Carboxylic Acids
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CARBOXYLIC ACID
What are carboxylic acids?
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
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.
Note:
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The bond between the sodium and the ethanoate is ionic. Don’t draw a line between the two (implying a covalent bond). That’s absolutely wrong!
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Although the name is written with the sodium first, the formula is always written in one of the ways shown. This is something you just have to get used to.
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.
Boiling points
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.
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.