Question

A solution of \((+2)-2\) -chloro-2-phenylethane in toluene racemises slowly in the presence of small amounts of \(\mathrm{SbCl}_{5}\), due to the formation of: (a) carbanion (b) carbene (c) free radical (d) carbocation

Short answer

The formation of a carbocation leads to racemization.

Step-by-step solution

Understanding the Problem

The problem describes a compound (+2)-2-chloro-2-phenylethane in a solution with toluene that racemizes (changes optical activity) in the presence of SbCl_{5}. We need to determine the chemical species responsible for this racemization process.

Recognizing Chemical Reactions

Racemization is often associated with the formation of an intermediate species that allows for the compound's structure to change its chirality. The options provided suggest different types of intermediates that could be formed.

Identifying the Likely Intermediate

In the presence of strong Lewis acids like SbCl_{5}, common reactions include the formation of a carbocation. Carbocations are known to be planar, allowing for the free rotation around the single bonds, which can lead to racemization.

Confirming Carbocation Formation

The chloride group in (+2)-2-chloro-2-phenylethane can leave under the influence of SbCl_{5}, forming a positively charged carbon atom (carbocation). This can allow the subsequent molecules to rotate, changing their chiral orientation and resulting in racemization.

Checking Other Options

Carbanions, carbenes, and free radicals are less likely in this scenario. Carbanions are usually formed in basic conditions, carbenes require different reaction mechanisms, and free radicals require homolytic bond cleavage rather than a nucleophilic attack.

Key concepts

Racemization

Racemization is a process where an optically active compound becomes a racemic mixture, losing its optical activity. This occurs when a molecule can interconvert between its chiral forms, leading to a mixture of equal amounts of enantiomers.
In the context of the given problem, the racemization of (+2)-2-chloro-2-phenylethane is prompted by the formation of a carbocation. This carbocation is an intermediate species that allows the molecule to rearrange in a way that its chirality is altered, resulting in racemization.
Factors such as temperature, solvents, and the presence of catalysts like Lewis acids can accelerate racemization. By breaking and reforming bonds, these catalysts make it easier for the molecule to switch its configurations.

Lewis Acids

Lewis acids are chemical species that can accept electron pairs from Lewis bases. They are often used in various chemical reactions as catalysts. In our context, SbCl_5 functions as a Lewis acid. It interacts with the substrate by accepting an electron pair, facilitating the departure of a leaving group, which in turn forms a carbocation.
The presence of a Lewis acid like SbCl_5 is crucial in this reaction as it helps to stabilize the carbocation intermediate. This stabilization is necessary to enable the subsequent changes in molecular configuration, leading to racemization.
Lewis acids are especially useful in reactions involving the formation of carbocations because they enhance the electrophilic character of the substrate, making it more reactive.

Chirality

Chirality refers to the property of a molecule that makes it non-superimposable on its mirror image. Think of it like how your left and right hands are mirror images but cannot be perfectly overlaid. Chiral molecules often have a carbon atom with four different substituents, known as a chiral center.
In the 2-chloro-2-phenylethane molecule, the chirality initially exists due to its specific arrangement of atoms around the chiral center. However, when racemization occurs, the chirality is disturbed, and the molecule's configuration changes.
The formation of a carbocation allows for the free rotation of substituents around a formerly chiral carbon, effectively "erasing" its chirality for a brief moment, and as a result, the molecule can end up in different enantiomeric forms.

Intermediate Species

Intermediate species are transient structures that form during chemical reactions. These species are not the initial reactants nor the final products, but they play a crucial role in the progression and transition of a chemical reaction.
In the case of racemization of 2-chloro-2-phenylethane, the carbocation acts as an intermediate species. This positively charged species forms when chloride is cleaved from the molecule under the influence of the Lewis acid, SbCl_5.
The carbocation intermediate makes it possible for the molecule to adopt new spatial orientations, which ultimately leads to the interconversion between enantiomers. Understanding the nature of intermediate species helps chemists design and predict the outcomes of various reactions, including racemization.