Chapter 6, Problem 27P

Interpretation Introduction

Interpretation:

The reaction which occurs faster in each of the given reactions is to be determined and the reason is to be explained.

Concept introduction:

Alkyl iodides are several times more reactive than alkyl bromides. These reactivity differences can be related to the carbon-halogen bond strength and the basicity of the halide anion. Alkyl iodides have the weakest carbon-halogen bond and require the lowest activation energy to break.

Regarding basicity of the halide leaving the group, iodide is the weakest base. Generally, it is true that, the less basic the leaving group, the smaller the energy requirement for cleaving its bond to carbon and the faster the rate.

Alkyl groups that are adjacent to the carbon atom to the point of nucleophilic attack decrease the rate of the ${\text{S}}_{\text{N}}\text{2}$ reaction.

Protic solvents having $\text{-OH}$ groups form hydrogen bonds to anionic nucleophiles. This solvation around an anion suppresses its nucleophilicity and retards the rate of bimolecular substitution (${\text{S}}_{\text{N}}\text{2}$).

In ${\text{S}}_{\text{N}}\text{1}$ reaction, the rate determining step is the formation of an intermediate carbocation.

Answer to Problem 27P

Solution:

a) ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{I }$ reacts faster than ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{Br}$ by the ${\text{S}}_{\text{N}}\text{2}$ mechanism.

b) $\text{1-chloropentane}$ is more reactive than $\text{1-chloro-2-methylbutane }$

c) Cyclohexyl chloride reacts faster than hexyl chloride by the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

d) Tert-butyl bromide reacts faster than $\text{1-bromo-2,2-dimethylpropane}$ by the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

e) sec-butyl bromide reacts faster than isobutyl bromide by the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

f) The reaction of $\text{1-chlorobutane}$ is faster with sodium methoxide in methanol than with sodium acetate in acetic acid.

g) The reaction of $\text{1-chlorobutane}$ is faster with sodium azide than with sodium p-toluene sulfonate in aqueous ethanol.

Explanation of Solution

a)

Both the given alkyl halides are primary alkyl halides. The reagent is sodium cyanide in dimethyl sulfoxide. Both these suggest an ${\text{S}}_{\text{N}}\text{2}$ reaction. $\text{1-iodobutane}$ undergoes nucleophilic substitution several times faster than $\text{1-bromobutane}$. The carbon-iodine bond length is more than the carbon-bromine bond length. Hence, the carbon-iodine bond is weaker than the carbon-bromine bond. Also, iodide is a weaker base than bromide. Hence iodide has a smaller energy requirement for cleaving its bond to carbon and reacts faster than bromide. Hence, the reaction of 1-iodobutane will occur faster than that of 1-bromobutane.

b)

Both the given alkyl halides are primary alkyl halides. The reagent is sodium iodide in acetone. Both these suggest an ${\text{S}}_{\text{N}}\text{2}$ reaction. $\text{1-chloro-2-methylbutane }$ is slightly more hindered as compared to $\text{1-chloropentane}$. In $\text{1-chloro-2-methylbutane }$, the methyl group on the carbon atom adjacent to the carbon bearing the chlorine atom decreases the rate of the ${\text{S}}_{\text{N}}\text{2}$ reaction.

c)

Hexyl chloride is a primary alkyl halide whereas cyclohexyl chloride is a secondary alkyl halide. The solvent is ethanol, which is a polar protic solvent. The nucleophile is the azide ion, which is a good nucleophile. Solvation of the azide ion by ethanol reduces the rate of bimolecular substitution. Polar protic solvents favor ${\text{S}}_{\text{N}}\text{1}$ the type of reaction. In ${\text{S}}_{\text{N}}\text{1}$ reaction, the rate determining step is the formation of a carbocation intermediate. Secondary carbocations are more stabilized than primary carbocations. Cyclohexyl chloride forms a secondary carbocation while hexyl chloride forms a primary carbocation. Hence cyclohexyl chloride reacts faster than hexyl chloride by the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

d)

The solvent is ethanol, which is a polar protic solvent. It favors the ${\text{S}}_{\text{N}}\text{1}$ reaction. In ${\text{S}}_{\text{N}}\text{1}$ reaction, the rate determining step is the formation of a carbocation intermediate. $\text{1-bromo-2,2-dimethylpropane}$ is a primary alkyl halide whereas tert-butyl bromide is a tertiary alkyl halide. Tert-butyl bromide forms tertiary carbocation while $\text{1-bromo-2,2-dimethylpropane}$ forms a primary carbocation. Tertiary carbocations are more stabilized than primary carbocations. Hence tert-butyl bromide reacts faster than $\text{1-bromo-2,2-dimethylpropane}$ by the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

e)

The solvent is aqueous formic acid, which is a polar protic solvent. It favors the ${\text{S}}_{\text{N}}\text{1}$ reaction. In ${\text{S}}_{\text{N}}\text{1}$ reaction, the rate determining step is the formation of a carbocation intermediate. Isobutyl bromide is a primary alkyl halide whereas sec-butyl bromide is a secondary alkyl halide. Sec-butyl bromide forms secondary carbocation while isobutyl bromide forms a primary carbocation. Secondary carbocations are more stabilized than primary carbocations. Hence, sec-butyl bromide reacts faster than isobutyl bromide by the ${\text{S}}_{\text{N}}\text{1}$ reaction mechanism.

f)

$\text{1-chlorobutane}$ is a primary alkyl halide. It reacts by the ${\text{S}}_{\text{N}}\text{2}$ mechanism. This mechanism requires a strong nucleophile. The two given nucleophiles are acetate ion (sodium acetate) and methoxide ion (from sodium methoxide).

As long as the nucleophilic atom is the same, the more basic the nucleophile, the more reactive it is. The methoxide ion is more basic and more nucleophilic than the acetate ion. Thus, the reaction of $\text{1-chlorobutane}$ is faster with sodium methoxide in methanol than with sodium acetate in acetic acid.

g)

$\text{1-chlorobutane}$ is a primary alkyl halide. It reacts by the ${\text{S}}_{\text{N}}\text{2}$ mechanism. This mechanism requires a strong nucleophile. The two given nucleophiles are azide ion (sodium azide) and p-toluene sulfonate ion.

Comparing the nucleophilic atoms, the azide ion is more nucleophilic than the p-toluene sulfonate ion since nitrogen is less electronegative than oxygen. Thus, the reaction of $\text{1-chlorobutane}$ is faster with sodium azide than with sodium p-toluene sulfonate in aqueous ethanol.