Chemistry · Ch 8 — Aldehydes, Ketones and Carboxylic Acids

Summary

Carbonyl group ( $\ce{C=O}$ ) is the functional core: aldehydes have it at chain end ( $\ce{-CHO}$ ), ketones within chain ( $\ce{-CO-}$ ).
Nomenclature: suffix -al for aldehydes, -one for ketones; carboxylic acids use -oic acid ( $\ce{-COOH}$ ).
Preparation:
- Aldehydes from oxidation of primary alcohols (PCC, $\ce{CrO3}$ ), Rosenmund reduction ( $\ce{RCOCl + H2/Pd-BaSO4}$ ), or hydration of alkynes (Markovnikov with $\ce{Hg^{2+}/H+}$ ).
- Ketones from oxidation of secondary alcohols, Friedel–Crafts acylation, or hydration of alkynes (anti-Markovnikov via $\ce{B2H6}$ then $\ce{H2O2/OH-}$ ).
- Carboxylic acids from oxidation of primary alcohols/aldehydes, hydrolysis of nitriles ( $\ce{RCN + H3O+}$ ), or Grignard with $\ce{CO2}$ .
Nucleophilic addition (aldehydes/ketones): $\ce{C=O}$ $C = O$ is polar; $\ce{CN-}$ $CN X^{-}$ , $\ce{HSO3-}$ $HSO X_{3} X^{-}$ , $\ce{NH3}$ $NH X_{3}$ derivatives add to form cyanohydrins, bisulfite adducts, imines.
- Aldehydes are more reactive than ketones (less steric hindrance, greater $\delta+$ on carbon).
Reduction: $\ce{NaBH4}$ or $\ce{LiAlH4}$ gives alcohols; Clemmensen ( $\ce{Zn-Hg/HCl}$ ) or Wolff–Kishner ( $\ce{N2H4/KOH}$ ) reduces $\ce{C=O}$ to $\ce{CH2}$ .
Oxidation:
- Aldehydes oxidise to acids (Tollens’ test: silver mirror; Fehling’s test: red $\ce{Cu2O}$ ).
- Ketones resist mild oxidation; strong oxidants cleave $\ce{C-C}$ bonds.
Alpha-hydrogen reactions: enolate formation leads to aldol condensation (crossed or self), haloform reaction ( $\ce{CH3CO-}$ gives $\ce{CHX3}$ ).
Carboxylic acids: acidic ( $\ce{pKa \approx 4-5}$ $p Ka \approx 4 - 5$ ) due to resonance-stabilised conjugate base; form salts, esters, amides, acid chlorides, anhydrides.
- Decarboxylation: $\ce{RCOOH}$ loses $\ce{CO2}$ on heating with soda lime ( $\ce{NaOH/CaO}$ ) or via Hunsdiecker (Ag salt + $\ce{Br2}$ ).
Distinguishing tests: …

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