sn1/sn2 – Putting It Together

by James

This last post on the SN1/SN2/E1/E2 aims to help you put it all together.

There are a few major components to successfully answering these types of questions.

Part 1 is recognizing situations where you are supposed to apply SN1/SN2/E1/E2 in the first place. You want to look for alkyl halides, alcohols, or any other compound with a good leaving group. If you don’t see that, you’re not looking at a question in this category.

Part 2 is identifying whether the reaction is SN1/SN2/E1/E2 according to the “4 questions” approach.

Once you figure out what type of reaction you’re dealing with, Part 3 is applying the bond forming/breaking patterns of the SN1/SN2/E1/E2  in turn.

Here are the 4 patterns:

  • In the SN1, you’re breaking C-(leaving group) and forming C-(nucleophile). If you start off with a stereocenter, you’re going to get a mix of retention and inversion. Since you’re going through a carbocation, rearrangements are possible.
  • In the SN2, you’re breaking C-(leaving group) and forming C-(nucleophile). If you start off with a stereocenter, you’re going to get inversion exclusively. 
  • In the E1, you’re breaking C-(leaving group) as well as the C-H bond on the most substituted adjacent carbon (Zaitsev!) , and forming a C-C Pi bond. There’s no requirement for the C-H and the C-(leaving group) to be anti to each other. Since you’re going through a carbocation, rearrangements are possible. 
  • In the E2, you’re breaking C-(leaving group) as well as the C-H bond on the most substituted adjacent carbon (Zaitsev! unless you’re using a bulky base)  and forming a C-C Pi bond. The C-(leaving group) bond and the C-H bond that you break MUST be anti to each other. 

Try applying everything in these examples below. I know they’re simple: the point is to start at a basic level. Answers below.


Thanks for reading! James

Also: check out The Quick N’ Dirty Guide To SN1/SN2/E1/E2
P.S. For many more examples check out Ian Gould’s site.