7. Substitution Reactions

What stereoisomers do the following reactions form?

d.

Write the mechanism for the reaction of a cysteine side chain with iodoacetic acid.

Amines are good nucleophiles, even though they are neutral. How would the rate of an S_N2 reaction between an amine and an alkyl halide be affected if the polarity of the solvent is increased?

How will the rate of each of the following S_N2 reactions change if it is carried out in a more polar solvent?

a.

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

a. Both cis reactants form an optically active trans product, but each cis reactant forms a different trans product.

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

c. A trans reactant is more reactive than a cis reactant.

Which substitution reaction takes place more rapidly?

b.

Which substitution reaction takes place more rapidly?

c.

What nucleophiles would form the following compounds as a result of reacting with 1-iodobutane?

g.

What nucleophiles would form the following compounds as a result of reacting with 1-iodobutane?

h.

Starting with an alkyl halide, how could the following compounds be prepared?

a. 2-methoxybutane

Starting with an alkyl halide, how could the following compounds be prepared?

b. 1-methoxybutane

2-Acetoxycyclohexyl tosylate reacts with acetate ion to form 1,2-cyclohexanediol diacetate. The reaction is stereospecific—that is, the stereoisomers obtained as products depend on the stereoisomer used as a reactant. Recall that because 2-acetoxycyclohexyl tosylate has two asymmetric centers, it has four stereoisomers—two are cis and two are trans. Explain the following observations:

b. Both trans reactants form the same racemic mixture.

The rate of the reaction of methyl iodide with quinuclidine was measured in nitrobenzene, and then the rate of the reaction of methyl iodide with ­triethylamine was measured in the same solvent. The concentration of the reagents was the same in both experiments.

b. The same experiment was done using isopropyl iodide instead of methyl iodide. Which reaction had the larger rate constant?

The rate of the reaction of methyl iodide with quinuclidine was measured in nitrobenzene, and then the rate of the reaction of methyl iodide with ­triethylamine was measured in the same solvent. The concentration of the reagents was the same in both experiments.

c. Which alkyl halide has the larger k_quinuclidine/k_{triethylamine} ratio?