Nomenclature, Nature of Carbonyl Group — Core Principles

Core Principles

The carbonyl group ($C=O$) is a central functional group, featuring a carbon double-bonded to an oxygen. This carbon is $sp^2$ hybridized, resulting in a trigonal planar geometry with bond angles of approximately $120^circ$.

Due to oxygen's higher electronegativity, the $C=O$ bond is highly polar, with a partial positive charge on carbon and a partial negative charge on oxygen, making the carbonyl carbon electrophilic. Compounds containing this group are classified as aldehydes if the carbonyl carbon is bonded to at least one hydrogen and an alkyl/aryl group ($R-CHO$), and ketones if it's bonded to two alkyl/aryl groups ($R-CO-R'$).

IUPAC nomenclature systematically names aldehydes by replacing the parent alkane's '-e' with '-al' (e.g., ethanal), always numbering the aldehyde carbon as C-1. Ketones are named by replacing '-e' with '-one', with the carbonyl carbon's position indicated by a number (e.

g., propan-2-one). Common names like 'formaldehyde' and 'acetone' are also prevalent. Understanding both naming systems and the inherent polarity of the carbonyl group is crucial for NEET.

Important Differences

vs Ketones

Aspect	This Topic	Ketones
Functional Group Position	Terminal (at the end of the carbon chain)	Internal (within the carbon chain)
Groups attached to Carbonyl Carbon	At least one hydrogen and one alkyl/aryl group ($R-CHO$)	Two alkyl/aryl groups ($R-CO-R'$)
IUPAC Suffix	'-al' (e.g., ethanal)	'-one' (e.g., propanone)
Carbonyl Carbon Numbering	Always C-1, position not explicitly stated in name (unless cyclic carbaldehyde)	Position must be indicated by a number (e.g., butan-2-one)
Oxidation	Easily oxidized to carboxylic acids	Resistant to oxidation (require strong oxidizing agents under harsh conditions, leading to C-C bond cleavage)
Reactivity towards Nucleophilic Addition	Generally more reactive (less steric hindrance, stronger electrophilicity)	Generally less reactive (more steric hindrance, weaker electrophilicity)

Aldehydes and ketones both contain the carbonyl group, but their structural differences lead to distinct chemical properties and nomenclature. Aldehydes have at least one hydrogen attached to the carbonyl carbon, making them terminal and easily oxidizable to carboxylic acids. Ketones have two alkyl or aryl groups, making them internal and more resistant to oxidation. These structural variations also influence their reactivity towards nucleophilic addition, with aldehydes typically being more reactive due to less steric hindrance and greater electrophilicity at the carbonyl carbon. IUPAC naming reflects these differences, using '-al' for aldehydes and '-one' for ketones, with specific numbering rules for each.