Topic 15 · page 6 of 28

Physical properties of carbonyl compounds

Aldehydes and ketones have no O–H bond, so they cannot hydrogen-bond to each other and boil lower than alcohols. Their oxygen atom can accept a hydrogen bond from water, so small ones dissolve.

12 min read15.7(i)15.7(ii)

Read first: Naming aldehydes and ketones

Boiling temperatures 15.7(i)

The C=O bond is polar. Oxygen is more electronegative than carbon, so the carbon is δ+ and the oxygen is δ−. Molecules of aldehydes and ketones attract each other through permanent dipole–dipole forces, as well as London forces.

Left: propanone with a C=O dipole, delta plus on the carbon and delta minus on the oxygen, and a note that there is no O–H bond. Right: a hydrogen bond drawn as a dotted line from a hydrogen atom of a water molecule to a lone pair on the carbonyl oxygen.
The carbonyl group is polar, but its hydrogen atoms are bonded to carbon, so molecules of a carbonyl compound cannot hydrogen-bond to each other.

A hydrogen bond needs a hydrogen atom bonded to oxygen, nitrogen or fluorine. In an aldehyde or ketone the hydrogen atoms are all bonded to carbon, so the molecules cannot form hydrogen bonds with each other. The strongest force between them is permanent dipole–dipole.

A bar chart of boiling temperatures in degrees Celsius: butane minus 1, propanal 48, propanone 56 and propan-1-ol 97. Under each bar the strongest force between molecules: London forces for butane, permanent dipole-dipole for propanal and propanone, and hydrogen bonds for propan-1-ol.
Four molecules with about the same relative molecular mass. Stronger forces between molecules mean a higher boiling temperature.
Compound MrM_\text{r} Strongest force between molecules Boiling temperature / °C
butane 58.0 London forces −1
propanal 58.0 permanent dipole–dipole 48
propanone 58.0 permanent dipole–dipole 56
propan-1-ol 60.0 hydrogen bonds 97

All four have almost the same MrM_\text{r}, so the London forces are similar. The carbonyl compounds boil higher than the alkane because dipole–dipole attractions are added to the London forces. They boil lower than the alcohol because hydrogen bonds are stronger than dipole–dipole forces.

Solubility in water 15.7(ii)

Water molecules can hydrogen-bond to the carbonyl oxygen. A δ+ hydrogen of water is attracted to a lone pair on the oxygen of the C=O group.

So an aldehyde or ketone molecule can accept a hydrogen bond from water, although it cannot donate one. This is enough for small molecules to mix with water completely: methanal, ethanal and propanone are all miscible with water.

As the carbon chain gets longer the non-polar hydrocarbon part grows. It cannot form hydrogen bonds with water, and it breaks up the hydrogen bonds between water molecules without replacing them. Solubility therefore falls as the chain length increases, and compounds with about six or more carbon atoms are only slightly soluble.

The solubility practical for aldehydes, ketones and carboxylic acids, with the expected observations, is on the page about hydrogen bonding in carboxylic acids.

Check yourself

  1. Explain why propanal boils at a higher temperature than butane even though they have the same relative molecular mass.
  2. Explain why propan-1-ol boils at a higher temperature than propanal.
  3. Draw a diagram showing a hydrogen bond between a water molecule and a propanone molecule. Label the lone pair and the partial charges.
  4. Explain why propanone mixes with water but a ketone with a long carbon chain does not.
  5. State why molecules of ethanal cannot form hydrogen bonds with each other.

Answers cover these until you have tried the questions

  1. Propanal molecules attract each other by permanent dipole–dipole forces as well as London forces, because the C=O bond is polar. Butane has only London forces. Dipole–dipole attractions are additional and need more energy to overcome, so propanal has the higher boiling temperature.
  2. Propan-1-ol has an O–H bond and forms hydrogen bonds between its molecules. These are stronger than the permanent dipole–dipole forces in propanal, so more energy is needed to separate the molecules.
  3. A water molecule with its δ+ hydrogen pointing at the oxygen of the propanone C=O, joined by a dotted line (the hydrogen bond) that ends on a lone pair drawn on the carbonyl oxygen. Label the carbonyl carbon δ+ and the carbonyl oxygen δ−, and the water oxygen δ−.
  4. Propanone's carbonyl oxygen forms hydrogen bonds with water, and its two CH₃ groups are small. In a long-chain ketone the non-polar hydrocarbon chain is large. It cannot hydrogen-bond to water and it disrupts the hydrogen bonds between water molecules, so the energy released is not enough to make up for the bonds broken.
  5. Hydrogen bonds need a hydrogen atom bonded to O, N or F. In ethanal all the hydrogen atoms are bonded to carbon, so there is no suitable hydrogen.