INTRODUCTION TO CARBOXYLIC ACIDS AND THEIR PHYSICAL PROPERTIES

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 CH3COONa+ or just CH3COONa.

Notice:

  • 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!
  • 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

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.

For example:

propan-1-olCH3CH2CH2OH97.2°C
ethanoic acidCH3COOH118°C

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

Condensed Structural FormulaName of AcidMelting Point (°C)Boiling Point (°C)Solubility (g/100 g of Water)
HCOOHformic acid8100miscible
CH3COOHacetic acid17118miscible
CH3CH2COOHpropionic acid–22141miscible
CH3(CH2)2COOHbutyric acid–5163miscible
CH3(CH2)3COOHvaleric acid–351875
CH3(CH2)4COOHcaproic acid–32051.1
C6H5COOHbenzoic acid1222490.29

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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