Nomenclature and Properties of Aldehydes and Ketones - SS2 Chemistry Lesson Note
Aldehydes and ketones are important classes of organic compounds that contain the carbonyl functional group (-C=O). They play crucial roles in various biological, industrial, and synthetic applications.
Nomenclature of Aldehydes:
The systematic nomenclature of aldehydes follows the rules established by the International Union of Pure and Applied Chemistry (IUPAC). Aldehydes are named by replacing the final "e" in the name of the corresponding parent hydrocarbon with the suffix "-al."
IUPAC Naming Rules:
● Identify the longest continuous carbon chain that contains the carbonyl group. This chain becomes the parent hydrocarbon.
● Number the carbon atoms in the chain, starting from the end nearest to the carbonyl group.
● Indicate the position of the carbonyl group with the lowest possible number.
● Add the suffix "-al" to the name of the parent hydrocarbon.
Examples of IUPAC Names:
● Methanal: The simplest aldehyde, also known as formaldehyde.
● Ethanal: An aldehyde with two carbon atoms, also known as acetaldehyde.
Nomenclature of Ketones:
The systematic nomenclature of ketones also follows the IUPAC rules. Ketones are named by replacing the final "e" in the name of the corresponding parent hydrocarbon with the suffix "-one."
IUPAC Naming Rules:
● Identify the longest continuous carbon chain that contains the carbonyl group. This chain becomes the parent hydrocarbon.
● Number the carbon atoms in the chain, starting from the end nearest to the carbonyl group.
● Indicate the position of the carbonyl group with the lowest possible number.
● Add the suffix "-one" to the name of the parent hydrocarbon.
Examples of IUPAC Names:
● Propanone: The simplest ketone, also known as acetone.
● Butanone: A ketone with four carbon atoms, also known as methyl ethyl ketone (MEK).
Properties of Aldehydes and Ketones:
Aldehydes and ketones possess several important properties that arise from the presence of the carbonyl group.
● Polar Nature: The carbonyl group is a polar functional group, with oxygen being more electronegative than carbon. This polarity gives rise to dipole-dipole interactions and influences the physical and chemical properties of aldehydes and ketones.
● Boiling Points: Aldehydes and ketones have higher boiling points compared to hydrocarbons of similar molecular weight due to the presence of polar carbonyl groups, which promote intermolecular interactions.
● Solubility: Aldehydes and ketones with smaller carbon chains are soluble in water to some extent due to the ability to form hydrogen bonds with water molecules. However, solubility decreases with increasing carbon chain length.
● Odour: Many aldehydes and ketones have distinctive odours, which contribute to their use in perfumes and flavourings.
Reactivity of Aldehydes and Ketones:
Aldehydes and ketones exhibit characteristic reactivity due to the presence of the carbonyl group, which undergoes various reactions.
● Nucleophilic Addition Reactions: Aldehydes and ketones can undergo nucleophilic addition reactions, where a nucleophile adds to the carbonyl carbon, resulting in the formation of a new carbon-oxygen bond. This reaction is important in the synthesis of alcohols, ethers, and other functional groups.
● Oxidation Reactions: Aldehydes can be further oxidised to carboxylic acids, while ketones are relatively resistant to oxidation under normal conditions.
● Reduction Reactions: Aldehydes and ketones can be reduced to form primary and secondary alcohols, respectively, using reducing agents such as sodium borohydride (NaBH4) or lithium aluminium hydride (LiAlH4).
● Condensation Reactions: Aldehydes and ketones can undergo condensation reactions, such as the formation of hemiacetals and acetals, with alcohols in the presence of acid catalysts.
Applications of Aldehydes and Ketones:
Aldehydes and ketones are widely utilised in various fields, including:
● Organic Synthesis: They serve as important intermediates in the synthesis of complex organic compounds and pharmaceuticals.
● Flavourings and Fragrances: Many aldehydes and ketones contribute to the flavours and fragrances of foods, beverages, and perfumes.
● Industrial Solvents: Some ketones, such as acetone and MEK, are widely used as solvents in various industries.
● Polymer Production: Aldehydes and ketones are employed in the synthesis of polymers, such as polyesters and polycarbonates.
Aldehydes and ketones are essential classes of organic compounds with diverse applications. Understanding their nomenclature and properties, including their systematic naming conventions and characteristic reactivity, enables chemists to effectively identify, synthesise, and utilise these compounds in various fields, ranging from organic synthesis to flavourings, fragrances, and industrial processes.