7. Substitution Reactions

Which compound is more reactive in an S_N1 reaction? In each case, you can assume that both alkyl halides have the same stability.

a.

b.

Which of the following rate laws describes the kinetics of an $S{N}_1$ reaction?

Provide the mechanism and final products for the following reactions.

Which haloalkane would you expect to undergo the fastest S_N1 reaction? Why?

Allylic halides have the structure

a. Show how the first-order ionization of an allylic halide leads to a resonance-stabilized cation.

Rank the following from most reactive to least reactive in an S_N1 reaction:

Give the substitution products expected from solvolysis of each compound by heating in ethanol.

(c)

(d)

Optically active butan-2-ol racemizes in dilute acid. Propose a mechanism for this racemization.

When tert-butyl bromide is heated with an equal amount of ethanol in an inert solvent, one of the products is ethyl tert-butyl ether.

b. What happens to the rate if the concentration of tert-butyl bromide is tripled and the concentration of ethanol is doubled?

c. What happens to the rate if the temperature is raised?

Because the S_N1 reaction goes through a flat carbocation, we might expect an optically active starting material to give a completely racemized product. In most cases, however, S_N1 reactions actually give more of the inversion product. In general, as the stability of the carbocation increases, the excess inversion product decreases. Extremely stable carbocations give completely racemic products. Explain these observations.

Propose a mechanism for the reaction of benzyl bromide with ethanol to give benzyl ethyl ether.

tert-Butyl chloride undergoes solvolysis in both acetic acid and formic acid. Solvolysis occurs 5000 times faster in one of these two solvents than in the other. In which solvent is solvolysis faster? Explain your answer. (Hint: Formic acid is more polar than acetic acid.)

Furfuryl chloride can undergo substitution by both S_N2 and S_N1 mechanisms. Because it is a 1° alkyl halide, we expect S_N2 but not S_N1 reactions. Draw a mechanism for the S_N1 reaction shown below, paying careful attention to the structure of the intermediate. How can this primary halide undergo S_N1 reactions?

Give the S_N1 mechanism for the formation of 2-ethoxy-3-methylbutane, the unrearranged product in this reaction.

The allylic bromide below gives two S_N1 products. Justify the formation of each.