Summary
Summary Topic 8 - Carboxylic acids
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ChemistryTopic 8: Carboxylic Acids
Carboxylic acids R-COOH, contain the functional group -COOH. Aldehydes and Ketones contain the functional
group C=O. Since carboxylic acids also contain C=O and C-O-H you might expect them to have a blend of the
properties of aldehydes, ketones and alcohols, however this is not the case.
The general formula for carboxylic acids is: CnH2nO2
Carboxylic acid derivatives include: acyl chlorides R-COCl; esters RCOOR’ and amides RCONHR’
Physical Properties
The physical properties (i.e. boiling point and solubility) of the carboxylic acids and are governed by their
ability to form hydrogen bonds.
Boiling Points
The boiling points of alcohols are higher than those of alkanes of similar size because the alcohols can form
hydrogen bond with each other as well as van der Waals dispersion forces and dipole-dipole interactions.
The boiling points of carboxylic acids of similar size are higher still than their similar sized alcohols
counterparts:
Propan-1-ol CH3CH2CH2OH 97.2oC
Ethanoic acid CH3COOH 118oC
The higher boiling points of the carboxylic acids are still
caused by hydrogen bonding, but this is because in a pure
carboxylic acid, intermolecular hydrogen bonding can occur
between two molecules of acids to produce a dimer.
This immediately doubles the size of the molecules and so
increases the van der Waals dispersion forces between one
of these dimers and its neighbours – resulting in a high
boiling point.
Solubility
In the presence of water, carboxylic acids do not dimerise with
each other. Instead, hydrogen bonds are formed between water
molecules and individual molecules of acid.
The carboxylic acids with up to four carbon atoms will mix with
water in any proportion. When you mix the two together, the
energy is released when the new hydrogen bonds form is much
the same as needed to break the hydrogen bonds in the pure
liquids.
The solubility of the bigger acids decreases very rapidly with size. This is because the longer hydrocarbon ‘tails’
of the molecules get between water molecules and break hydrogen bonds. In this case, these broke hydrogen
bonds are only replaced by much weaker van der Waals dispersion forces.
Summary
Carboxylic acids have a sharp, irritating odour
Short chain carboxylic acids are liquids – the polar OH group causes hydrogen bonds between
molecules, which require a lot of energy to break them
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