Question

A solution of \((+2)-2\)-chloro-2-phenylethane in toluene racemises slowly in the presence of small amounts of \(\mathrm{SbCl}_{6}\), due to the formation of (a) carbanion (b) carbene (c) free radical (d) carbocation

Short answer

The formation of a carbocation (option d) is responsible for racemization.

Step-by-step solution

Understanding Racemization

The process of racemization involves converting an optically active compound into an inactive mixture, often through a loss of stereochemistry at a chiral center. This usually involves the formation of an intermediate that can equilibrate between different stereoisomers.

Analyzing the Reaction Environment

The solution contains \( (+2)-2\)-chloro-2-phenylethane in toluene with \( \mathrm{SbCl}_{6} \). The presence of a halogen (chlorine) group suggests that the compound may undergo nucleophilic substitution or elimination, often involving a planar carbocation intermediate which allows free rotation, leading to racemization.

Identifying Likely Intermediates

Considering the reaction environment and the presence of \( \mathrm{SbCl}_{6} \), a strong Lewis acid, it is likely that \( Cl^- \) is removed from 2-chloro-2-phenylethane, leading to the formation of a stable carbocation. Carbocations are planar and allow for free rotation around the single bond, which leads to the potential inversion of configuration.

Conclusion on the Intermediate

Of the given choices, the formation of a carbocation is most likely to cause racemization, given it provides a mechanism for the conversion between enantiomers through a planar intermediate state. The other options do not facilitate this stereochemical rearrangement as effectively as a carbocation.

Key concepts

Chirality

Chirality is an essential concept in chemistry that describes a molecule's asymmetry. When a molecule is chiral, it has a non-superimposable mirror image, much like our left and right hands. This property is often due to the presence of a chiral center, usually a carbon atom with four different substituents.

In the context of the exercise, the molecule "+2-2-chloro-2-phenylethane" has a chiral center. The chirality ensures that the molecule can exist in two forms, called enantiomers, which are mirror images of each other. These enantiomers can rotate plane-polarized light in different directions, which is a property we'll discuss in Optical Activity.

During racemization, the molecule loses this chirality, resulting in a mix of the two enantiomers. This process happens because of changes at or near the chiral center, often through some intermediate stage where the molecule can adopt a symmetrical form. Understanding chirality helps us predict how a molecule will behave during reactions, especially those involving enantiomers.

Carbocation

A carbocation is a type of intermediate molecule that features a carbon atom bearing a positive charge. This occurs when a group, such as a halogen like chlorine, is removed from a carbon atom, resulting in the carbon atom having fewer electrons than protons.

In the step-by-step solution, the exercise suggests the formation of a carbocation when "+2-2-chloro-2-phenylethane" is placed in the presence of SbCl_6. The halogen atom is removed, leading to the creation of a positively charged carbon.

• This formation is critical as carbocations are planar.
• Planarity means that the molecule can rotate freely around double bonds.
• This rotation equalizes the distribution of the stereochemical configuration.

Consequently, this free rotation allows for both enantiomeric forms to be reached equally, explaining the racemization as the molecule shifts between forms. Understanding carbocations is key to appreciating how certain reactions allow for the conversion back and forth between enantiomers.

Optical Activity

Optical activity refers to a molecule's ability to rotate the plane of polarized light. Molecules that exhibit chirality typically display optical activity, with each enantiomer rotating light in opposite directions.

In the original exercise, this concept plays a big part. The "+2-2-chloro-2-phenylethane" is naturally optically active due to its chiral center. However, as the carbocation intermediate forms during racemization, the mixture becomes a 50:50 mix of the two enantiomers. This mix is optically inactive because the enantiomers' effects on polarized light cancel each other out.

Optical activity can provide valuable insights into the nature of a solution and help determine whether a mixture contains an equal mix of enantiomers or if one form is more prevalent. Hence, it becomes a critical element of understanding the behavior and outcome of racemization processes.