Question

In which of the following reactions, rearrangement is possible? (A) \(\mathrm{E}^{1}\) (dehydration of alcohol) (B) \(\mathrm{S}_{\mathrm{N}} 1\) (C) \(\mathrm{S}_{\mathrm{N}} 2\) (D) Both (A) and (B)

Short answer

The correct answer is (D) Both (A) and (B), as rearrangement is possible in \(E^{1}\) (dehydration of alcohol) and \(S_{N}1\) reactions due to the formation of carbocation intermediates.

Step-by-step solution

Reviewing the \(E^{1}\) mechanism

In the first order elimination or \(E^{1}\) mechanism, the reaction typically undergoes two steps: 1) Leaving group leaves, creating a carbocation and, 2) The base removes a proton from the most substituted carbon next to the carbocation and the electrons move to form a π bond. Given the formation of a carbocation intermediate, this makes rearrangement possible in \(E^{1}\) reactions.

Understanding the \(S_{N}1\) mechanism

The \(S_{N}1\) reaction is a two-step reaction involving the formation of a carbocation intermediate, just like in \(E^{1}\) reactions. In the first step, the leaving group leaves, forming a carbocation. In the second step, a nucleophile attacks the carbocation and a substitution takes place. Since this mechanism also involves the formation of a carbocation, rearrangement is possible when the carbocation rearranges to a more stable one before it is attacked by the nucleophile.

Examining the \(S_{N}2\) mechanism

The \(S_{N}2\) mechanism is a one-step process where a nucleophile attacks the substrate and forces the leaving group to leave immediately. Since there is no carbocation formed and the mechanism does not allow for an opportunity for the atoms in the molecule to reorder, rearrangement is not possible in \(S_{N}2\) reactions.

Collecting Information and Concluding

To answer the question, rearrangement is possible in \(E^{1}\) (dehydration of alcohol) and \(S_{N}1\) reactions due to the formation of carbocation intermediates. Thus the correct answer to the exercise is (D) Both (A) and (B).

Key concepts

E1 mechanism

The E1 mechanism is a type of elimination reaction where two major steps occur. First, the leaving group detaches from the molecule, forming a positively charged carbocation. This step is crucial as it sets the stage for the next phase.
Following this, a base removes a proton from the adjacent carbon, resulting in the migration of electrons to form a π bond and complete the formation of the alkene. This two-step process is highly sensitive to the structure of the starting material, especially the stability of the formed carbocation.
Rearrangement is a characteristic possibility during E1 reactions due to the unstable nature of the carbocation intermediate. As molecules seek greater stability, they can rearrange themselves—often moving to more substituted carbons or forming more stable structures altogether. Therefore, in E1 mechanisms, rearrangement is always a possibility.

SN1 mechanism

The SN1 mechanism is a nucleophilic substitution reaction, following a similar two-step pattern to the E1 mechanism. Initially, the leaving group departs, which results in the formation of a highly reactive carbocation intermediate. Stability of this carbocation is vital in determining the reaction's success.
Subsequently, a nucleophile approaches and bonds with the electron-deficient carbocation, completing the substitution reaction. Due to the configuration of the intermediate stage, the process can sometimes offer a mixture of products if the attacking nucleophile approaches from different angles.
This mechanism permits rearrangements. The carbocation formed can shift to more stable conformations—like a more substituted carbon—prior to being targeted by the nucleophile. This aspect renders rearrangements a distinct feature of SN1 reactions.

Carbocation Intermediate

A carbocation intermediate plays a crucial role in both E1 and SN1 mechanisms. It is a molecule where a carbon atom bears a positive charge due to the loss of an electron-bonded group.
Carbocations are inherently unstable because of their electron deficiency. Their stability is influenced by several factors including hyperconjugation and resonance, which often lead to rearrangements in pursuit of rapid stabilization.
Potential rearrangements include alkyl shifts or hydride shifts. These shifts will transform the carbocation into a more stable state, typically switching from a less substituted to a more substituted location or taking advantage of resonance stabilization.
Recognizing the potential for rearrangement in reactions involving carbocation intermediates is key to understanding the diversity of possible reaction pathways in organic chemistry.