Alkene Reactions

By James Ashenhurst

Summary: Three Key Families Of Alkene Reaction Mechanisms

Last updated: November 28th, 2022 |

The Three Key Families Of Alkene Addition Reaction Mechanisms (Plus Two Minor Ones)

In this post we’ll do a final review of alkene addition reactions and sum up the three major families of mechanisms (and two minor classes of reactions worth paying attention to).

Table of Contents

  1. Reaction Family #1: The Carbocation Pathway
  2. Reaction Family #2: The “3-Membered Ring” Pathway
  3. Reaction Family #3: The “Concerted” Pathway
  4. Two Miscellaneous Minor Pathways: Oxidative Cleavage of Alkenes And Free-Radical Addition To Alkenes
  5. Notes

1. Alkene Reaction Family #1 – The Carbocation Pathway

In the Carbocation Pathway, the alkene acts as a nucleophile and attacks an electrophile, resulting in the formation of a carbocation. The regioselectivity is Markovnikov and the stereochemistry of the reaction is a mixture of syn and anti products. Since carbocations are formed, be alert for rearrangements ! This is the only family where this can happen.

carbocation pathway in alkene addition example hbr to alkenes markovnikov selective syn plus anti stereochemistry rearrangements can happen

2. Alkene Reaction Family #2 – The 3-Membered Ring Pathway

In the so called  “3-membered ring pathway” the alkene attacks an electrophile and forms a 3-membered ring intermediate. This intermediate is then attacked at the most substituted carbon by a nucleophile via a backside attack, giving rise to anti stereochemistry:

3 membered ring pathway in alkene addition eg br2 br2 h2o anti stereochemistry and markovnikov selective

3. Alkene Reaction Family #3 – The Concerted Pathway

The “concerted” pathway is not meant to describe a single reaction mechanism, but it does describe similar consequences. In this class of reaction the regioselectivity is generally not relevant (except for hydroboration with BH3, which is anti-Markovnikov). The stereochemistry of the reaction is syn, meaning the two new bonds form on the same face of the alkene.

concerted pathway in alkene additions syn stereochemistry anti markov for bh3 epoxidation dihydroxylation etc

4. Two Miscellaneous Minor Alkene Reaction Families: Oxidative Cleavage of Alkenes And Free-Radical Addition To Alkenes

In addition there is a fourth pathway which goes through a free radical addition of HBr in the presence of peroxides. The regiochemistry is anti-Markovnikov and the stereochemistry is a mixture of syn and anti.

Finally in the presence of strong oxidants such as KMnO4 or O3 alkenes undergo a reaction called oxidative cleavage which results in the complete breakage of C=C to form carbonyl compounds.

two additional pathways in alkene addition reactions oxidative cleavage and free radical addition

This sums up the series on alkenes for now. In the next series, we’ll go through the reactions of alkynes.

Reaction Map – Reactions Of Alkenes



Comment section

6 thoughts on “Summary: Three Key Families Of Alkene Reaction Mechanisms

  1. Isn’t it important to know whether KMnO4 is cold or hot? Doesn’t cold KmNO4 allow the -OH groups to bond onto the location of the double bond, while hot KMnO4 does the oxidative cleavage you’re referring to? Thanks!

    1. Correct: KMnO4 oxidative strength depends on temperature. Cis diol at low temperature, and Carboxylic acids at high temperature (acidic workup)

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    1. Thanks Sanjay. Writing a book is a pretty time consuming endeavor and I already have my hands really, really full. I can imagine writing a smaller book on a very niche topic though.

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