In this post we’ll do a final review of alkene addition reactions and sum up the three major pathways (and two minor classes of reactions worth paying attention to).
Reaction Pathway #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.
Reaction Pathway #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:
Reaction Pathway #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.
Two Miscellaneous Minor Pathways
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.
This sums up the series on alkenes for now. In the next series, we’ll go through the reactions of alkynes.
