CHAPTER 18: Carbonyl Compounds
18.1 Introduction to Aldehydes and Ketones
18.2 Reactions Aldehydes and Ketones
18.3 Tests for Aldehydes and Ketones
Learning outcomes:
(a) describe:
(i) the formation of aldehydes and ketones from primary and secondary alcohols respectively
using Cr2O7²⁻/H⁺.
(ii) the reduction of aldehydes and ketones, e.g. using NaBH4 or LiAlH4.
(iii) the reaction of aldehydes and ketones with HCN and NaCN.
(b) describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with
aldehydes and ketones.
(c) describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) reagent to detect the presence of
carbonyl compounds.
(d) deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of
simple tests (i.e. Fehling’s and Tollens’ reagents; ease of oxidation).
, 18.1 Introduction to Aldehydes and Ketones
What are carbonyl compounds?
1) Carbonyl compounds are compounds that contain the C=O(carbonyl) group.
Examples are aldehydes and ketones.
Physical properties of carbonyl compounds
1) i. The boiling point of carbonyl compounds is higher than the alkanes with
similar Mr.
ii. The boiling point increases with increasing number of carbon atom. This is
because there are more electrons, hence more temporary dipoles can be set
up. More energy is required to overcome these forces.
ii. Besides temporary dipoles, permanent dipole-dipole forces are
also present due to carbonyl compounds being polar.
iii. Methanal and ethanal are gases at room temperature, while others are liquids.
2) i. Carbonyl compounds are soluble in water. This is because they are able to
form hydrogen bond with water molecules.
ii. The solubility decreases with increasing number of carbon atoms. This is
because the long hydrocarbon chain disrupts the hydrogen bonding.
18.1 Introduction to Aldehydes and Ketones
18.2 Reactions Aldehydes and Ketones
18.3 Tests for Aldehydes and Ketones
Learning outcomes:
(a) describe:
(i) the formation of aldehydes and ketones from primary and secondary alcohols respectively
using Cr2O7²⁻/H⁺.
(ii) the reduction of aldehydes and ketones, e.g. using NaBH4 or LiAlH4.
(iii) the reaction of aldehydes and ketones with HCN and NaCN.
(b) describe the mechanism of the nucleophilic addition reactions of hydrogen cyanide with
aldehydes and ketones.
(c) describe the use of 2,4-dinitrophenylhydrazine (2,4-DNPH) reagent to detect the presence of
carbonyl compounds.
(d) deduce the nature (aldehyde or ketone) of an unknown carbonyl compound from the results of
simple tests (i.e. Fehling’s and Tollens’ reagents; ease of oxidation).
, 18.1 Introduction to Aldehydes and Ketones
What are carbonyl compounds?
1) Carbonyl compounds are compounds that contain the C=O(carbonyl) group.
Examples are aldehydes and ketones.
Physical properties of carbonyl compounds
1) i. The boiling point of carbonyl compounds is higher than the alkanes with
similar Mr.
ii. The boiling point increases with increasing number of carbon atom. This is
because there are more electrons, hence more temporary dipoles can be set
up. More energy is required to overcome these forces.
ii. Besides temporary dipoles, permanent dipole-dipole forces are
also present due to carbonyl compounds being polar.
iii. Methanal and ethanal are gases at room temperature, while others are liquids.
2) i. Carbonyl compounds are soluble in water. This is because they are able to
form hydrogen bond with water molecules.
ii. The solubility decreases with increasing number of carbon atoms. This is
because the long hydrocarbon chain disrupts the hydrogen bonding.
