7. Substitution Reactions

Which reacts faster in an S_N1 reaction?

Practice your electron-pushing skills by drawing a mechanism for the following S_N1 reactions.

(b)

Rationalize the ranking of increasing reaction rate of the benzylic halides shown.

Predict the product of the following reaction:

Predict the product of the following reaction:

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.

Allylic halides have the structure

c. Show the products expected from S_N1 solvolysis of these halides in ethanol.

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

(a)

(b)

A reluctant first-order substrate can be forced to ionize by adding some silver nitrate (one of the few soluble silver salts) to the reaction. Silver ion reacts with the halogen to form a silver halide (a highly exothermic reaction), generating the cation of the alkyl group.

Give mechanisms for the following silver-promoted rearrangements.

(a)

Propose a mechanism involving a hydride shift or an alkyl shift for each solvolysis reaction. Explain how each rearrangement forms a more stable intermediate.

Hint: Most rearrangements convert 2° (or incipient 1°) carbocations to 3° or resonance-stabilized carbocations.

(b)

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

Propose an S_N1 mechanism for the solvolysis of 3-bromo-2,3-dimethylpentane in ethanol.

Predict the compound in each pair that will undergo solvolysis (in aqueous ethanol) more rapidly.

(a) (CH₃CH₂)₂CH—Cl or (CH₃)₃C—Cl

(b)

Choose the member of each pair that will react faster by the S_N1 mechanism.

a. 1-bromopropane or 2-bromopropane

b. 2-bromo-2-methylbutane or 2-bromo-3-methylbutane

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?