Two Key Reactions of Enolates

by Kiley Lynch

Yesterday I said that enolates are great nucleophiles.

The next question is: OK, so what electrophiles does it react with?

Here’s two important ones.

  1. alkyl halides
  2. aldehydes/ketones.

Think about alkyl halides for a second. What kind of reactions do they do?

If you said the SN2, congratulate yourself. Enolates can react with alkyl halides and do SN2 reactions.

 

Second: what kind of reactions do aldehydes and ketones do?

Addition reactions!

So let’s walk through the possibilities with each of these reactions.

The reaction where we 1) form the enolate, and 2) add an alkyl halide that does an SN2 is called “enolate alkylation”, or simply:  “Alkylation”.

The reaction where we 1) form the enolate, 2) do an addition to the aldehyde/ketone, and 3) protonate also has a name…..

it’s called the Aldol Reaction.

Here it is, in graphical format.

This is why it’s so important that we only form one enolate…. if we have multiple enolates present, then we can get multiple products. Nobody likes getting a mixture. So if you’re planning a reaction that involves an enolate, try to use conditions that will only lead to the formation of a single product.

Thanks for reading! James

P.S. Note that there’s an extra wrinkle in the Aldol… if you heat with base (or acid), you end up doing an extra elimination reaction (break C-OH, break C-H, form C-C (pi). ). This is called the Aldol “condensation”, since we’re removing a molecule of water.  Topic for another day.