C_nH_2n

Alkenes have one or more carbon-carbon double bond.

sp² hybridisation

In an alkene, each carbon atom involved in the double bond forms three single bonds, either with hydrogen atoms (C-H) or other carbon atoms (C-C), and one double bond with the adjacent carbon atom. The three single bonds arrange themselves in a trigonal planar geometry, lying in a single plane with bond angles of ≈120°.

The trigonal planar arrangement of the bonds is due to the equal repulsion between the bonding pairs of electrons.

The pi (π) bond formed by the sideways overlap of the p-orbitals on the carbon atoms locks the carbons in position and prevents rotation around the double bond.

Alkenes undergo addition reactions because they are unsaturated molecules - a molecule can add across the C=C double bond to form a single saturated compound.

Alkenes react with electrophiles (electron pair-acceptors) because there is an area of high electron density around the C=C double bond.

The high reactivity of alkenes is due to the presence of the electron-rich π-bond in the C=C double bond, which readily attracts electrophiles and enables addition reactions.

1. Elimination of hydrogen halides from halogenoalkanes using ethanolic sodium hydroxide and heat.
2. Dehydration of alcohols using a heated aluminum oxide catalyst or concentrated sulfuric acid.
3. Cracking of longer chain alkanes using heat and an aluminum oxide catalyst.