Problem 18 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

(d) carbocation

Step 1: Understanding Racemisation

Racemisation in a chemical solution refers to the process where an optically active compound transforms into a racemic mixture, losing its optical activity. This process often involves the formation of intermediates that allow for changes in the stereochemistry of the compound.

Step 2: Identifying the Intermediates Involved

To determine the intermediate responsible for the racemisation in the solution of chloropropane in the presence of \(\mathrm{SbCl}_{5}\), we must consider what intermediate is stabilized by \(\mathrm{SbCl}_{5}\). Typically, \\(\mathrm{SbCl}_{5}\) is a Lewis acid and can take part in reactions that form carbocations.

Step 3: Role of \(\mathrm{SbCl}_{5}\) as a Lewis Acid

\(\mathrm{SbCl}_{5}\) can accept a chlorine ion from the compound to form \(\mathrm{SbCl}_6^-\), leaving behind a positively charged intermediate - a carbocation.

Step 4: Carbocation Formation

The presence of the Lewis acid \(\mathrm{SbCl}_{5}\) leads to the formation of a carbocation as it stabilizes the positively charged center. As the chlorine is removed, the two positively charged centers can form, facilitating racemisation as the planar carbocation allows for attack from either side.

Key Concepts

Intermediate Formation

In many chemical reactions, intermediate formations are pivotal. Intermediates are temporary species formed during a reaction, leading to the product. In the context of racemization, intermediates play a crucial role in changing the stereochemistry, which affects the optical activity of compounds. The presence of the intermediate provides an opportunity for the molecule to rearrange its structure. In the case of (+2)-2-chloro-2-phenylethane in the presence of \(\mathrm{SbCl}_{5}\), we have intermediate formation in the form of a carbocation. Recognizing this helps us understand why the compound loses its optical activity, eventually forming a racemic mixture.

Lewis Acid

A Lewis acid is a chemical species that accepts an electron pair. This definition is the opposite of a Lewis base, which donates an electron pair. Lewis acids are crucial in forming new chemical bonds. \(\mathrm{SbCl}_{5}\) acts as a Lewis acid by accepting a chloride ion from another molecule, effectively forming \(\mathrm{SbCl}_{6}^{-}\). The removal of the chloride ion facilitates the creation of a carbocation. Many reactions involving racemization rely on Lewis acids to stabilize intermediates. In these reactions, the Lewis acid's role is integral, as it ensures the smooth transition from one state to another, allowing for rearrangements needed for the process to complete.

Carbocation

Carbocations are molecules that contain a carbon atom with a positive charge. They are highly reactive intermediates, often formed when a bond breaks and a pair of electrons from the bond remains with one of the atom partners. In the context of (+2)-2-chloro-2-phenylethane, the presence of \(\mathrm{SbCl}_{5}\) leads to the loss of a chloride ion and the formation of a carbocation. This positively charged species is a key player in racemization processes. The carbocation, being planar and electron-deficient, allows for attack from either side. This characteristic leads to the interconversion of stereochemistry, which ultimately results in the loss of optical activity.

Optical Activity

Optical activity refers to a compound's ability to rotate the plane of polarized light. Compounds that are optically active have chiral centers—meaning they have a non-superimposable mirror image. However, during racemization, this optical activity is lost. When a carbocation intermediate forms during a reaction, the resulting molecule can transform into an equal mix of enantiomers—a racemic mixture. This mixture doesn't rotate plane-polarized light because the effects of the enantiomers cancel each other out. Understanding optical activity and its loss during racemization help explain why some reactions result in compounds without optical rotation.