Attacking aldehydes and ketones with carbanions to make alcohols

Attacking formaldehyde is particularly useful, since this gives a primary alcohol - with one extra carbon atom:

You should be able to visualize that attacking aldehydes give secondary alcohols, and attacking ketones gives tertiary ones.

Making secondary alcohols gives a choice of two possible routes, for example:

It depends on which fragment you put the C=O and which one you put the metallic group.

The reaction used usually depends on what has the highest yield andor the cheapest starting materials.

Making a tertiary alcohol from a ketone gives three routes, example: