Chapter 8 — Aldehydes, Ketones and Carboxylic Acids
One small group, C=O, gives perfumes, preservatives, vinegar and soap their chemistry. How aldehydes and ketones are made and how they react, the tests that tell them apart, and why carboxylic acids are acids.
Open a bottle of vanilla essence, crush a cinnamon stick, or peel a bitter almond, and the smell you notice comes from an aldehyde: vanillin, cinnamaldehyde and benzaldehyde. The nail-polish remover on a dressing table is propanone (acetone), a ketone; so are the camphor in a cold rub and the muscone of musk perfumes. Formalin, a solution of methanal (formaldehyde) in water, preserves biological specimens, and methanal is also a raw material for adhesives and plastics. Vinegar owes its sourness to ethanoic acid, the sting of an ant bite to methanoic acid, and soap is made of the sodium salts of long-chain carboxylic acids.
All of these compounds contain the carbonyl group, $\mathrm{C{=}O}$. Its carbon is electron-poor and its oxygen electron-rich, and almost everything in this chapter follows from that polarity. You will first name aldehydes and ketones and learn the reactions that make them. Then you will see how nucleophiles add to the carbonyl carbon, how aldehydes are oxidised so easily that simple test-tube reactions (Tollens', Fehling's) pick them out, and how the hydrogen atoms next to the carbonyl group make the aldol and related reactions possible. Finally, joining an $\mathrm{OH}$ to the carbonyl carbon gives the carboxylic acids, whose acidity you will explain and compare with that of phenols and alcohols.
| Question | Key idea | Section |
|---|---|---|
| Where does a nucleophile attack? | $\mathrm{\overset{\delta+}{C}{=}\overset{\delta-}{O}}$: at the carbon | 8.1, 8.2 |
| How do I make an aldehyde without over-oxidising it? | PCC; Rosenmund; Stephen; DIBAL-H | 8.1 |
| Is it an aldehyde or a ketone? | Tollens' silver mirror, Fehling's red precipitate | 8.2 |
| Aldol or Cannizzaro? | α-hydrogen present or absent | 8.2 |
| Which acid is stronger? | smaller $\mathrm{p}K_{a}$; $-I$ groups stabilise $\mathrm{RCOO^{-}}$ | 8.3 |
- give IUPAC and common names of aldehydes, ketones and carboxylic acids, and explain the structure and polarity of the carbonyl group;
- prepare aldehydes and ketones from alcohols, alkenes, alkynes, acyl chlorides, nitriles, esters and aromatic hydrocarbons, naming the reagent for each step;
- write the mechanism of nucleophilic addition, explain why aldehydes are more reactive than ketones, and predict the products with HCN, sodium hydrogensulphite, Grignard reagents, alcohols and ammonia derivatives;
- reduce carbonyl compounds to alcohols or hydrocarbons (Clemmensen, Wolff–Kishner), and distinguish aldehydes, ketones and methyl ketones by Tollens', Fehling's and iodoform tests;
- predict aldol, cross-aldol and Cannizzaro products from the presence or absence of α-hydrogens;
- prepare carboxylic acids, rank their acid strengths from substituent effects, and write their reactions: salts, anhydrides, esters, acyl chlorides, amides, reduction, decarboxylation, the Hell–Volhard–Zelinsky reaction and ring substitution.
Sections in this chapter
Aldehydes and Ketones: Structure and Preparation
Naming, the structure and polarity of the carbonyl group, preparation from alcohols, alkenes, alkynes, acyl chlorides, nitriles, esters and aromatic hydrocarbons, and physical properties.
Reactions of Aldehydes and Ketones
Nucleophilic addition and addition–elimination, reduction and oxidation, Tollens', Fehling's and iodoform tests, aldol and Cannizzaro reactions, and ring substitution.
Carboxylic Acids: Preparation, Acidity and Reactions
Naming and preparation, hydrogen-bonded dimers, acid strength and substituent effects, and the reactions of the carboxyl group and of the ring.