Animated Solution for Chemistry - Organic Chemistry: $R-\text{CN}\underset{\text{(ii) }{\text{H}}_2\text{O}}{\overset{\text{(i) DIBAL-H}}{\to }}R-Y$ Consider the above reaction and identify $Y$.

Have you ever been on a train that just wouldn't stop where you wanted it to? In organic chemistry, strong reducing agents like Lithium Aluminium Hydride (${\text{LiAlH}}_4$) are exactly like that runaway train. If you feed them a nitrile ($R-\text{C}\equiv \text{N}$), they will relentlessly reduce it all the way down to a primary amine ($R-{\text{CH}}_2{\text{NH}}_2$). But what if you just want to stop halfway and get an aldehyde?

Enter DIBAL-H (Diisobutylaluminium hydride).

DIBAL-H is a specialized, mild reducing agent. Its secret lies in its structure: it has two bulky isobutyl groups attached to the aluminium atom. This steric bulk acts like a set of brakes. When DIBAL-H reacts with a nitrile, it can only deliver one hydride ion ($H^{-}$) before the sheer size of the molecule prevents any further reduction.

When the first hydride attacks the electrophilic carbon of the nitrile, the triple bond breaks into a double bond, forming an intermediate called an imine ($R-\text{CH}=\text{NH}$).

Because of the bulky nature of DIBAL-H and the typically low temperatures used for this reaction, the reduction halts right here. The runaway train has successfully stopped at the intermediate station!

But an imine isn't our final destination. The second step of our reaction involves adding water (${\text{H}}_2\text{O}$). This is where the magic of hydrolysis happens.

Water attacks the imine carbon, and through a series of proton transfers, the nitrogen is kicked out as ammonia (${\text{NH}}_3$), leaving behind a pristine carbon-oxygen double bond.

And just like that, our nitrile has been elegantly transformed into an aldehyde ($R-\text{CHO}$).

Looking back at our original question, the functional group $Y$ produced by the reaction of a nitrile with DIBAL-H followed by hydrolysis is an aldehyde group ($-\text{CHO}$).

Always remember: choosing the right reagent is like choosing the right tool for a job. ${\text{LiAlH}}_4$ is your sledgehammer, but DIBAL-H is your scalpel!