Animated Solution for Chemistry - Organic Chemistry: The major product of following reaction is $R-C\equiv N\underset{\text{(ii) }{H}_{2}O}{\overset{\text{(i) }AlH(i-Bu{)}_2}{\to }}?$

This is a classic transformation in organic synthesis. The reaction we are looking at involves a nitrile ($R-C\equiv N$) reacting with a very special reagent: DIBAL-H, followed by hydrolysis with water ($H_{2}O$).

DIBAL-H stands for Diisobutylaluminium hydride. It is a bulky, electrophilic reducing agent. Unlike standard reducing agents like Lithium aluminum hydride ($LiAl{H}_4$) which are nucleophilic, DIBAL-H has an aluminum atom with an empty p-orbital. This makes the aluminum atom electron-deficient, meaning it acts as a Lewis acid or an electrophile.

A Dance of Electrons Let's dive into the mechanism. The nitrogen atom in the nitrile group possesses a lone pair of electrons. Because the aluminum in DIBAL-H is electron-deficient, the nitrogen's lone pair eagerly attacks the aluminum atom.

This initial attack forms a bond between nitrogen and aluminum, but it leaves the aluminum atom with a formal negative charge. To stabilize itself, the aluminum atom immediately transfers a hydride ion ($H^{-}$) to the adjacent electrophilic carbon atom of the nitrile group.

This elegant transfer of the hydride ion reduces the carbon-nitrogen triple bond to a double bond. The resulting intermediate is an aluminum imine complex ($R-CH=N-Al(i-Bu{)}_2$). Because DIBAL-H is incredibly bulky, the reaction typically stops at this imine stage, preventing further reduction to an amine.

The Hydrolysis Finale The final step of our reaction is hydrolysis. We introduce water ($H_{2}O$) into the system. The water molecules attack the imine complex, cleaving the carbon-nitrogen double bond.

Through this hydrolysis process, the nitrogen is completely removed (eventually forming ammonia or aluminum salts), and a carbon-oxygen double bond is established in its place.

This reaction is a beautiful demonstration of how choosing the right reagent—a bulky, electrophilic hydride donor like DIBAL-H—allows us to selectively pause a reduction halfway and trap the intermediate to get our desired aldehyde!