Topic 15 · page 13 of 28

Hydrogen bonding in carboxylic acids

A carboxylic acid has both a C=O oxygen to accept and an O–H hydrogen to give hydrogen bonds. This gives high boiling temperatures and, for small acids, complete solubility in water.

14 min read15.10

Read first: Naming carboxylic acids, Physical properties of carbonyl compounds

Boiling temperatures 15.10

A carboxylic acid molecule has a δ+ hydrogen on oxygen (the O–H) and oxygen atoms with lone pairs (the C=O and the O–H). It can both donate and accept hydrogen bonds, so it forms strong hydrogen bonds with other acid molecules.

Two molecules of ethanoic acid joined by two hydrogen bonds shown as dotted lines. The hydrogen of the O–H group of each molecule points at the oxygen of the C=O group of the other, forming a ring.
Two ethanoic acid molecules can pair up through two hydrogen bonds.

The result is a high boiling temperature. Compare four molecules of about the same relative molecular mass.

A bar chart of boiling temperatures in degrees Celsius: butane minus 1, propanal 48, propan-1-ol 97 and ethanoic acid 118. Ethanoic acid and propan-1-ol have almost the same relative molecular mass, but the acid boils about 20 degrees higher.
Ethanoic acid boils higher than propan-1-ol because its molecules form more hydrogen bonds.
Compound MrM_\text{r} Strongest force Boiling temperature / °C
butane 58.0 London forces −1
propanal 58.0 permanent dipole–dipole 48
propan-1-ol 60.0 hydrogen bonds 97
ethanoic acid 60.0 hydrogen bonds 118

Ethanoic acid and propan-1-ol have the same MrM_\text{r} and both hydrogen-bond. The acid boils about 20 °C higher because its O–H hydrogen is more strongly δ+ (the neighbouring C=O draws electrons away from it), so each hydrogen bond is stronger, and the C=O oxygen gives a second site for hydrogen bonding. Pairs of molecules held in a ring need more energy to separate.

Boiling temperature also rises along the series as the chain grows, because the London forces become stronger with more electrons.

Acid Boiling temperature / °C
methanoic acid 101
ethanoic acid 118
propanoic acid 141
butanoic acid 164

Solubility in water 15.10

An acid molecule forms hydrogen bonds with water in two ways:

Ethanoic acid with two water molecules. The carbonyl oxygen of the acid accepts a hydrogen bond from a hydrogen of one water molecule, and the O–H hydrogen of the acid forms a hydrogen bond to the oxygen of the other water molecule. Both bonds are drawn as dotted lines.
An acid hydrogen-bonds to water through both its C=O oxygen and its O–H hydrogen.

Methanoic, ethanoic, propanoic and butanoic acids all mix completely with water. As the chain gets longer the non-polar hydrocarbon chain grows, and it disrupts the hydrogen bonding in water without forming bonds of its own, so solubility falls. Hexanoic acid, with six carbons, is only slightly soluble.

PracticalSolubility of acids, aldehydes and ketones

This is the further suggested practical on solubility.

  1. Label six test tubes and add about 5 cm³ of water to each.
  2. Add about 1 cm³ (or 10 drops) of one of methanoic acid, ethanoic acid, propanoic acid, hexanoic acid, propanone and pentan-3-one to each tube. Use a fume cupboard for the acids with strong smells.
  3. Shake each tube for a few seconds and stand it for a minute. Record whether the liquid mixes completely (one layer) or forms two layers.
  4. Test the water layer of each tube with universal indicator paper.
Sample Expected result in water pH of the mixture
methanoic acid, ethanoic acid, propanoic acid one layer, mixes completely acidic
hexanoic acid two layers, slightly soluble slightly acidic
propanone one layer, mixes completely about neutral
pentan-3-one two layers if a lot is added, partly soluble about neutral

The pattern is the same for every family: short chains dissolve, long chains do not, because the hydrocarbon part grows while the polar group stays the same.

Check yourself

  1. Explain why ethanoic acid has a higher boiling temperature than propanal.
  2. Explain why ethanoic acid has a higher boiling temperature than propan-1-ol, although their relative molecular masses are the same.
  3. Draw a diagram showing two hydrogen bonds between a molecule of ethanoic acid and water molecules. Mark the partial charges on the atoms involved.
  4. State and explain the trend in solubility in water of methanoic, propanoic and hexanoic acids.
  5. A student adds propanone and hexanoic acid to separate tubes of water. Predict and explain what she sees.

Answers cover these until you have tried the questions

  1. Ethanoic acid molecules form hydrogen bonds with each other because they have an O–H group. These are stronger than the permanent dipole–dipole forces between propanal molecules, which have no O–H, so more energy is needed to overcome them.
  2. Both molecules hydrogen-bond, but an ethanoic acid molecule has a C=O oxygen as well as the O–H, so it can form two hydrogen bonds to a neighbour (a ring of two). Its O–H is also more polarised than the O–H of an alcohol, so each hydrogen bond is stronger. More energy is needed to separate the acid molecules, so its boiling temperature is higher (118 °C compared with 97 °C).
  3. One water molecule with its δ+ H pointing at the δ− carbonyl O of the acid, joined by a dotted line to a lone pair. A second water molecule with its δ− O pointing at the δ+ H of the acid's O–H, joined by a dotted line. Mark δ+ on both of those hydrogen atoms and δ− on both oxygen atoms involved.
  4. Solubility decreases from methanoic acid (completely miscible) to propanoic acid (completely miscible) to hexanoic acid (only slightly soluble). The polar –COOH group forms hydrogen bonds with water in each, but the non-polar hydrocarbon chain gets longer, and this part cannot hydrogen-bond to water and disrupts water's hydrogen bonds.
  5. Propanone mixes completely with water (one layer), because its C=O oxygen accepts hydrogen bonds from water and the chain is short. Hexanoic acid forms two layers, because the six-carbon chain is too large for the –COOH group's hydrogen bonds to make up for the disruption of water's hydrogen bonding.