In the presence of a small amount of bromine, cyclohexene undergoes the following light-promoted reaction:

d. Explain why cyclohexene reacts with bromine much faster than cyclohexane, which must be heated to react.

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)

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.

(c)

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.

(d)

For each reaction, give the expected substitution product, and predict whether the ­mechanism will be predominantly first order (S_N1) or second order (S_N2).

a. 2-chloro-2-methylbutane + CH₃COOH

b. isobutylbromide + sodium methoxide

For each reaction, give the expected substitution product, and predict whether the ­mechanism will be predominantly first order (SN1) or second order (SN2).

c. 1-iodo-1-methylcyclohexane + ethanol

For each reaction, give the expected substitution product, and predict whether the ­mechanism will be predominantly first order (SN1) or second order (SN2).

d. cyclohexylbromide + methanol

e. cyclohexylbromide + sodium ethoxide

Under certain conditions, when (R)-2-bromobutane is heated with water, the S_N1 substitution proceeds twice as fast as the S_N2. Calculate the e.e. and the specific rotation expected for the product. The specific rotation of (R)-butan-2-ol is −13.5°. Assume that the S_N1 gives equal amounts of the two enantiomers.

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)

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.

(b)

Show how you would convert (in one or two steps) 1-phenylpropane to the three products shown below. In each case, explain what unwanted reactions might produce undesirable impurities in the product.

Show how you would convert (in one or two steps) 1-phenylpropane to the three products shown below. In each case, explain what unwanted reactions might produce undesirable impurities in the product.

Draw the structures of the following compounds.

a. sec-butyl chloride

b. isobutyl bromide

Predict the compound in each pair that will undergo the S_N2 reaction faster.

(a)

(b)

Predict the compound in each pair that will undergo the S_N2 reaction faster.

(c)

(d)

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

(a) (CH₃CH₂)₂CH—Cl or (CH₃)₃C—Cl

(b)

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

(c)

(d)

Show how each compound might be synthesized by the SN2 displacement of an alkyl halide.

e. H₂C=CH—CH₂CN

f. H—C≡C—CH₂CH₂CH₃

Give two syntheses for (CH₃)₂CH—O—CH₂CH₃, and explain which synthesis is better.

When ethyl bromide is added to potassium tert-butoxide, the product is ethyl tert-butyl ether.

CH₃CH₂–Br + (CH₃)₃C–O^–K⁺ → (CH₃)₃C–O–CH₂CH₃ ethyl bromide potassium tert-butoxide ethyl tert-butyl ether

a. What happens to the reaction rate if the concentration of ethyl bromide is doubled?

When ethyl bromide is added to potassium tert-butoxide, the product is ethyl tert-butyl ether.

CH₃CH₂–Br + (CH₃)₃C–O^–K⁺ → (CH₃)₃C–O–CH₂CH₃ ethyl bromide potassium tert-butoxide ethyl tert-butyl ether

b. What happens to the rate if the concentration of potassium tert-butoxide is tripled and the concentration of ethyl ­bromide is doubled?

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

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.

a. What happens to the reaction rate if the concentration of ethanol is doubled?

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?

Chlorocyclohexane reacts with sodium cyanide (NaCN) in ethanol to give cyanocyclohexane. The rate of formation of cyanocyclohexane increases when a small amount of sodium iodide is added to the solution. Explain this acceleration in the rate.

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

(a)

(b)

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

(c)

(d)

Allylic halides have the structure

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

Allylic halides have the structure

b. Draw the resonance structures of the allylic cations formed by ionization of the following halides.

(i)

(ii)

Allylic halides have the structure

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

List the following carbocations in decreasing order of their stability.