Chemistry
Topic 7: Aldehydes and Ketones
Physical Properties
Boiling Points
Methanal is a gas (boiling point -21oC)
Ethanal has a boiling point of +21oC. That means that ethanal boils at close to room temperature.
The other aldehyde and the ketones are liquids, with boiling points rising as the molecules get
bigger. The size of the boiling point is governed by the strengths of the intermolecular forces:
Van der Waals Attractions
These attractions get stronger as the molecules get longer and have more electrons which increase
the strength of the Van der Waals attractions between the molecules. This is why the boiling points
increase as the number of carbon atoms in the chains increases – irrespective of whether you are
dealing with aldehydes or ketones.
Dipole-dipole interactions
Both aldehydes and ketones are polar molecules because of the presence of the carbon-oxygen
double bond. In addition to the VDW attractions, there will also be interactions between the
permanent dipoles on nearby molecules.
That means that the boiling points will be higher than those of similarly sized hydrocarbons – which
only experience VDW attractions.
Molecule Type Boiling point (oC) Types of Bonding
CH3CH2CH3 Alkane -42 VDW only
CH3CHO Aldehyde +21 VDW and Dipole-
dipole
CH3CH2OH Alcohol +78 VDW and Hydrogen
Bonds
N.B: Similar RMM so therefore have a similar VDW
Although the aldehydes and ketones are highly polar molecules, they don’t have any hydrogen
atoms attached directly to the oxygen, therefore they cannot hydrogen bond with each other.
Solubility in water
Small aldehydes and ketones are freely soluble in water
Solubility falls with chain length
The reason for the solubility is that although aldehydes and ketones can’t hydrogen
bond with themselves, they can hydrogen bond with water molecules.
The lone pair of electrons on the oxygen atom (on the carbonyl) can form a
hydrogen bond with the hydrogen atom on the water molecule (attached to
an electronegative oxygen atom).
As chain lengths increase, the hydrocarbon “tails” of the molecules (all the
hydrocarbon bits apart from the carbonyl group) start to get in the way and
solubility decreases.
Topic 7: Aldehydes and Ketones
Physical Properties
Boiling Points
Methanal is a gas (boiling point -21oC)
Ethanal has a boiling point of +21oC. That means that ethanal boils at close to room temperature.
The other aldehyde and the ketones are liquids, with boiling points rising as the molecules get
bigger. The size of the boiling point is governed by the strengths of the intermolecular forces:
Van der Waals Attractions
These attractions get stronger as the molecules get longer and have more electrons which increase
the strength of the Van der Waals attractions between the molecules. This is why the boiling points
increase as the number of carbon atoms in the chains increases – irrespective of whether you are
dealing with aldehydes or ketones.
Dipole-dipole interactions
Both aldehydes and ketones are polar molecules because of the presence of the carbon-oxygen
double bond. In addition to the VDW attractions, there will also be interactions between the
permanent dipoles on nearby molecules.
That means that the boiling points will be higher than those of similarly sized hydrocarbons – which
only experience VDW attractions.
Molecule Type Boiling point (oC) Types of Bonding
CH3CH2CH3 Alkane -42 VDW only
CH3CHO Aldehyde +21 VDW and Dipole-
dipole
CH3CH2OH Alcohol +78 VDW and Hydrogen
Bonds
N.B: Similar RMM so therefore have a similar VDW
Although the aldehydes and ketones are highly polar molecules, they don’t have any hydrogen
atoms attached directly to the oxygen, therefore they cannot hydrogen bond with each other.
Solubility in water
Small aldehydes and ketones are freely soluble in water
Solubility falls with chain length
The reason for the solubility is that although aldehydes and ketones can’t hydrogen
bond with themselves, they can hydrogen bond with water molecules.
The lone pair of electrons on the oxygen atom (on the carbonyl) can form a
hydrogen bond with the hydrogen atom on the water molecule (attached to
an electronegative oxygen atom).
As chain lengths increase, the hydrocarbon “tails” of the molecules (all the
hydrocarbon bits apart from the carbonyl group) start to get in the way and
solubility decreases.
