Reactions of Alkenes

Alkenes are hydrocarbons containing a carbon-carbon double bond. The hybridisation at carbon is said to be sp², i.e. one s and two p orbitals combine, leaving one p-orbital, perpendicular to the plane of the sp² hybrid. Two adjacent sp² hybrids overlap and form the familiar s-bonds (C-C, C-H etc.). Two adjacent p-orbitals can overlap and form a p-bond. the p-bond is weaker than the s-bond because the degree of overlap is significantly reduced.

The average bond dissociation energy for a C-C s-bond is ~350 kJ mol^-1 and for a C=C bond is ~610 kJ mol^-1. The difference (~ 260 kJ mol^-1) can be attributed approximately to the overlap of p-orbitals which form the p-bond. We can therefore see that the p-bond is somewhat weaker than the s-bond and this gives rise to a range of reactions with a large number of reagents.

The C=C bond is electron-rich and most of the reactions of simple alkenes are with electrophilic reagents. The most common of these are hydrogen halides, HX, which have a permanent dipole with the hydrogen atom carrying the partial positive charge. The first step in these reactions is protonation of the C=C bond which forms a carbocation intermediate. The carbocation, which in certain circumstances can rearrange, is then quenched rapidly by the halide anion. The order of reactivity of the hydrogen halides towards alkenes follows the order of acidity:

The mechanism for electrophilic addition of HX to an alkene can be written as follows:

If the alkene is unsymmetrically substituted, e.g. propene, there are two possible products of addition of a hydrogen halide, e.g. HBr:

There is thus the question of regioselectivity, i.e. which way around does the HBr add? In practice, in an ionic mechanism, the positive end of the dipole (in this case hydrogen) is found to add to the carbon with the greater number of hydrogens. This is known as Markownikoff's Rule. In practice, most examples are totally regioselective, i.e. the minor isomer is not formed at all. Markownikoff's Rule can be rationalised on mechanistic grounds. Addition according to the rule involves the more stable carbocation but addition contrary to the rule involves a less stable carbocation.

There are certain reactions which do not appear to follow Markownikoff's Rule. This is usually because the mechanism does not involve a carbocation intermediate. A good example of this is hydroboration, developed by HC Brown. In a hydroboration reaction, a B-H fragment adds across the double bond to give an organoborane. This intermediate is then oxidised by hydrogen peroxide in alkaline conditions to an alcohol. The overall effect of this sequence is indirect addition of water across the C=C bond. It differs from acid hydrolysis or oxymercuration, however, in that the product is the Anti-Markownikoff alcohol.

This reaction follows a different mechanism to the addition of HX in that the initial addition of diborane (B₂H₆) is not stepwise, but concerted. The boron bonds to the less hindered carbon and the C-B bond is then oxidatively cleaved by peroxide ion, giving the Anti-Markownikoff product alcohol. This is one example of how choice of synthetic method can enable access to the desired product.

Alkenes can also undergo radical additions under certain conditions but the problems in this section occur only via ionic or concerted mechanisms. If you encounter an unfamiliar reagent or alkene, remember that alkenes are electron-rich and will react at the electrophilic part of the reagent.