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).
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.
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:
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.