Problem 36 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction, \(\mathrm{CH}_{3} \mathrm{CHO}+\mathrm{HCN} \longrightarrow \mathrm{X}\), a chiral centre is introduced. The product \(X\) is (a) laevo rotatory (b) dextro rotatory (c) meso compound (d) racemic mixture

Step-by-Step Solution

Answer

The product \(X\) is a racemic mixture (option d).

Step 1: Understanding the reaction

The reaction is between acetaldehyde (\(\mathrm{CH}_3\mathrm{CHO}\)) and hydrogen cyanide (\(\mathrm{HCN}\)). This reaction is a nucleophilic addition, where the \(\text{CN}^-\) ion from \(\mathrm{HCN}\) attacks the carbonyl carbon of acetaldehyde, forming a cyanohydrin.

Step 2: Determining the chirality

When \(\mathrm{HCN}\) adds to acetaldehyde, a new carbon-carbon bond is formed. The carbonyl carbon in acetaldehyde becomes a new chiral center in the product since it will be attached to four different groups: a hydrogen, a hydroxyl group, a methyl group, and a cyanohydrin group.

Step 3: Identifying product configuration

Since the product, which is a cyanohydrin, has a chiral center, it can exist in two enantiomeric forms (mirror images). These are not the same compound and have opposite optical activities.

Step 4: Deciding between optical activities

In typical reactions forming chiral centers from symmetrical starting materials, such as this one (acetaldehyde is symmetric), the product is usually a racemic mixture. This is because both enantiomers of the chiral product are formed in equal probability, resulting in an overall optical rotation of zero.

Key Concepts

Nucleophilic Addition Reaction

In the reaction between acetaldehyde (\[ \mathrm{CH}_3\mathrm{CHO} \]) and hydrogen cyanide (\[ \mathrm{HCN} \]), we encounter a fascinating chemical process known as a nucleophilic addition reaction. This type of reaction is common when dealing with carbonyl compounds like aldehydes and ketones. The defining feature here is the 'nucleophile', which is generally a negatively charged or neutral species with a pair of spare electrons.

• In our example, the nucleophile is the \[ \text{CN}^- \] ion from \[ \mathrm{HCN} \], seeking to attach itself to a positively charged or electron-deficient region.
• The target is the carbonyl carbon in acetaldehyde, which is partially positive due to the electronegativity of the oxygen atom.
• The \[ \text{CN}^- \] ion attacks this carbon, forming a new compound known as a cyanohydrin.

This reaction introduces a new feature—a chiral center—into the molecule, significantly affecting the chemical properties of the resulting product. It's essential to understand that in nucleophilic addition reactions, the nucleophile transforms the structural characteristics of the initial compound, sometimes endowing it with new traits such as chirality.

Racemic Mixture

The product of the reaction between \[ \mathrm{CH}_3\mathrm{CHO} \] and \[ \mathrm{HCN} \] turns out to be a racemic mixture. This term might sound complex at first, but it's actually quite logical.

• A racemic mixture is a combination of two enantiomers that are mirror images of each other and is produced in equal parts.
• When equal amounts of these mirror-image enantiomers are combined, their optical activities cancel each other out, resulting in no net rotation of plane-polarized light.
• This situation occurs frequently during reactions involving the formation of chiral centers from unsymmetrical substrates like acetaldehyde.

The resulting racemic mixture doesn't exhibit any optical activity. This balance indicates that both enantiomers were produced with the same probability, creating a mixture that fails to interact with polarized light in a chiral environment. Understanding racemic mixtures helps in recognizing why certain chemical reactions yield optically inactive products even when chiral centers are introduced.

Enantiomers

Enantiomers are an intriguing concept brought to light in the product of the nucleophilic addition reaction we've been discussing. These are pairs of compounds that are mirror images of each other, much like your left and right hands.

• Though they resemble each other closely, enantiomers cannot be superimposed on one another, meaning you can't place one on top of the other and have all parts align.
• These molecules, being chiral, have the remarkable property of rotating plane-polarized light, albeit in opposite directions.
• One enantiomer will rotate light in a clockwise direction (dextrorotatory), while the other will cause a counterclockwise rotation (levorotatory).

When chiral centers are formed from a symmetric molecule like acetaldehyde, both enantiomers are usually formed in equal amounts. This is critical in understanding why a racemic mixture, like the one formed in the reaction, exhibits no net optical activity. Enantiomers are essential in chemistry, especially in the field of pharmaceuticals, where the activity of drugs can differ vastly between enantiomers.