Addition Pattern 1 – Carbocations

by Kiley Lynch

OK, now that I’ve hopefully sensitized you to the importance of regiochemistry and stereochemistry, let’s dig into some of the key patterns you’ll see in addition reactions to alkenes.

There are 3 main patterns you’ll see in the addition reactions of alkenes. It’s a slight oversimplification, but not too much of one. If you can get these 3 different patterns straight, you’ll be in good shape.

The first pattern – what I’ll talk about today – is what you could call theCarbocation Pathway.

A perfect example is the reaction of an alkene with a hydrogen halide such as HBr.

In the first step, the alkene acts as a nucleophile, attacking the electrophilic hydrogen of H-Br. This leads to formation of a carbocation.

Important! Think back to the SN1 – what factors were important for carbocation stability? – remember that carbocation stability increases as we go from primary to secondary to tertiary?

The “big barrier” for this reaction is also carbocation stability – so we’re going to preferentially add the hydrogen to the alkene so as to provide the most stable carbocation.

Once we form the carbocation, a nucleophile (in this case, bromide) can come in and attack it. The product will have the bromine attached to the most substituted carbon of the alkene.

In other words, it follows Markovnikoff’s rule!

What about stereochemistry?

Let’s remember the SN1 again. Remember how in the SN1, the carbocation was flat, so the nucleophile could attack from either side? Same thing applies here!

This means that the orientation of the hydrogen and the bromide could be syn or anti – they have no strong preference. In other words, it’s random.

One last thing. We’re forming a carbocation. So be on the lookout for the possibility of rearrangements. If the carbocation is formed adjacent to a carbon that could rearrange to give  a more substituted carbocation, it will happen.

Putting it all together, here’s a summary of what we’ve talked about. The cool thing is that this exact pattern applies to HCl, HBr, and HI. It also applies to the addition of H3O(+) to alkenes, although there’s an extra book-keeping step at the end with this one to give neutral OH.

 

Tomorrow: the second important pattern.

Thanks for reading! James

P.S. Addition of HBr to alkenes – Reaction Guide

P.P.S. Addition with rearrangement