Alkynes: Deprotonation and SN2

by James

On to the last topic these emails will cover for Org 1 – Alkynes. Thankfully, there will be a lot of similarity between the reactions of alkynes and those of alkenes. More on that soon.

But first, let’s talk about the most important reaction of alkynes you’ll learn. It’s also a reaction that’s unique to alkynes.

OK, here it is: Alkynes are acidic.

Not strongly acidic, mind you. But acidic enough. If you treat a “terminal alkyne” (an alkyne that ends in a C-H bond) with a strong base such as NaNH2, it will be deprotonated to form its conjugate base – called an “acetylide”.

OK, so what? What’s the big deal?

Here’s the big deal. Acetylides are great nucleophiles. They are particularly useful for SN2 reactions. This means we can treat acetylides with alkyl halides and form carbon-carbon bonds. 

OK, again: why is this a big deal?

It’s a big deal because there are very few reactions you learn in Org 1 that form carbon-carbon bonds. This is the most prominent example. You will see this in synthesis problems a LOT. Anytime you’re going from a molecule with a short carbon chain to a molecule with a longer one, chances are good that it will involve this reaction.

If you only learn one reaction of alkynes, learn this one!

Thanks for reading! James

P.S. Why are alkynes more acidic than alkanes? Alkynes are acidic because the C-H bond in this case is sp hybridized. There’s 50% s character (compared to 25% for sp3 hybridized alkanes) And because the s orbital is closer to the nucleus than the p orbital, negative charge will be more stable.