Aldehydes, ketones, and carboxylic acids constitute an important family of carbonyl compounds that occupy a central position in organic chemistry. Aldehydes and ketones contain the carbonyl group (C=O), with aldehydes having the carbonyl carbon attached to at least one hydrogen atom and ketones having it attached to two carbon groups. Carboxylic acids contain the carboxyl group (-COOH), which combines a carbonyl and a hydroxyl group. These compounds are widespread in nature, occurring in flavours, fragrances, preservatives, and metabolic intermediates.
The carbonyl group is polarised with the carbon carrying a partial positive charge, making it the site of nucleophilic attack. This polarity governs the characteristic reactions of aldehydes and ketones, particularly nucleophilic addition reactions. Aldehydes are generally more reactive than ketones because the carbonyl carbon of an aldehyde is more electrophilic, having only one alkyl group to hinder attack and release electron density.
The chapter covers the nomenclature, preparation, and reactions of these three classes, including oxidation and reduction, nucleophilic addition, aldol condensation, Cannizzaro reaction, and the distinctive reactions of carboxylic acids such as esterification and decarboxylation. Distinguishing tests such as Fehling's and Tollens' tests separate aldehydes from ketones, while the acidity of carboxylic acids explains their salt formation and derivative reactions.
Aldehydes are named by replacing the -e of the parent alkane with -al. The simplest aldehyde is methanal (HCHO), followed by ethanal (CH3CHO) and propanal. The -CHO group is given the lowest possible locant.
Ketones are named by replacing the -e of the parent alkane with -one. The simplest ketone is propanone (CH3COCH3), commonly called acetone. The position of the carbonyl group is indicated by a number.
Carboxylic acids are named by replacing the -e of the parent alkane with -oic acid. Methanoic acid (HCOOH), ethanoic acid (CH3COOH), and benzoic acid (C6H5COOH) are common examples.
The carbonyl group is planar with sp2 hybridisation at carbon. The electronegative oxygen atom polarises the C=O bond, making the carbon electrophilic and the oxygen nucleophilic.
Ozonolysis of alkenes cleaves the double bond to give carbonyl compounds.
The Rosenmund reduction converts acid chlorides to aldehydes using hydrogen over a poisoned palladium catalyst.
Nitriles react with Grignard reagents and then undergo hydrolysis to give ketones.
The carbonyl carbon is attacked by nucleophiles. Important examples include: - Addition of hydrogen cyanide: Forms cyanohydrins, used in the synthesis of alpha-hydroxy acids. $$RCHO + HCN \rightarrow RCH(OH)CN$$ - Addition of sodium bisulphite: Forms bisulphite addition compounds; used to purify aldehydes and ketones. - Addition of Grignard reagents: Give secondary alcohols from aldehydes and tertiary alcohols from ketones. - Addition of alcohols: Formation of hemiacetals and acetals.
Aldehydes reduce to primary alcohols and ketones to secondary alcohols using H2/Ni, NaBH4, or LiAlH4.
Aldehydes are easily oxidised to carboxylic acids and are strong reducing agents. They give positive Tollens' (silver mirror) and Fehling's tests, while ketones do not. Ketones resist oxidation and give a mixture of acids only under drastic conditions.
Aldehydes and ketones with at least one alpha-hydrogen undergo aldol condensation in the presence of a dilute base to form beta-hydroxy aldehydes or ketones, which on heating dehydrate to give alpha,beta-unsaturated carbonyl compounds. $$2CH_3CHO \xrightarrow{dil.\ NaOH} CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH=CHCHO$$
Aldehydes without alpha-hydrogens, such as HCHO and C6H5CHO, undergo disproportionation in the presence of concentrated alkali, giving an alcohol and a carboxylic acid. $$2HCHO \xrightarrow{conc.\ NaOH} CH_3OH + HCOONa$$
Benzaldehyde undergoes meta-directed electrophilic substitution because the -CHO group is deactivating.
Carboxylic acids are weak acids and dissociate in water: $$RCOOH \rightleftharpoons RCOO^- + H^+$$ The carboxylate ion is stabilised by resonance, making carboxylic acids more acidic than phenols and alcohols. Electron-withdrawing substituents increase acidity (chloroacetic acid > acetic acid), while electron-donating substituents decrease it. Formic acid is more acidic than acetic acid.
| Test/Reagent | Aldehyde | Ketone |
|---|---|---|
| Tollens' reagent | Silver mirror | No reaction |
| Fehling's solution | Red precipitate | No reaction |
| Sodium bisulphite | Addition compound | Addition compound |
| Iodoform test | Positive (CH3CHO) | Positive (CH3COCH3) |
| Oxidation | Easily oxidised | Resists oxidation |
| Product | Method | Reagent/Condition |
|---|---|---|
| Aldehyde | Rosenmund reduction | RCOCl + H2/Pd-BaSO4 |
| Aldehyde | Oxidation of 1° alcohol | Cu/O2 or PCC |
| Ketone | Oxidation of 2° alcohol | CrO3/H+ |
| Aldehyde/Ketone | Ozonolysis | O3 then Zn/H2O |
| Acid | Oxidation of aldehyde | K2Cr2O7/H2SO4 |
| Acid | Grignard + CO2 | RMgX then H3O+ |
The carbonyl compounds, including aldehydes, ketones, and carboxylic acids, are among the most important classes in organic chemistry. Their characteristic reactivity, dominated by the electrophilic carbonyl carbon, gives rise to nucleophilic addition reactions that are the foundation of numerous synthetic routes. Distinguishing reactions such as Tollens', Fehling's, and the iodoform test allow chemists to identify these compounds, while condensation reactions like the aldol and Cannizzaro reactions demonstrate carbon-carbon bond formation. Carboxylic acids, with their resonance-stabilised carboxylate ions, display the acidity and reactivity needed to form esters, acid chlorides, amides, and other derivatives. This chapter provides essential knowledge for understanding metabolic processes, pharmaceuticals, and polymers, and builds directly on the study of alcohols and haloalkanes to complete the picture of organic functional group chemistry.