Carbonyls

Carbonyls refer to the carbonyl group, a carbon atom double-bonded to oxygen (C=O), a defining feature of many organic compounds. The carbonyl carbon is typically sp2-hybridized and planar; the C=O bond is highly polarized, with partial positive charge on carbon and partial negative charge on oxygen. This polarization makes the carbonyl carbon electrophilic and influences reactivity, spectroscopy, and physical properties.

The carbonyl family includes aldehydes and ketones, in which the C=O is the primary functional unit, and carboxylic acid derivatives such as esters, amides, anhydrides, and acyl chlorides. Aldehydes bear at least one hydrogen on the carbonyl carbon; ketones bear two carbon substituents. Inorganic chemistry also features metal carbonyls, where metal centers coordinate multiple carbonyl ligands (e.g., Fe(CO)5, Ni(CO)4).

Spectroscopic and physical characteristics: carbonyl groups show strong infrared absorptions (C=O stretches ~1650–1850 cm−1; shifts with substitution and conjugation). In 13C NMR, carbonyl carbons resonate around 160–210 ppm. Conjugation with double bonds or aromatics lowers the C=O frequency and can alter reactivity.

Reactivity: aldehydes are generally more reactive toward nucleophiles than ketones. Common transformations include nucleophilic additions (Grignard or cyanide), oxidation of primary alcohols to aldehydes and then acids, and reduction to alcohols. Carboxylic derivatives undergo acyl substitution (esters, amides, anhydrides, acyl chlorides). Hydrolysis and condensations such as esterification and aldol reactions are widespread.

Applications and occurrence: carbonyl groups are central in biology (sugars, metabolism), industry (solvents, flavors, polymers), and synthesis. Safety depends on the compound, as some carbonyls are irritants or toxic.