Q.Read the following passage carefully and answer the questions given below –
Carboxylic acids are reduced to corresponding primary alcohols by Lithium aluminium hydride (LiAlH₄) or with diborane (B₂H₆). Diborane does not easily reduce functional groups such as ester, nitro, halo etc. Carboxylic acids having an α-hydrogen, on treatment with chlorine or bromine in the presence of small amount of red phosphorus give α-halocarboxylic acids. Sodium salts of carboxylic acids upon heating with soda lime (NaOH and CaO in the ratio of 3:1) lose carbon dioxide and hydrocarbons are formed. Aromatic carboxylic acids undergo electrophilic substitution reactions in which the carboxyl group acts as a deactivating and meta-directing group. They however, do not undergo Friedel-Crafts reaction because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group.

DIBAL-H Reduction: Stopping the Reduction at Aldehyde

You already know that LiAlH₄ reduces an ester all the way to a primary alcohol. But what if you want to stop at the aldehyde — the intermediate step? That is exactly what DIBAL-H does. The trick is not a stronger reagent, but a weaker, bulkier one that runs out of steam halfway.

The Intuition: A Bulky Hammer That Hits Once

Think of an ester as a two‑storey building. LiAlH₄ is a wrecking ball that smashes both floors at once — you get the alcohol. DIBAL-H is a much larger, clumsier hammer. It can swing hard enough to break the first floor (the ester → aldehyde step), but it is so bulky that it cannot easily reach the second floor (aldehyde → alcohol). So the reaction stops at the aldehyde.

The same logic applies to nitriles. A nitrile has a triple bond that can be reduced in two stages: first to an imine (C=N), then to an amine. DIBAL-H stops at the imine, which hydrolyses to an aldehyde during work‑up.

The Precise Chemistry

DIBAL-H stands for diisobutylaluminium hydride, formula $(i\text{-Bu})_2\text{AlH}$. It is a Lewis acid (the aluminium is electron‑deficient) and a hydride donor. The key is that it is used at low temperature (typically –78 °C, dry ice/acetone bath) and with one equivalent of reagent.

For an ester (say ethyl benzoate):

1. DIBAL-H coordinates to the carbonyl oxygen (Lewis acid–base interaction).
2. A hydride is transferred to the carbonyl carbon, forming a hemiacetal intermediate — a tetrahedral species with an –OAlR₂ group and an –OR group.
3. At low temperature, this hemiacetal is stable. It does not collapse to the aldehyde until you add water during work‑up.
4. Upon aqueous work‑up (often with a mild acid like Rochelle salt or dilute HCl), the hemiacetal hydrolyses to the aldehyde. The second hydride never gets a chance to attack because the aluminium is already tied up.

For a nitrile (say benzonitrile):

1. DIBAL-H adds a hydride to the electrophilic nitrile carbon, forming an imine intermediate (R–CH=N–AlR₂).
2. This imine is stable at low temperature.
3. Aqueous work‑up hydrolyses the imine to the aldehyde (R–CHO + NH₃).

Why Not Just Use LiAlH₄ and Stop?

LiAlH₄ is too reactive. It reduces the aldehyde faster than it reduces the ester — so you never see the aldehyde. DIBAL-H is milder and bulkier. The aluminium in DIBAL-H is only bonded to two bulky isobutyl groups (plus one H), so it is less nucleophilic than the AlH₄⁻ ion. It also coordinates strongly to oxygen, stabilising the hemiacetal intermediate.

A Common Exam Point …