Alkenes: bonding and reactivity

In a nutshell

Alkenes are unsaturated hydrocarbons. They consist of both a sigma bond and a pi pond. The presence of the pi-bond makes them quite reactive, serving as a good starting material to produce other organic molecules.

Alkenes

Alkenes are classed as unsaturated hydrocarbons as they contain carbon-to-carbon double bonds ($$C=C$$​). They have the general formula $$C_nH_{2n}.$$​ 

​Here are some simple alkenes:

Bonding

The overlap of atomic orbitals and the sharing of electrons between two different atoms leads to the formation of a covalent bond.

Sigma bonds $$(\sigma-bond)$$ are formed by the overlap of s orbitals between two atoms. There is a high electron density in this region giving rise to a strong electrostatic attraction between the nuclei and the shared pair of electrons. This results in a high bond enthalpy. 

A double bond consists of a sigma bond and a pi bond $$(\pi-bond)$$. Pi bonds are formed by the sideways overlap of adjacent p-orbitals, above and below the plane of the atoms. The electrons are less localised than those in the sigma bond. There is a weaker force of attraction between the $$\pi$$ electrons and nuclei, therefore a lower bond enthalpy compared to sigma bonds. 

Reactivity of alkenes

The low bond enthalpy and high electron density in the pi bond of alkenes makes the $$C=C$$ bonds reactive and thus prone to an attack by electrophiles. The reactivity of alkenes make them a good starting material to produce other organic molecules.