Problem 108 - A Complete Resource Book in Chemistry for JEE Main

Question

The most reactive compound towards formation of cyanohydrin on treatment with KCN followed by acidification is (a) p-nitrobenzaldehyde (b) p-hydroxybenzaldehyde (c) benzaldehyde (d) phenylacetaldehyde

Step-by-Step Solution

Answer

(a) p-nitrobenzaldehyde is the most reactive compound towards cyanohydrin formation.

Step 1: Understand Cyanohydrin Formation

Cyanohydrin formation occurs when an aldehyde or ketone reacts with hydrogen cyanide (HCN). The formation is facilitated by the presence of a negative charge which attacks the carbonyl carbon. More reactive carbonyl groups will form cyanohydrins more readily. This reactivity is influenced by the substituents attached to the carbonyl group.

Step 2: Evaluate Electron-Withdrawing and Electron-Donating Groups

Identify whether the substituents are electron-withdrawing or electron-donating. Electron-withdrawing groups increase the positive character of the carbonyl carbon, making it more reactive. Conversely, electron-donating groups decrease the positive character, making the carbonyl less reactive.

Step 3: Analyze Each Compound

- **(a) p-nitrobenzaldehyde**: The nitro group is strongly electron-withdrawing, increasing the positive charge on the carbonyl carbon. - **(b) p-hydroxybenzaldehyde**: The hydroxy group is electron-donating, reducing the positive character of the carbonyl carbon. - **(c) benzaldehyde**: No additional substituents to influence reactivity. - **(d) phenylacetaldehyde**: The presence of additional alkyl groups provides slight electron-donating effects.

Step 4: Determine the Most Reactive Compound

From the analysis, (a) p-nitrobenzaldehyde has an electron-withdrawing group which makes its carbonyl carbon highly electrophilic and reactive towards nucleophilic attack, leading to easier formation of cyanohydrin compared to the other options.

Key Concepts

Aldehydes and Ketones Reactivity

Aldehydes and ketones are crucial in organic chemistry reactions due to their carbonyl group \((\text{C}=\text{O})\). The carbon atom in the carbonyl group tends to be partially positive because the oxygen atom is more electronegative and attracts electrons toward itself. This polarity makes aldehydes and ketones reactive to nucleophiles, substances that are electron-rich and can donate an electron pair.In general, aldehydes are more reactive than ketones. This is because the carbonyl carbon in aldehydes is more exposed since it is attached to only one alkyl group, while in ketones, it is attached to two. This steric hindrance in ketones makes it harder for nucleophiles to attack. Therefore, the relative reactivity of these compounds differs based on their structural environment and any additional substituents attached to them.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are key players in determining the reactivity of molecules like aldehydes and ketones. They pull electrons away from nearby atoms or groups, effectively increasing the positive charge on the carbonyl carbon. This increase in positive charge makes the carbonyl carbon more susceptible to attack by nucleophiles.Common electron-withdrawing groups include:

• Nitro groups \((\text{NO}_2)\)
• Carbonyl groups \((\text{C}=\text{O})\)
• Cyanide \((\text{CN})\)

When electron-withdrawing groups are present, they make the carbonyl compound more reactive. For instance, in the case of p-nitrobenzaldehyde, the nitro group increases the electrophilicity of the carbonyl carbon, making it a prime candidate for reactions like cyanoaddition.

Nucleophilic Attack

Nucleophilic attack is a fundamental concept in organic chemistry where a nucleophile donates a pair of electrons to an electron-deficient center, such as a carbonyl carbon. In the case of cyanohydrin formation, the nucleophile is the cyanide ion \((\text{CN}^-)\).Here is how it works:

• Firstly, the cyanide ion approaches the partially positive carbonyl carbon.
• The carbonyl carbon, being an electrophile, accepts the electron pair from the cyanide ion, leading to the formation of a new carbon-carbon bond.
• This addition eliminates the double bond of the carbonyl group and creates a tetrahedral intermediate.

This nucleophilic attack is a critical step in many carbonyl compound reactions, enabling the formation of various products like cyanohydrins.

Cyano Group Formation

The formation of a cyano group is a central aspect when forming cyanohydrins. When aldehydes or ketones are treated with potassium cyanide \((\text{KCN})\) followed by an acid, the cyano group \((\text{CN})\) is introduced into the molecule.Here's what happens:

• The cyanide ion \((\text{CN}^-)\) acts as a nucleophile, attacking the electrophilic carbonyl carbon.
• Subsequently, the intermediate formed is treated with acid, which protonates the alkoxide ion formed from the nucleophilic addition, resulting in a stable cyanohydrin structure.

This selective reaction is widely utilized in organic synthesis to extend carbon chains and introduce functionality. Cyano group formation is thus a valuable tool for chemists looking to modify or build complex organic compounds.