4.6 Reactions of Alkenes

Most alkene reactions fall into the class of electrophilic addition to alkenes. Many different reagents could add to the double bond to form more stable products. In some cases catalyst has to be added to have convenient reaction rates. The double bond in an alkene has loosely held π-bonding electrons which have affinity towards a strong electrophile. First, a strong electrophile attracts the loosely held electrons from the π-bond of an alkene and forms carbocation. The carbocation reacts with a nucleophile to form an addition product (Scheme 1).

Scheme 1

4.6.1 Addition of Hydrogen Halides

The proton in HBr is electrophilic; thus, it reacts with the alkene to form a carbocation. Bromide ion reacts rapidly with the carbocation to give a stable product in which the elements of HBr have added to the ends of the double bond. For example, 2-methyl-2-butene reacts with hydrogen bromide to give 2-bromo-2-methylbutane (Scheme 2).

Scheme 2

Markovnikov's Rule : when a hydrogen halide adds to an unsymmetrical alkene, the addition occurs in such a manner that the halogen attaches itself to the double-bonded carbon atom of the alkene bearing the lesser number of hydrogen atoms. When the proton adds to the secondary carbon, a tertiary carbocation results. When the proton adds to the tertiary carbon atom, a secondary carbocation results. The tertiary carbocation is more stable, so the corresponding product is favored (Figure 1).

Figure 1

4.6.2 Addition of Halogens

Halogens add to alkenes to form vicinal dihalides. The nucleophilic alkene attacks the electrophilic nucleus of one halogen atom, and the other halogen serves as the leaving group, departing as halide ion. For example, the reaction of propene with bromine follows: