Bulky Bases

by Kiley Lynch

Here’s an annoying exception for elimination reactions you have to watch out for.

With the E2 reaction, *most* of the time we obtain the “Zaitsev” product. That is, we form the most substituted alkene. This is because, all things being equal, alkenes become more stable as the number of attached carbons increases.

However, there’s one big exception.

If we use a very sterically hindered base, the normal (Zaitsev) product becomes disfavored, since the bulky base has to reach into a tight spot to rip off the proton.

The end result: the base takes the proton from the least substituted carbon, resulting in the least substituted alkene. Sometimes this is called the “Hoffman” or “anti-Zaitsev” product.

Thankfully, for our purposes there’s only one bulky base to keep track of. If you see the base  [written  t-butoxide,  (CH3)3CO(-), or t-BuO(-)], watch out for this situation. Make the less substituted alkene instead.

 


Thanks for reading! James

P.S. The 8 Most annoying exceptions in Org 1