Question

Consider the following statements in reference to \(S_{N} 1, S_{N} 2, E 1\), and E2 reactions of haloalkanes. To which mechanism(s), if any, does each statement apply? (a) Involves a carbocation intermediate. (b) Is first order in haloalkane and first order in nucleophile. (c) Involves inversion of configuration at the site of substitution. (d) Involves retention of configuration at the site of substitution. (e) Substitution at a stereocenter gives predominantly a racemic product. (f) Is first order in haloalkane and zero order in base. (g) Is first order in haloalkane and first order in base. (h) Is greatly accelerated in protic solvents of increasing polarity. (i) Rearrangements are common. (j) Order of reactivity of haloalkanes is \(3^{\circ}>2^{\circ}>1^{\circ}\). (k) Order of reactivity of haloalkanes is methyl \(>1^{\circ}>2^{\circ}>3^{\circ}\).

Short answer

Answer: S_N1 and E1 mechanisms involve a carbocation intermediate.

Step-by-step solution

(a) Involves a carbocation intermediate.

This statement applies to \(S_{N}1\) and \(E1\) mechanisms, as both of these reactions involve the formation of a carbocation intermediate during their processes.

(b) Is first order in haloalkane and first order in nucleophile.

This statement applies to the \(S_{N}2\) mechanism, as the rate of the reaction depends on both the haloalkane and the nucleophile concentrations.

(c) Involves inversion of configuration at the site of substitution.

This statement applies to the \(S_{N}2\) mechanism, where nucleophilic substitution occurs with inversion of configuration (Walden inversion) at the site of substitution.

(d) Involves retention of configuration at the site of substitution.

This statement applies to the \(S_{N}1\) mechanism, as the nucleophile can attack the carbocation from either side, leading to retention of configuration in the product.

(e) Substitution at a stereocenter gives predominantly a racemic product.

This statement applies to the \(S_{N}1\) mechanism, as the nucleophile can attack the carbocation from either side, producing a racemic mixture of products.

(f) Is first order in haloalkane and zero order in base.

This statement applies to the \(E1\) mechanism, as the rate of the reaction depends only on the haloalkane concentration and does not depend on the base concentration.

(g) Is first order in haloalkane and first order in base.

This statement applies to the \(E2\) mechanism, as the rate of the reaction depends on both the haloalkane and the base concentrations.

(h) Is greatly accelerated in protic solvents of increasing polarity.

This statement applies to the \(S_{N}1\) and \(E1\) mechanisms, as protic solvents favor the formation of carbocation intermediates and stabilize them.

(i) Rearrangements are common.

This statement applies to the \(S_{N}1\) and \(E1\) mechanisms, as both involve a carbocation intermediate that can rearrange to form more stable carbocations.

(j) Order of reactivity of haloalkanes is \(3^{\circ}>2^{\circ}>1^{\circ}\).

This statement corresponds to the \(S_{N}1\) and \(E1\) mechanisms, as tertiary haloalkanes form more stable carbocations leading to higher reaction rates.

(k) Order of reactivity of haloalkanes is methyl \(>1^{\circ}>2^{\circ}>3^{\circ}\).

This statement corresponds to the \(S_{N}2\) and \(E2\) mechanisms, as steric hindrance in tertiary haloalkanes makes it difficult for nucleophiles or bases to approach the reactive center.