Day 85

Just realized today that we're kind of just two weeks out from the final exam. That's crazy.

This afternoon I did an enolate chemistry problem set; a nice change from the aromatic chemistry I was focused on for the past few days.

This problem set wasn't really that great, there were a few questions where I was like "why would you do that - do this instead". I guess we've gotten to the point where there's more than one way to solve a problem, and often times, when you don't get the same answer as the answer sheet, you get discouraged. But in actuality, there's never one answer in chemistry.

Today, I was hardcore reminded that enolate equivalents (LDA, enamines, silyl enol ethers, etc.) are primarily used when you are reacting carbonyls with one another. If you're reacting a carbonyl with an alkyl halide, for example, then you can just use a weak base and get on with your day.

However, what annoys me is that they'll use a weak base and latch on an alkyl group through your typical enolate reaction. But then, when they need a kinetic enolate, they'll go ahead and use LDA. Why not just use any old bulky - but strong - base like potassium tert-butoxide? Why do we have to use an equivalent? All we really need is something bulky, do we not?

I have a gut feeling that it has something to do with the fact that LDA is reallllly strong. It doesn't have time to "think" about which proton it snatches, it just takes the first one. And I'm guessing it's something to do with lithium making LDA very reactive, as compared to the Potassium counterpart in potassium tert-butoxide. Let's take a look at some pKa's... 

• pKa of the conjugate acid of tert-butoxide: 17
• pKa of the conjugate acid of LDA: 36

Therefore, LDA is a much stronger base than t-butoxide - this explains why we use LDA in alkylations -- it's not that we need an equivalent, it's that we need a really, really, strong bulky base that will definitely ensure that the kinetic enolate is formed. Perhaps using t-butoxide won't always give the kinetic enolate...

Who knows.

Enolate Problem Set (Answers)