Home Molecules Alkenes

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Hydrocarbons are compounds containing hydrogen and carbon atoms only. The alkenes are a homologous series of hydrocarbons with the general formula CnH2n.

The models show unbranched members of this series with 2, 3 or 4 carbon atoms. You can also see the structural and displayed formulae for these alkenes.

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chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

ethene, C2H4

propene, C3H6

but-1-ene, C4H8

but-2-ene, C4H8

Homologous series

A homologous series is a series of compounds which:

  • have the same functional group
  • have the same general formula
  • differ by CH2 in molecular formulae from neighbouring compounds
  • show a gradual variation in physical properties, such as their boiling points
  • have similar chemical properties.

Functional group

A functional group is an atom, group of atoms or a bond responsible for the characteristic chemical reactions of a substance. The functional group in alkenes is the double carbon–carbon double bond, C=C.

General formula

A general formula is a chemical formula used to represent the members of a homologous series. The general formula for the alkanes is CnH2n.

Hexene molecules, for example, contain six carbon atoms:

n = 6
2n = (2 × 6) = 12

So the molecular formula for hexene is C6H12.

Structural and displayed formulae

Name of alkene
Structural formula
Displayed formula
ethene CH2=CH2 displayed formula of ethene
propene CH3CH=CH2 displayed formula of propene
but-1-ene CH3CH2CH=CH2 displayed formula of but-1-ene
but-2-ene CH3CH=CHCH3 displayed formula of but-2-ene

The model and displayed formula of but-2-ene show trans-but-2-ene. This is one isomer of but-2-ene. The other isomer is cis-but-2-ene. Find out about cis-trans isomers here.