Alkenes have the general formula CnH2n

Alkenes are a class of hydrocarbons containing carbon-carbon double bonds. Their general formula is C_nH_2n, where n represents the number of carbon atoms.

Key features of alkenes:

• They consist of carbon and hydrogen atoms (hydrocarbons).
• Each alkene molecule contains at least one carbon-carbon double bond.
• The double bond makes them "unsaturated", enabling them to participate in addition reactions.

Examples of alkenes:

Note that cyclic alkenes such as cyclohexene (C₆H₁₀) have two fewer hydrogen atoms compared to acyclic alkenes with an equivalent number of carbon atoms. The general formula of cyclic alkenes is C_nH_2n-2.

A double bond consists of a sigma bond and a pi bond

The carbon-carbon double bond in alkenes comprises:

1. A sigma (σ) bond:

• This is a strong bond formed by the head-on overlap of carbon s orbitals.
• It has a high electron density situated directly between the nuclei.
• It is very strong, possessing the highest bond enthalpy among single bonds.

2. A pi (π) bond:

• This bond is created by the sideways overlap of adjacent p orbitals.
• Its electron density is distributed above and below the bond axis.
• It has a lower electron density and bond enthalpy compared to the sigma bond.
• The presence of the pi bond restricts rotation around the C=C bond, as rotation would disrupt the parallel overlap of the p orbitals, leading to a significant increase in energy.