Alkenes are hydrocarbons containing at least one carbon-carbon double bond. The carbons are sp2 hybridised, i.e. for each carbon, 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 a C-C σ -bond. Two adjacent p-orbitals can overlap and form a C-C π -bond. So there are two C-C bonds i.e. a double bond.
The π -bond is weaker than the σ -bond because the degree of overlap is significantly reduced. 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 is then quenched rapidly by the halide anion. The order of reactivity of the hydrogen halides towards alkenes follows the order of acidity:
HF < HCl < HBr < HI
The mechanism for electrophilic addition of HX to an alkene is as follows:
If the alkene is unsymmetrically substituted, e.g. propene, there are two possible products of addition of a hydrogen halide, as shown below:
Which product is favoured? In practice, 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 Markovnikov’s Rule. In practice, most examples are totally regioselective, i.e. the minor isomer is not formed at all. Markovnikov’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 Markovnikov’s Rule. This is usually because the mechanism does not involve a carbocation intermediate.
Alkynes are hydrocarbons containing at least one carbon-carbon triple bond. The carbons are sp hybridised i.e. for each carbon, one s and one p orbital combine, leaving two p-orbitals perpendicular to the plane of the sp hybrid. Two adjacent sp hybrids overlap and form a C-C σ -bond. Two adjacent p-orbitals can overlap and form a π -bond. Two other adjacent p-orbitals can overlap to form another π -bond. So there are three C-C bonds i.e. a triple bond!
Alkynes undergo electrophilic addition reactions with the same mechanism as for alkenes, but because they have a triple bond, 2 molecules of HX can add to each molecule of alkyne.
To practice drawing mechanisms for reactions of alkenes and alkynes, go to curlyarrows.org and select ‘Alkenes and Alkynes’.
If you want to read more about electrophilic addition to alkenes, a good start would be chapter 19 of Organic Chemistry (second edition) – Clayden, Greeves and Warren.
Acknowledgement: Notes provided by Sarah Piggott, KLabs Student Author