Mechanism (LAH reduction ester to alcohol)

Below is the mechanism of the reduction of an ester to alcohol using LiAlH₄ (LAH).  I have kept it simple by not going into details.

Step 1: The nucleophilic H from the hydride reagent adds to the electrophilic C in the polar carbonyl group of the ester. Electrons from the C=O move to the electronegative O creating the tetrahedral intermediate a metal alkoxide complex.

Step 2: The tetrahedral intermediate collapses and displaces the alcohol portion of the ester as a leaving group, in the form of the alkoxide, RO^-. This produces an aldehyde as an intermediate.

Step 3: Now we are reducing an aldehyde.  The nucleophilic H from the hydride reagent adds to the electrophilic C in the polar carbonyl group of the aldehyde. Electrons from the C=O move to the electronegative O creating an intermediate metal alkoxide complex.

Step 4: This is the work-up step, a simple acid/base reaction. Protonation of the alkoxide oxygen creates the primary alcohol product from the intermediate complex.