Chapter 8, Problem 8.1P

Interpretation Introduction

(a)

Interpretation:

Using curved arrow notation, an ${\text{S}}_N\text{2}$ mechanism for the given reaction is to be drawn.

Concept introduction:

The ${\text{S}}_N\text{2}$ reaction is a bimolecular nucleophilic substitution single step reaction in which a strong nucleophile attacks the substrate from the side opposite the leaving group. The attacking species, the nucleophile used in an ${\text{S}}_N\text{2}$ reaction, must be strong. A polar aprotic solvent is used as a solvent in an ${\text{S}}_N\text{2}$ reaction mechanism. Secondary alkyl halides can undergo all four possible substitution and elimination reactions.

Answer to Problem 8.1P

Mechanism for the given ${\text{S}}_N\text{2}$ reaction using curved arrow notation is

Explanation of Solution

The given reaction is

The given substrate is a secondary alkyl halide. Chlorine is a moderately good leaving group. Bromide ion is a strong nucleophile. An ${\text{S}}_N\text{2}$ reaction for this would look like this:

In the above reaction, the bromide ion attacks the carbon, to which the leaving group, chlorine, is attached from the side opposite the leaving group. In the product, the chlorine atom is replaced by a bromine atom.

Conclusion

The ${\text{S}}_N\text{2}$ reaction is the nucleophilic substitution single step reaction in which a strong nucleophile attacks the substrate from the side opposite the leaving group.

Interpretation Introduction

(b)

Interpretation:

Using curved arrow notation, an ${\text{S}}_{N}1$ mechanism for the given reaction is to be drawn.

Concept introduction:

The ${\text{S}}_{N}1$ reaction is a unimolecular nucleophilic substitution single two-step reaction. In the first step known as heterolysis, the bond between carbon and the laving group breaks to yield a carbocation intermediate. This carbocation intermediate formed can rearrange itself via 1, 2-hydride shift or 1, 2-methyl shift to be a more stable carbocation intermediate. In the second step, that is the coordination step, the nucleophile attacks the intermediate carbocation from the either side to yield a mixture of stereoisomers if there exists a chiral center.

Answer to Problem 8.1P

Mechanism for the given ${\text{S}}_{N}1$ reaction using curved arrow notation is

Explanation of Solution

The given reaction is

The first step is the breaking of carbon-chlorine bond. This will generate a secondary carbocation intermediate. In the second step, the electron rich ${\text{Br}}^{\text{-}}$ attacks the carbocation to form a new $\text{C}-\text{Br}$ bond. The curved arrow notation for the ${\text{S}}_{N}1$ mechanism for the give reaction is as follows:

The leaving group is lost and forms a carbocation as follows:

Conclusion

The ${\text{S}}_{N}1$ reaction is a unimolecular nucleophilic substitution single two-step reaction. In the first step known as heterolysis, the bond between carbon and the laving group breaks to yield a carbocation intermediate. In the second step, that is the coordination step, the nucleophile attacks the intermediate carbocation from either side to yield a mixture of stereoisomers if there exists a chiral center.