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

Short answer

The most reactive compound is (a) p-nitrobenzaldehyde.

Step-by-step solution

Understanding Cyanohydrin Formation

Cyanohydrin formation involves the reaction of a compound with hydrogen cyanide (HCN) which is generally generated in situ by reacting potassium cyanide (KCN) with an acid. The reaction typically involves an aldehyde or ketone that reacts with HCN to form a cyanohydrin. The electrophilicity of the carbonyl carbon determines the reactivity of the compound towards cyanohydrin formation.

Assessing Electrophilicity of the Compounds

To determine which compound would be most reactive in forming a cyanohydrin, we must assess the electrophilicity of each compound's carbonyl carbon. Electrophilicity is increased by electron-withdrawing groups that make the carbonyl carbon more positive and hence more susceptible to nucleophilic attack by cyanide ion.

Analyzing Each Compound

- (a) p-nitrobenzaldehyde: It contains a nitro group which is a strong electron-withdrawing group, making the carbonyl carbon highly electrophilic. - (b) p-hydroxybenzaldehyde: The hydroxyl group can donate electrons via resonance, potentially decreasing the electrophilicity of the carbonyl carbon. - (c) benzaldehyde: This molecule lacks additional electron-withdrawing or donating groups affecting reactivity. - (d) phenylacetaldehyde: The additional methylene group may slightly reduce the electrophilicity compared to benzaldehyde.

Drawing Conclusion on Reactivity

Considering the presence of functional groups, the p-nitrobenzaldehyde with its strong electron-withdrawing nitro group increases the electron deficiency at the carbonyl carbon the most, thereby increasing its reactivity towards nucleophilic attack by the cyanide ion in the cyanohydrin formation reaction.

Key concepts

Electrophilicity

Electrophilicity is a crucial concept in understanding chemical reactivity, particularly in reactions like cyanohydrin formation. In simple terms, electrophiles are species that seek electrons. An electrophile has an electron-deficient atom or group, often positively charged or polarized, making it an attractive target for nucleophiles (electron-rich species).
In the context of cyanohydrin formation, the electrophilic site is the carbonyl carbon of an aldehyde or ketone. The reactivity depends on how electrophilic this carbon is, which can be altered by nearby substituents:

• Electron-withdrawing groups increase electrophilicity by making the carbon more positive.
• Electron-donating groups decrease electrophilicity by providing extra electron density.

In our example, the presence of a nitro group in p-nitrobenzaldehyde significantly enhances the electrophilicity of its carbonyl carbon, making it the most reactive compound in forming cyanohydrin.

Nucleophilic Attack

A nucleophilic attack is when a nucleophile, a molecule with available electron pairs, interacts with an electrophile. In the formation of cyanohydrins, this process involves the nucleophilic cyanide ion ( abla C^- ) attacking the electrophilic carbonyl carbon of aldehydes or ketones.
This mechanism involves several steps:

• The electron-rich cyanide ion seeks out the electron-poor carbonyl carbon.
• Once it approaches, the electrons from the carbonyl group shift, creating a new bond between the cyanide ion and the carbonyl carbon.
• This results in a tetrahedral intermediate that eventually stabilizes, forming a cyanohydrin.

The rate and success of this attack highly depend on the carbonyl carbon's electrophilicity, emphasized in electrophilicity discussions.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are atoms or groups of atoms that reduce electron density around other atoms. They play a pivotal role in enhancing the reactivity of electrophilic centers, such as in carbonyl groups during cyanohydrin formation.
Let's consider how EWGs influence reactions:

• They draw electron density away from the carbonyl carbon, increasing its positive character.
• This makes the carbonyl more susceptible to attack by nucleophiles, like in cyanohydrin formation.
• Examples of strong electron-withdrawing groups include nitro groups ( abla NO_2 ), carbonyls ( abla CO ), and halogens.

In the scenario given, p-nitrobenzaldehyde's nitro group acts as a potent EWG. This significantly boosts the aldehyde's electrophilicity, making it the most reactive towards cyanohydrin formation compared to the other options.