Alkene Reactions

By James Ashenhurst

Summary: Three Key Families Of Alkene Reaction Mechanisms

Last updated: January 21st, 2020 |

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

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. Wow I just read through a couple of your articles and they are damn good. If you haven’t already write a book on organic chemistry sell it in India cus a lot of people( by that I mean millions) don’t have regular internet facilities. But people a lot of books especially educational books among others( our culture promotes reading). By writing a book not only can you help so many students as your stuff is a million times better than any indian writer ( trust me thee are a lot of,indian writers) its so simple and to the point. I understand your attempts of making it free which is a noble qestures but for a low price a book by you could help so many( and indian parents will buy a book regardless of the cost its just how the society works no compramises towards an education). Oh.. If myrant is pointless cus you already have one please send a link on the mentioned email address.

    Keep on the amazing work your helping scores of students I literally have printouts of your articles now, compileing a book myself :P.


    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.

  2. 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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