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Alcohols

What is here?

Alcohols are organic compounds that:

  • contain the hydroxyl functional group, —OH
  • have molecules based on alkanes in which one of the H atoms is replaced by an —OH group
  • form a homologous series which has the general formula CnH2n+1OH.

The models show members of this series with 1 – 4 carbon atoms. You can also see the structural and displayed formulae for these alcohols.

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

methanol, CH3OH

ethanol, C2H5OH

propan-1-ol, C3H7OH

propan-2-ol, C3H7OH

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 alcohols is most easily shown as —OH.

General formula

A general formula is a chemical formula used to represent the members of a homologous series. The general formula for the alcohols is CnH2n+1OH.

Hexanol molecules, for example, contain six carbon atoms:

n = 6
2n + 1 = (2 × 6) + 1 = 13

So the molecular formula for hexanol is C6H13OH.

Structural and displayed formulae

Name of alkane
Structural formula
Displayed formula
methanol CH3OH displayed formula of methanol
ethanol CH3CH2OH displayed formula of ethanol
propan-1-ol CH3CH2CH2OH displayed formula of propan-1-ol
propan-2-ol CH3CH2OHCH3 displayed formula of propan-2-ol

Isomers of butanol

Isomers are substances that have the same molecular formula, but a different arrangement of atoms:

  • propan-1-ol and propan-2-ol are position isomers in which the hydroxy group is found in different positions on the carbon chain.

The molecular formula of butanol is C4H9OH. Butanol has four isomers:

  • butan-1-ol and butan-2-ol
  • 2-methylpropan-1-ol and 2-methylpropan-2-ol.

The molecular models here show these four isomers. Two of them have branched carbon chains.

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butan-1-ol

2-methylpropan-1-ol

butan-2-ol

2-methylpropan-2-ol

Unbranched isomers of butanol

Name of alcohol
Structural formula
Displayed formula
butan-1-ol CH3CH2CH2CH2OH displayed formula of butan-1-ol
butan-2-ol CH3CH2CH2OHCH3 displayed_butan-2-ol.gif

Branched isomers of butanol

Name of alcohol
Structural formula
Displayed formula
2-methylpropan-1-ol (CH3)2CHCH2OH displayed formula of 2-methylpropan-1-ol
2-methylpropan-2-ol (CH3)3COH displayed formula of 2-methylpropan-2-ol