Carbonyl Compounds
Carbonyl Compounds
Organic Chemistry
1
, Carbonyl Compounds
Aldehydes and ketones are known as carbonyl compounds and they contain carbonyl
group.
Functional Group
Aldehyde Ketone
At least one H attached to the
carbonyl group.
Two carbons attached to the
carbonyl group.
Aliphatic Aldehyde
1. CH3CHO – Ethanal
2. C4H9CHO – Pentanal
3. CH3CH2CHO – Propanal
Aromatic Aldehyde
Ketones
2
, Carbonyl Compounds
Aldehydes and ketones contribute to the distinctive odour of food and plants. For example
carvanone is reponsible for the mint odour.
(a) Is carvanone optically active? Yes
(b) State the number of carbon atoms in carvanone: 9C
(C) Is carvanone an aldehyde or a ketone? Ketone
(d) Calculate the Mr carvanone: 152
PHYSICAL PROPERTIES
The C = O group polarity affects the physical properties.
1. The aldehydes and ketones are less volatile and have higher boiling points than the
corresponding alkanes.
Example: Ethanal is a volatile liquid at room temperature but ethane is a gas.
Ethanal consists of discrete molecules held by permanent dipole – permanent dipole
attraction while ethane consists of discrete molecules held by temporary dipole – induced
dipole (td – id). At RT, the energy is enough to separate ethane molecules but not high
enough to separate ethanal molecules.
2. The aldehydes and ketones, however, are more volatile than corresponding alcohols
which have hydrogen bonding between molecules.
The aldehydes cannot form hydrogen bonds between molecules as the hydrogen is bonded to
carbon and not to oxygen.
Explain why the boiling point of ethanal is 20oC whereas boiling point of ethanol is 78oC
Ethanol molecules are held by hydrogen bonding whereas ethanal molecules are held by
p.d – p.d.
Since hydrogen bond is greater than p.d – p.d, more energy id needed to separate ethanol
molecules than to separate ethanal molecules.
Note: As the molecular mass increases, so the strength of the intermolecular forces
increases.
3
, Carbonyl Compounds
Consider the following table of boiling points.
Compounds Boiling Point oC
Methanal -21
Ethanal 20
Propanal 49
Propanone 56
Butanal 76
Butanone 80
3. The Smaller members are soluble in water because of their polarity and also they can
form H-bond with water molecules.
The solubility decreases as the hydrocarbon chain gets longer because of the non-polar
nature of the hydrocarbon chain.
The smaller members of the series are solvents for polar and non-polar solutes.
As the molecules increases in molecular mass, so the influence of the carbonyl group
diminishes and the solubility in water decreases.
Compound Solubility in water
Methanal Very soluble in water
Ethanal Very soluble in water
Propanal Soluble in water
Propanone Soluble in water
Pentanone Slightly soluble in water
The aldehydes and ketones are all soluble in non-polar solvents. Propanone is a widely
used solvent in industry.
3. Solubility decreases as Mr increases as influence of carbonyl group decreases.
Reminder
4
Carbonyl Compounds
Organic Chemistry
1
, Carbonyl Compounds
Aldehydes and ketones are known as carbonyl compounds and they contain carbonyl
group.
Functional Group
Aldehyde Ketone
At least one H attached to the
carbonyl group.
Two carbons attached to the
carbonyl group.
Aliphatic Aldehyde
1. CH3CHO – Ethanal
2. C4H9CHO – Pentanal
3. CH3CH2CHO – Propanal
Aromatic Aldehyde
Ketones
2
, Carbonyl Compounds
Aldehydes and ketones contribute to the distinctive odour of food and plants. For example
carvanone is reponsible for the mint odour.
(a) Is carvanone optically active? Yes
(b) State the number of carbon atoms in carvanone: 9C
(C) Is carvanone an aldehyde or a ketone? Ketone
(d) Calculate the Mr carvanone: 152
PHYSICAL PROPERTIES
The C = O group polarity affects the physical properties.
1. The aldehydes and ketones are less volatile and have higher boiling points than the
corresponding alkanes.
Example: Ethanal is a volatile liquid at room temperature but ethane is a gas.
Ethanal consists of discrete molecules held by permanent dipole – permanent dipole
attraction while ethane consists of discrete molecules held by temporary dipole – induced
dipole (td – id). At RT, the energy is enough to separate ethane molecules but not high
enough to separate ethanal molecules.
2. The aldehydes and ketones, however, are more volatile than corresponding alcohols
which have hydrogen bonding between molecules.
The aldehydes cannot form hydrogen bonds between molecules as the hydrogen is bonded to
carbon and not to oxygen.
Explain why the boiling point of ethanal is 20oC whereas boiling point of ethanol is 78oC
Ethanol molecules are held by hydrogen bonding whereas ethanal molecules are held by
p.d – p.d.
Since hydrogen bond is greater than p.d – p.d, more energy id needed to separate ethanol
molecules than to separate ethanal molecules.
Note: As the molecular mass increases, so the strength of the intermolecular forces
increases.
3
, Carbonyl Compounds
Consider the following table of boiling points.
Compounds Boiling Point oC
Methanal -21
Ethanal 20
Propanal 49
Propanone 56
Butanal 76
Butanone 80
3. The Smaller members are soluble in water because of their polarity and also they can
form H-bond with water molecules.
The solubility decreases as the hydrocarbon chain gets longer because of the non-polar
nature of the hydrocarbon chain.
The smaller members of the series are solvents for polar and non-polar solutes.
As the molecules increases in molecular mass, so the influence of the carbonyl group
diminishes and the solubility in water decreases.
Compound Solubility in water
Methanal Very soluble in water
Ethanal Very soluble in water
Propanal Soluble in water
Propanone Soluble in water
Pentanone Slightly soluble in water
The aldehydes and ketones are all soluble in non-polar solvents. Propanone is a widely
used solvent in industry.
3. Solubility decreases as Mr increases as influence of carbonyl group decreases.
Reminder
4
