Chapter 9, Problem 9.1P

Interpretation Introduction

Interpretation:

The complete and detailed mechanisms for the ${\text{S}}_{\text{N}}{\text{2, S}}_{\text{N}}\text{1, E2, and E1}$ reactions between cyclohexane and ammonia are to be drawn.

Concept introduction:

${\text{S}}_{\text{N}}\text{1}$ and ${\text{S}}_{\text{N}}2$ reactions are nucleophilic substitution reactions. $\text{E1}$ and $\text{E2}$ are elimination reactions.

${\text{S}}_{\text{N}}\text{1}$ and $\text{E1}$ reactions are both two step, unimolecular reactions. They share the first, rate-determining step in which the bond between the leaving group and the carbon atom breaks heterolytically, generating a carbocation. In the second step of the ${\text{S}}_{\text{N}}\text{1}$ mechanism, the nucleophile forms a bond with the carbocation to give the substitution product. If the nucleophile is neutral, an additional deprotonation step is necessary to give the final product. In the second step of the E1 mechanism, a proton attached to a carbon adjacent to the carbocation is extracted by a base, and the $\text{C}-\text{H}$ bond pair moves toward the carbocation to form a $\text{π}$ bond between the two carbon atoms.

${\text{S}}_{\text{N}}2$ and $\text{E2}$ reactions are both single-step, bimolecular reactions. In an ${\text{S}}_{\text{N}}\text{1}$ reaction, the nucleophile forms a bond with the electrophilic carbon in the substrate and the carbon-leaving groups bond breaks simultaneously. If the nucleophile is neutral, an additional deprotonation step is necessary to give the final product. In an $\text{E2}$ reaction, a base extracts a proton from a carbon adjacent to the carbon attached to the leaving group. The $\text{C}-\text{H}$ bond pair moves toward the carbon with the leaving group to form a $\text{π}$ bond, forcing the leaving group to depart with its bond pair.

Answer to Problem 9.1P

The mechanism for S_N2 reaction:

The mechanism for S_N1 reaction:

The mechanism for E2 reaction:

The mechanism for E1 reaction:

Explanation of Solution

In the case of an S_N2 mechanism, the nucleophile attacks the reactant, and at the same time, the leaving group departs. So the S_N2 mechanism is a one-step reaction. As the nucleophile ammonia is neutral, an unstable charged species is formed.

The base, another molecule of ammonia, extracts a proton from the charged species to form the stable, final product.

Therefore, the complete mechanism can be shown as below:

An S_N1 mechanism is a two-step mechanism. In the first step, the leaving group departs along with the electron pair from its bond with the carbon atom. This generates a carbocation.

In the second step, the nucleophile ammonia uses the lone pair on nitrogen to form a bond with the carbocation to form a charged species.

In the final step, another molecule of ammonia acts as a base and extracts a proton from the ${\text{-NH}}_{\text{3}}{}^{\text{+}}$ group to give the final substitution product.

Therefore, the complete mechanism can be shown as below:

In the case of an E2 mechanism, ammonia acts as a base and abstracts a proton from a carbon atom adjacent to the one attached to the leaving group. The $\text{C}-\text{H}$ bond pair moves toward the carbon attached to the leaving group to form a $\text{π}$ bond. At the same time, the leaving group departs along with its bond pair.

Therefore, the complete mechanism can be shown as below:

In the case of an E1 mechanism, the leaving group iodine departs with its bond pair, as an iodide anion. This forms the carbocation.

In the second step, ammonia acts as a base and extracts a proton from a carbon adjacent to the positively charged carbon. The $\text{C}-\text{H}$ bond pair moves toward the carbocation to form a $\text{π}$ bond.

Therefore, the complete mechanism for the E1 reaction can be shown as below:

Conclusion

In a nucleophilic substitution reaction, the attacking species acts as a nucleophile while in an elimination reaction, it acts as a base.