[Nemesisof99]

IN base catalyzed aldol condensation of 2-methyl propanal, in the first step the base abstracts an alpha hydrogen from it, thereby forming a carbanion. Doesn't this carbanion (being tert.) undergo rearrangement to form a primary carbanion?

[renge ishyo]

It doesn't really form a carbanion for long because the lone pair of electrons is delocalized towards the carbonyl carbon. This forms a double bond that forces a lone pair up onto the oxygen (forming an enolate intermediate). See the first step in the image below obtained from the wiki  for an example (http://commons.wikimedia.org/wiki/File:Enolate_aldol_mechanism_Commons.png). This image doesn't even show the carbanion forming; it just skips straight to the enolate before the condesation step in step 2 (some mechanisms on the other hand show just the carbanion and skip the enolate forming step entirely proving that there is more than one way for a chemist to be lazy when drawing mechanisms):

[Dan]

To put it another way, despite the fact that the carbanion is tertiary it is stabilised by resonance (enolate form in renge ishyo's post). The possible primary carbanions would not be resononace stabilised and are of higher energy as a result. It's another case of resonance effects outweighing inductive effects.

[Nemesisof99]

Thanks! That made sense