7. Substitution Reactions

Which reacts faster in an S_N2 reaction?

Explain why the alkyl halide shown here reacts much more rapidly with guanine than does a primary alkyl halide (such as pentyl chloride).

Under appropriate conditions, (S)-1-bromo-1-fluoroethane reacts with sodium methoxide to give pure (S)-1-fluoro-1-methoxyethane.

a. Why is bromide rather than fluoride replaced?

b. Draw perspective structures (as shown on the previous page for 2-bromobutane) for the starting material, the transition state, and the product.

c. Does the product show retention or inversion of configuration? d. Is this result consistent with reaction by the SN2 mechanism?

Which of the following S_N2 reactions should proceed at a faster rate? Justify your answer on a reaction coordinate diagram.

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