Question

In the reaction, \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{HCN} \longrightarrow \mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CN}\) a chiral centre is produced. This product would be (a) racemic mixture (b) meso compound (c) dextrorotatory (d) laevorotatory

Short answer

The product will be a racemic mixture (option a).

Step-by-step solution

Identify the Reaction Type

The reaction shown is an addition of hydrogen cyanide (HCN) to ethanal (acetaldehyde, \( \mathrm{CH}_3\mathrm{CHO} \)). This is a typical example of a nucleophilic addition reaction.

Analyze the Formation of the Product

In the given reaction, the \( \mathrm{CN}^- \) attacks the carbonyl carbon of acetaldehyde. The oxygen acquires a hydrogen, forming a hydroxyl group \( \mathrm{OH} \). The resulting product is \( \mathrm{CH}_3\mathrm{CH(OH)CN} \) which has a new chiral center at the carbon atom bonded to \( \mathrm{OH} \), \( \mathrm{CN} \), \( \mathrm{H} \), and \( \mathrm{CH}_3 \).

Determine the Stereochemistry

Since a chiral center is formed and the reagents used are symmetric and non-chiral, the product formed can exist in two enantiomeric forms. These two forms are non-superimposable mirror images and will be present in equal amounts due to the lack of any inherent directionality or chiral influence in the starting materials.

Conclude the Nature of the Product

In the absence of any chiral influence when forming a new chiral center, a racemic mixture is formed. A racemic mixture consists of equal quantities of both enantiomers, so there's no overall optical activity in the product mixture.

Key concepts

Chiral Center

A chiral center, also known as a stereocenter, is an atom in a molecule that is bonded to four different substituents. This unique arrangement creates a point at which the molecule becomes asymmetrical. In simpler terms, you can think of a chiral center as a site where the molecule can have two non-superimposable mirror image forms, like your left and right hands.
In the nucleophilic addition reaction of hydrogen cyanide (HCN) to ethanal, a chiral center is created at the carbon atom that connects to the hydroxyl group (OH), cyano group (CN), a methyl group (CH₃), and a hydrogen atom. This formation happens because each of the attached groups is different, giving rise to two possible spatial arrangements, or stereoisomers.
Chiral centers are significant because they often influence the physical and chemical properties of compounds, including biological activity. This is why understanding the concept of a chiral center is essential when studying organic chemistry and its applications in pharmaceuticals and other fields.

Racemic Mixture

A racemic mixture is a 50:50 blend of two enantiomers, which are molecules that are mirror images of each other. These enantiomers are often labeled as 'R' and 'S' configurations based on their spatial arrangement.
The key aspect of a racemic mixture is that it contains equal parts of both enantiomers, leading to no net optical activity. This means that a beam of plane-polarized light will pass through the mixture without being rotated, hence making it appear optically inactive. The reason for this behavior is that each enantiomer rotates light in equal and opposite directions, thereby canceling out the optical activity.
In the context of the nucleophilic addition reaction mentioned in the exercise, because the starting materials (ethanal and HCN) do not have any inherent chirality, the formation of a chiral center results in the creation of both enantiomers in equal proportions, leading to a racemic mixture.

Stereochemistry

Stereochemistry is a branch of chemistry that focuses on the three-dimensional arrangement of atoms within molecules. It is crucial for understanding how molecules behave and interact with one another, particularly in biological systems.
One of the central themes of stereochemistry is chirality, which is all about the spatial arrangement of atoms around a chiral center. This spatial arrangement causes molecules to exist as different stereoisomers, each having distinct properties and biological activities.
When looking at nucleophilic addition reactions, such as the one in this exercise, stereochemistry plays a pivotal role in determining the configuration of the resulting product. Factors like the presence of chiral centers and the formation of racemic mixtures help predict the outcome of these reactions, influencing everything from the physical properties to the reactivity and interaction with other biological molecules.
In summary, understanding stereochemistry is integral for predicting how molecules will form, behave, and react, making it a fundamental concept in both academic and practical applications in the world of chemistry.