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Alkenes

Alkenes have at least one double bond between carbon atoms. The double bond is reactive: it can open, and new atoms can be added. That is called addition. A straight-chain alkene with one double bond has two hydrogens fewer than the alkane with the same number of carbons, so the formula is twice as many hydrogens as carbons. Ethene is the smallest. It is used to make plastics. The name ends in -ene, while alkanes end in -ane and alkynes in -yne.

CnH2n\mathrm{C}_n\mathrm{H}_{2n}straight-chain alkene with one double bond

Symbols

nnnumber of carbon atoms

Example

Ethene has 2 carbon atoms and one double bond.

Hydrogen: 2⋅2=42 \cdot 2 = 4, so the formula is C2H4.

Bromine water is decolourised by alkenes because bromine adds across the double bond. Alkanes do not do that.
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← Alkanes · Ethanol →

Part of Chemistry 2: Organic chemistry.