MCQ Detailed View

Aldehydes are organic compounds that contain the carbonyl group (C=O) with at least one hydrogen atom attached to the carbonyl carbon. The carbonyl carbon is sp² hybridised and has a planar structure. In the carbonyl group, oxygen is more electronegative than carbon, so it pulls electron density toward itself. This creates a partial positive charge on the carbon and a partial negative charge on the oxygen. As a result, the carbonyl carbon becomes highly electrophilic, making it a target for nucleophiles.

Because of this polarity, the typical reaction of aldehydes is nucleophilic addition. A nucleophile attacks the electron-deficient carbonyl carbon, breaking the π bond and forming a new bond. Later, a proton is usually added to complete the reaction.

One of the most common examples is the addition of hydrogen cyanide (HCN) to an aldehyde, forming a cyanohydrin. Another example is the addition of alcohols to form hemiacetals and acetals. In reduction reactions, nucleophilic hydride ions (from reagents like NaBH₄ or LiAlH₄) attack the carbonyl carbon to give primary alcohols.

The other options represent incorrect mechanisms for aldehydes. Electrophilic addition is typical of alkenes, where an electrophile attacks the electron-rich double bond. Nucleophilic substitution is common in haloalkanes, where a leaving group is replaced by a nucleophile. Nucleophilic elimination is seen in specific compounds but not in aldehydes, as they lack a good leaving group directly attached to the carbonyl carbon.

Therefore, nucleophilic addition is the general and most important reaction type for aldehydes. This mechanism explains why aldehydes are highly reactive toward nucleophiles and why they play a key role in many organic synthesis pathways. Understanding this helps students predict and explain the behavior of aldehydes in different organic reactions.