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What are carboxylic acids?
Carboxylic acids contain a -COOH group
Carboxylic acids are compounds which contain a -COOH group. For the purposes of this page we shall just look at compounds where the -COOH group is attached either to a hydrogen atom or to an alkyl group.
Examples of carboxylic acids
The name counts the total number of carbon atoms in the longest chain – including the one in the -COOH group. If you have side groups attached to the chain, notice that you always count from the carbon atom in the -COOH group as being number 1.
Salts of carboxylic acids
Carboxylic acids are acidic because of the hydrogen in the -COOH group. When the acids form salts, this is lost and replaced by a metal. Sodium ethanoate, for example, has the structure:
Depending on whether or not you wanted to stress the ionic nature of the compound, this would be simplified to CH3COO–Na+ or just CH3COONa.
Physical properties of carboxylic acids
The physical properties (for example, boiling point and solubility) of the carboxylic acids are governed by their ability to form hydrogen bonds.
Before we look at carboxylic acids, a reminder about alcohols:
The boiling points of alcohols are higher than those of alkanes of similar size because the alcohols can form hydrogen bonds 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.
These are chosen for comparison because they have identical relative molecular masses and almost the same number of electrons (which affects van der Waals dispersion forces).
The higher boiling points of the carboxylic acids are still caused by hydrogen bonding, but operating in a different way.
In a pure carboxylic acid, hydrogen bonding can occur between two molecules of acid to produce a dimer.
This immediately doubles the size of the molecule and so increases the van der Waals dispersion forces between one of these dimers and its neighbours – resulting in a high boiling point.
“Physical Constants of Carboxylic Acids” lists some physical properties for selected carboxylic acids. The first six are homologs. Notice that the boiling points increase with increasing molar mass, but the melting points show no regular pattern.
Table 15.2 Physical Constants of Carboxylic Acids
Solubility in water
In the presence of water, the carboxylic acids don’t dimerise. 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 released when the new hydrogen bonds form is much the same as is 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 broken hydrogen bonds are only replaced by much weaker van der Waals dispersion forces.
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