Chemistry · Ch 12 — Carbonyl Compounds and Carboxylic Acids
Chemical Properties of Aldehydes and Ketones
Chemical Properties of Aldehydes and Ketones
The carbonyl carbon's partial positive charge (Section 12.2) makes it the reactive site for essentially every reaction in this long section, and the reactions cluster into six mechanistic families, each covered as its own sub-topic below: (A) nucleophilic addition -- the single most characteristic carbonyl reaction, in which a nucleophile adds directly across the C=O pi bond; (B) oxidation -- of the aldehyde's own C-H to a carboxylic acid, or of a ketone's flanking C-C bond under forcing conditions; (C) reduction -- to an alcohol, all the way to a hydrocarbon, or (for ketones specifically) to a symmetrical diol (pinacol); (D) the haloform reaction, a special halogenation-plus-cleavage sequence unique to methyl ketones and acetaldehyde; (E) reactions at the alpha-carbon, principally a …
Haloform Reaction
Acetaldehyde and methyl ketones -- any carbonyl compound containing the CH3-CO- group -- when treated with a halogen and alkali, give the corresponding haloform (CHX3); this specific halogenation-plus-cleavage sequence is called the haloform reaction. Mechanistically, NaOH first drives successive alpha-halogenations of the methyl ketone's own methyl group (each alpha-hydrogen being progressively more acidic than the last, since each added halogen further activates the remaining ones), giving the fully trihalogenated ketone CX3-CO-R; the strongly electron-withdrawing CX3 group then makes that carbonyl carbon so electrophilic that hydroxide itself attacks it directly, and the resulting tetrahedral intermediate expels the stabilised CX3- carbanion (immediately protonated to CHX3, the haloform) rather than expelling hydroxide back out. Worked example: acetone (CH3-CO-CH3), treated with excess Cl2/NaOH, is first converted via CCl3-CO-CH3 and finally cleaves to give chloroform (CHCl3) + sodium acetate (CH3-COONa). Because the reaction absolutely requires the CH3-CO- fragment (or, equivalently for an alcohol precursor, a CH3-CH(OH)- fragment that NaOX first oxidises to the methyl ketone in situ), it doubles as a class …