What base so I have to use and when in an aldol condensation?

Question

My prof uses -OEt for some and LDA for others. I know that LDA is stronger, but I'm not sure when I have to use it vs. -OEt. Also, the book uses NaOH, confusing me even more.

Answer 1

To generate an enolate of a simple ketone, such as acetone (CH3-CO-CH3), you generally need a strong base such as LDA or LHMDS (lithium hexamethyldisilazide) is used. For activated enolatesm, such as beta-ketoesters (ROOC-CH2-CO-R, enolization can be accomplished using a weaker base since the enolizable proton (between the two carbonyl carbons) is much more acidic and, therefore, much more easily removed. Sodium ethoxide or even sodium carbonate will work in such cases. Therefore, my best answer to your question is estimate the pKa of the enolizable proton and used a base that is strong enough to abstact it...

Answer 2

Aldol condensation, actually aldol addition, is the base-catalyzed reaction of an aldehyde with itself to form the aldol. The original is reaction of acetaldehyde with OH-:

2CH3CHO ==(OH-)==> CH3-CH(OH)-CH2-CHO

You have to follow your professor's notes and the book for any other reactions very exactly. For example, the reaction of acetone to form diacetone alcohol specifically calls for Ba(OH)2.