a good leaving group; methoxide (CH3O-) is not. Note inversion of configuration.

Question 1(d)
1o benzylic carbon, weakly ionizing solvent favour SN2 substitution.

Question 1(e)
1o allylic carbon, acidic conditions (allowing protonation of OH), and strong nucleophile favour SN2 substitution. SN1 substitution might also be expected because of the strongly ionizing conditions, but the result is the same.

Question 1(f)
2o carbon bearing excellent leaving group, weakly ionizing solvent, and strong weakly basic nucleophile favour SN2 substitution.

2. Suggest a synthesis of (R)-2-butanol from (R)-2-bromobutane. Two steps may be required.

Question 2

Overall retention of configuration; thus, can't use SN1. SN2 gives inversion; two successive SN2's would result in overall retention.

Question 2

3. Hydroxide is a lousy leaving group. One way of converting an OH group in an alcohol to a "good" leaving group is to convert it to a para-toluenesulfonic acid ester (tosylate) or methanesulfonic acid ester (mesylate) ester. Nucleophilic substitution then proceeds readily, as the leaving group is now the tosylate (TsO-) or mesylate (MsO-) anion rather than hydroxide. For example,

Synthesis of cyclopentyl tosylate & reaction with cyanide

(a) Write a mechanism for the nucleophilic substitution reaction in the equation above.

Question 3a

(b) Carboxylic acid esters are synthesized by a similar procedure to that used for the synthesis of sulfonate esters. For example,

Synthesis of cyclopentyl acetate

Write a reasonable mechanism for the formation of cyclopentyl acetate from cyclopentanol and acetyl chloride, based on your knowledge of sulfonate ester formation.

Question 3b

(c) What would be the product of reaction of (1R,2S)-2-methylcyclopentanol with acetyl chloride in pyridine? (Give the structure and name)

Question 3c

(d) While tosylate and mesylate are very good leaving groups in nucleophilic substitution reactions, acetate and other carboxylate anions are not. Explain why.

Tosylate and mesylate have pKa < 0; they are good leaving groups because they are very weak bases. Acetic acid has pKa 4.8; acetate (CH3CO2-) is therefore a stronger base than TsO- or MsO- and is thus a poorer leaving group.

4. Show how each of the following compounds can be prepared from an alkyl halide.

Question 4a

Question 4b

Question 4c

Question 4d

Question 4e

Question 4f

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01nov96; wjl