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:

NAME	MOLECULAR FORMULA
Ethene	$C_2H_4$
Propene	$C_3H_6$
Butene	$C_4H_8$
Cyclopentene	$C_5H_{8}$

Bonding

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

Chemistry; Organic chemistry I; KS5 Year 12; Alkenes: bonding and reactivity

1.	s orbitals
2.	Overlap
3.	$\sigma$ - 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.

Chemistry; Organic chemistry I; KS5 Year 12; Alkenes: bonding and reactivity

1.	p orbitals
2.	Overlap
3.	$\pi$ - bond

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.