Aldehydes & Ketones

This is part of the HSC Chemistry course under the topic Reactions of Organic Acids and Bases.

HSC Chemistry Syllabus

investigate the structural formulae, properties and functional group including:

– primary, secondary and tertiary alcohols

– aldehydes and ketones (ACSCH127)

– amines and amides

– carboxylic acids

explain the properties within and between the homologous series of carboxylic acids, amines and amides with reference to the intermolecular and intramolecular bonding present

Carbonyl Compounds: Structure, Properties & Reactions

This video introduces a new group of organic compounds – carbonyl compounds, including the structure, properties and reactions of

Aldehydes and Ketones
Carboxylic acids (organic acid).

Structure and Nomenclature

Aldehyde, ketone and carboxylic acids all contain a carbonyl carbon that is sp² This means both functional groups contain a C=O bond, of which one is a reactive π-bond, the other is an unreactive s-bond.

Functional group	Suffix	Prefix	Generic structure	Example
Aldehyde	-al	Formyl-
Ketone	-one	Oxo-

Nomenclature priority

- In order of decreasing priority: aldehyde, ketone, alcohol, alkene, alkyne and alkanes.

Properties of Aldehydes and Ketones

Boiling and Melting Points

Smaller aldehydes and ketones are polar molecules and there can form dipole-dipole forces on top of dispersion forces.

Aldehydes and ketones generally have stronger intermolecular forces than hydrocarbons of similar molecular mass. Thus, they have higher boiling and melting points.

Compared to alcohols, aldehydes and ketones generally have weaker intermolecular forces because they cannot form hydrogen bonds. Alcohol molecules contain hydroxyl (–OH) groups that can participate in hydrogen bonding as either a donor or acceptor.

In their own homologous series, boiling and melting points of aldehydes and ketones increase with molecular mass due to stronger dispersion forces.

Solubility in water

Aldehydes and ketones are polar compounds, primarily due to the presence of an electronegative oxygen atom in their functional groups. Smaller aldehyde and keton molecules are soluble in water.
Aldehydes and ketones cannot form hydrogen bonds with themselves (among molecules of only aldehyde and ketone), they can however form hydrogen bonds with water molecules.
This enables small aldehyde and ketone molecules to dissolve in water (molecules with low number of carbon atoms).
As the length of non-polar carbon chain increases, the polarity of aldehydes and ketones decreases. This reduces the solubility of aldehydes and ketones in water.

Table: melting and boiling points of aldehydes and ketones increase with molecular weight (size) while their solubilities decrease with molecular weight.

Table: compounds that can form hydrogen bonds have, in general, stronger intermolecular force and higher boiling and melting points than those that do not.

Compound	Functional group	Molar mass (g mol^–1)	Type of intermolecular force	Boiling point (ºC)
Butane	Alkane	58	Dispersion	–1
Butanal	Aldehyde	72	Strong dipole	49
Butanone	Ketone	72	Stronger dipole	56
Butanol	Alcohol	74	Hydrogen bonding	97