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 C_nH_2n+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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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 C_nH_2n+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
ethanol	CH3CH2OH
propan-1-ol	CH3CH2CH2OH
propan-2-ol	CH3CH2OHCH3

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.

Unbranched isomers of butanol

Name of alcohol	Structural formula	Displayed formula
butan-1-ol	CH3CH2CH2CH2OH
butan-2-ol	CH3CH2CH2OHCH3

Branched isomers of butanol

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