Physical Properties of Aldehydes and Ketones (Carbonyl Compounds)

Physical properties of Carbonyl compounds (aldehydes and ketones) depends on several factors and different aldehydes and ketones show different characteristics such as some are soluble in water and some are not.

Physical properties of aldehydes, ketones depends on following.

• Polarization of C=O bond
• The size of unpolarized carbon chain
• Molecular mass of aldehyde and ketone compounds

Polarization of C=O group

Due to difference of electro-negativity of carbon and oxygen atoms, carbon and oxygen atoms are polarized. So aldehydes and ketones are considered as polar compounds.

Melting and boiling points

No hydrogen bonds in carbonyl compounds

In aldehydes and ketones, there are no hydrogen bonds because there is no hydrogen atom which is connected to the oxygen atom like alcohols and carboxylic acids. Therefore, polarization of aldehyde and ketone compounds are less when compares with alcohols and carboxylic acids.

Boiling points are less than alcohols and carboxylic acids

Therefore, melting and boiling points of aldehydes and ketones are less than corresponding alcohols and carboxylic acids. See boiling points of following example compounds which have three carbon atoms.

• Propanol (alcohol): 97⁰C
• Propanoic acid (carboxylic acid): 141.2⁰C
• Propanal (aldehyde): 48.8⁰C
• Propanone (ketone): 56⁰C

You see, propanal and propanone have low boiling points than propanol and propanoic acid.

Boiling points are greater than hydrocarbons

But, polarization of aldehyde and ketone compounds are greater when compares with hydrocarbons. So boiling points of aldehyde and ketones should have greater boiling points than alkanes, alkenes and alkynes.

See following three hydrocarbons which have three carbon atoms.

• Propane (alkane): -42⁰C
• Propene (alkene): -47.6⁰C
• Propyne (alkyne): -23.2⁰C

Boiling points of carbonyl compounds

Here, we are going to make a list of boling points of aldehydes and ketones.

Boiling points of aldehydes

• Methanal: -19⁰C
• Ethanal: 20.2⁰C
• Propanal: 48.8⁰C
• Butanal: 74.8⁰C
• Benzaldehyde: 178.1⁰C

Boiling points of ketones

• Propanone: 56⁰C
• Butanone: 79.64⁰C
• Acetaphenone: 202⁰C

You see when molecular mass increase (molecule gets large), boiling points also increase (see methnal to butanal).

Solubility of aldehydes and ketones

In water

Aldehydes which have less molecular mass are soluble in water. When number of carbon atom is more than five, solubility in water is very low.

• Formaldehyde, acetaldehyde and acetone are very much soluble in water.
• Propanal, butanal and butanone (methyl ethyl ketone) are soluble.
• Benzaldehyde and acetaphenone are insoluble in water.

But, aldehydes and ketones dissolve very well in organic solvents.

Questions

Why are the boiling points of aldehydes less than those of ketones?, Is this true?

This is correct at most times. But it is fir to compare equal number of carbon atoms in both aldehyde and ketone compound. If we consider propanal and propanol, boiling point of propanone is greater than propanal.

This is due to C=O group of ketone is more polarized than aldehyde group. So intermolecular forces between ketone molecules are greater than aldehyde molecules which higher boiling points of ketone compounds

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