Structure, Bonding and Reactivity of Alkenes

Full Notes

Alkenes and their bonding have been outlined in more detail here and sigma and pi bonding here.
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Structure of Alkenes

Alkenes are unsaturated hydrocarbons that contain a C=C double bond.

They are described as unsaturated because the carbon atoms in the double bond are only bonded to three other atoms (rather than four in saturated compounds).

Alkenes have a general formula of C_nH_2n

Example Ethene (C₂H₄)

Bonding in Alkenes

The C=C double bond is actually made up of two different types of covalent bond - a sigma (σ) and pi (π) bond.

Sigma (σ) bond:

• Formed by the end-to-end overlap of orbitals.
• Allows free rotation around single bonds.

Pi (π) bond:

• Formed by the sideways overlap of p orbitals above and below the plane.

• Prevents rotation around the C=C bond.
• Makes alkenes more reactive because the high electron density attracts electrophiles.

Reactivity of Alkenes

Alkenes are more reactive than alkanes due to the high electron density of the pi bond.

Electrophiles (electron pair acceptors) are attracted to the high electron density, meaning alkenes react readily with electrophiles.

Common reactions include:

• Electrophilic addition (reaction with Br₂, HBr, H₂SO₄).
• Polymerisation (formation of long-chain molecules).

Summary

• Alkenes are unsaturated hydrocarbons containing a C=C double bond (general formula C_nH_2n).
• The C=C bond consists of a σ bond and a π bond; the π bond prevents rotation and creates high electron density.
• The high electron density of the π bond attracts electrophiles, making alkenes more reactive than alkanes.
• Key reactions include electrophilic addition and polymerisation.