Aldehydes, Ketones and Carboxylic Acids

In this chapter: nomenclature and structure of the carbonyl group, preparation of aldehydes and ketones, nucleophilic addition, reduction and oxidation, tests with Tollens' and Fehling's reagents, the haloform reaction, aldol and Cannizzaro reactions, electrophilic substitution, and the preparation, acidity and reactions of carboxylic acids.

Nomenclature and structure

The carbonyl carbon is sp² hybridised and forms three σ bonds in a plane at about 120°, plus a π bond with oxygen. Oxygen is more electronegative, so the C=O bond is polar: the carbon is an electrophile (Lewis acid) and the oxygen a nucleophile (Lewis base). This polarity explains almost everything that follows.

Preparation of aldehydes and ketones

• Oxidation of alcohols: 1° alcohols give aldehydes (PCC stops the oxidation there); 2° alcohols give ketones.
• Dehydrogenation: alcohol vapour over heated Cu or Ag gives aldehydes (1°) or ketones (2°).
• Ozonolysis of alkenes: O₃ followed by Zn and water splits the double bond into two carbonyl compounds.
• Hydration of alkynes: with HgSO₄ and dilute H₂SO₄, ethyne gives ethanal; all other alkynes give ketones.
• Rosenmund reduction: acyl chloride + H₂ over Pd on BaSO₄ (the poisoned catalyst stops reduction at the aldehyde).
• Stephen reaction: a nitrile is reduced by SnCl₂ and HCl to an imine, which is hydrolysed to the aldehyde. Nitriles and esters are also reduced to aldehydes by DIBAL-H (diisobutylaluminium hydride).
• Aromatic aldehydes from hydrocarbons: toluene with chromyl chloride (CrO₂Cl₂) gives benzaldehyde (Etard reaction); toluene with CrO₃ in acetic anhydride gives benzylidene diacetate, which is hydrolysed; side-chain chlorination to benzal chloride followed by hydrolysis; benzene with CO and HCl in the presence of anhydrous AlCl₃ or CuCl (Gatterman-Koch reaction).
• Ketones: acyl chloride with a dialkylcadmium (2R′COCl + R₂Cd → 2R′COR + CdCl₂); a nitrile with a Grignard reagent followed by hydrolysis; Friedel-Crafts acylation of benzene with an acyl chloride and anhydrous AlCl₃.

Aldehydes and ketones boil higher than hydrocarbons and ethers of similar mass, because of dipole-dipole attractions, but lower than alcohols, since they cannot hydrogen bond with each other. The lower members dissolve in water by forming hydrogen bonds with it.

Nucleophilic addition

A nucleophile attacks the electrophilic carbonyl carbon from a direction roughly perpendicular to the plane of the sp² orbitals. The carbon becomes sp³ and a tetrahedral alkoxide intermediate forms, which takes up a proton to give the addition product.

Aldehydes are more reactive than ketones. Two alkyl groups in a ketone crowd the carbonyl carbon (steric effect) and reduce its positive charge by their +I effect. Benzaldehyde is less reactive than propanal, because the ring delocalises electrons towards the carbonyl carbon.

Reduction and oxidation

• To alcohols: NaBH₄, LiAlH₄ or catalytic hydrogenation. Aldehydes give 1° and ketones 2° alcohols.
• To hydrocarbons (C=O → CH₂): Clemmensen reduction with zinc amalgam and concentrated HCl; Wolff-Kishner reduction with hydrazine followed by heating with KOH or sodium ethoxide in a high boiling solvent such as ethylene glycol.
• Oxidation: aldehydes are oxidised to acids with the same number of carbon atoms, even by mild oxidants. Ketones need strong oxidants and high temperature, and then the carbon chain breaks, giving a mixture of acids with fewer carbon atoms.
• Tollens' test: warming an aldehyde with ammoniacal silver nitrate gives a silver mirror: RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag + 2H₂O + 4NH₃.
• Fehling's test: Fehling's solution A (aqueous CuSO₄) is mixed with solution B (alkaline sodium potassium tartrate, Rochelle salt). Aliphatic aldehydes give a reddish brown precipitate of Cu₂O. Aromatic aldehydes do not respond to Fehling's test.
• Haloform reaction: compounds with a CH₃CO- group (methyl ketones and ethanal), or alcohols that are oxidised to them (CH₃CH(OH)- group, including ethanol), react with sodium hypohalite to give haloform. With I₂ and NaOH, yellow iodoform (CHI₃) precipitates: the iodoform test. The C=C bond, if present, is not attacked.

Worked example: A compound C₃H₆O forms an orange precipitate with 2,4-DNP and gives a yellow precipitate with I₂/NaOH, but does not reduce Tollens' reagent. Identify it.
Solution: the 2,4-DNP test shows a C=O group. The two C₃H₆O carbonyl compounds are propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃). A negative Tollens' test rules out the aldehyde, and a positive iodoform test confirms a CH₃CO- group. The compound is propanone.

Reactions due to α-hydrogen: aldol and Cannizzaro

Hydrogen on the carbon next to C=O (α-hydrogen) is acidic, because the carbonyl group withdraws electrons and the resulting carbanion (enolate) is stabilised by resonance.

• Aldol reaction: aldehydes and ketones with at least one α-H, in dilute alkali, give β-hydroxy aldehydes (aldols) or β-hydroxy ketones (ketols). These lose water easily on heating to give α,β-unsaturated carbonyl compounds (aldol condensation products).
• Cross aldol: two different carbonyl compounds, both with α-H, give a mixture of four products. With one partner lacking α-H, the reaction is more useful: benzaldehyde and acetophenone give benzalacetophenone (1,3-diphenylprop-2-en-1-one).
• Cannizzaro reaction: aldehydes without α-H (HCHO, C₆H₅CHO) with concentrated alkali undergo self oxidation-reduction. One molecule is reduced to an alcohol and the other oxidised to the salt of an acid.

Electrophilic substitution: the carbonyl group is deactivating and meta-directing. Nitration of benzaldehyde gives 3-nitrobenzaldehyde.

Uses: 40% aqueous methanal (formalin) preserves biological specimens and is used to make bakelite and other resins; acetone and butanone are common solvents; benzaldehyde is used in perfumery and dyes.

Carboxylic acids

Preparation

• Oxidation of 1° alcohols and aldehydes with KMnO₄ or K₂Cr₂O₇.
• Vigorous oxidation of alkylbenzenes with chromic acid or acidic or alkaline KMnO₄: the whole side chain, whatever its length, becomes -COOH (benzoic acid). Primary and secondary alkyl groups are oxidised this way; a tertiary alkyl group is not affected.
• Hydrolysis of nitriles and amides (acid or base catalysed), of acyl halides and anhydrides (water), and of esters (acid, or alkali followed by acidification).
• Grignard reagent with CO₂ (dry ice), followed by acid: R-MgX → RCOOH, one carbon more than the alkyl halide.

Carboxylic acids boil higher than alcohols of comparable mass, because of more extensive hydrogen bonding; in the vapour phase and in aprotic solvents most exist as hydrogen-bonded dimers. The first four aliphatic acids mix with water in all proportions.

Acidity

The carboxylate ion is stabilised by resonance with the negative charge shared equally between two oxygen atoms. This makes carboxylic acids stronger than phenols (where the charge is delocalised onto less electronegative carbon atoms) and alcohols. Typical pKa values: HCOOH 3.75, C₆H₅COOH 4.19, CH₃COOH 4.76.

• Electron-withdrawing groups increase acidity and electron-donating groups decrease it.
• More halogens and more electronegative halogens mean a stronger acid: CF₃COOH > CCl₃COOH > CHCl₂COOH > ClCH₂COOH > CH₃COOH; and FCH₂COOH > ClCH₂COOH > BrCH₂COOH.
• The effect weakens with distance: 2-chlorobutanoic acid > 3-chlorobutanoic acid > 4-chlorobutanoic acid.
• HCOOH > C₆H₅COOH > CH₃COOH > CH₃CH₂COOH, since alkyl groups release electrons.

Reactions

Uses: methanoic acid in the rubber, textile, dyeing and leather industries; ethanoic acid as vinegar and as a solvent; sodium benzoate as a food preservative; higher fatty acids for soaps and detergents.

Common mistakes: (1) Writing an aldol reaction for HCHO or C₆H₅CHO; they have no α-H and give Cannizzaro. (2) Saying benzaldehyde gives Fehling's test. It gives Tollens' test but not Fehling's. (3) Forgetting that ethanol and ethanal give the iodoform test. (4) Confusing Clemmensen (acidic, Zn-Hg/HCl) and Wolff-Kishner (basic) conditions. (5) Using NaBH₄ to reduce a carboxylic acid.

JEE and NEET focus

• Named preparations: Rosenmund, Stephen, Etard, Gatterman-Koch, dialkylcadmium, DIBAL-H, Friedel-Crafts acylation.
• Relative reactivity towards nucleophilic addition, and the products with HCN, NaHSO₃, Grignard reagents and ammonia derivatives.
• Distinction tests: Tollens', Fehling's, iodoform, 2,4-DNP.
• Aldol, cross aldol and Cannizzaro: choosing which applies and writing products.
• Acidity order of substituted acids; decarboxylation, Kolbe electrolysis and HVZ reaction.

Practice questions