JEE Chemistry Aldehydes Ketones and Carboxylic Acids Notes Set 05

Preparation of Aldehydes and Ketones

1. Oxidation of primary alcohols (PCC) gives aldehydes and of secondary alcohols gives ketones. Dehydrogenation over Cu at 573 K does the same.
2. Ozonolysis of alkenes and hydration of alkynes: ethyne gives acetaldehyde and other alkynes give ketones (dil. H₂SO₄ with Hg²⁺).
3. Rosenmund reduction: RCOCl + H₂ over Pd/BaSO₄ gives RCHO. Stephen reaction: RCN with SnCl₂/HCl then hydrolysis gives RCHO. DIBAL-H reduces nitriles and esters to aldehydes.
4. Ketones: acid chlorides react with dialkyl cadmium or a Grignard reagent reacts with a nitrile (then hydrolysis) to give ketones. Friedel–Crafts acylation gives aryl ketones.
5. Aromatic aldehydes: Etard reaction (toluene with CrO₂Cl₂ then H₃O⁺), Gattermann–Koch (benzene, CO, HCl, AlCl₃/CuCl) and side-chain chlorination of toluene followed by hydrolysis.
6. Calcium salts of carboxylic acids on dry distillation give ketones; with calcium formate the product is an aldehyde.

Nucleophilic Addition Reactions

1. The carbonyl carbon is electron deficient, so aldehydes and ketones undergo nucleophilic addition. Aldehydes are more reactive than ketones because of less steric hindrance and less electron donation to the carbonyl carbon. Aromatic aldehydes are less reactive than aliphatic aldehydes because of delocalisation.
2. HCN gives cyanohydrins, which are racemic when a new stereocentre is formed. NaHSO₃ gives crystalline bisulphite adducts with aldehydes and methyl ketones, which can be used to purify carbonyl compounds.
3. Reactions with ammonia derivatives (NH₂OH gives oximes, NH₂NH₂ hydrazones, phenylhydrazine phenylhydrazones, semicarbazide semicarbazones, and 2,4-dinitrophenylhydrazine (Brady's reagent) orange precipitates) are addition–elimination condensation reactions. Primary amines give imines (Schiff bases).
4. Grignard reagents add to carbonyl compounds and hydrolysis gives alcohols, with HCHO giving primary, other aldehydes secondary and ketones tertiary alcohols.
5. Reduction: NaBH₄ and LiAlH₄ give alcohols, H₂/Pd reduces C=C as well, Clemmensen reduction (Zn–Hg/conc. HCl) and Wolff–Kishner reduction (NH₂NH₂, KOH, glycol, heat) convert C=O to CH₂.

Reactions Involving Alpha Hydrogen and Oxidation

1. Aldol condensation: aldehydes and ketones with α-hydrogen give β-hydroxy carbonyl compounds (aldols) in dilute alkali, and heating gives α,β-unsaturated compounds. Two acetaldehyde molecules give 3-hydroxybutanal and then crotonaldehyde. A crossed aldol between benzaldehyde and acetaldehyde gives cinnamaldehyde.
2. Intramolecular aldol reactions of dicarbonyl compounds form five- and six-membered rings (hexanedial gives cyclopentenecarbaldehyde).
3. Cannizzaro reaction: aldehydes without α-hydrogen (HCHO, benzaldehyde) disproportionate in conc. alkali to an alcohol and a carboxylate salt. In a crossed Cannizzaro reaction the formaldehyde is oxidised to formate.
4. Haloform (iodoform) test: compounds with CH₃CO– or CH₃CH(OH)– groups give yellow CHI₃ with I₂/NaOH. Acetaldehyde is the only aldehyde that gives it, and ethanol is the only primary alcohol.
5. Tollens' reagent ([Ag(NH₃)₂]⁺) gives a silver mirror with aldehydes. Fehling's solution gives red Cu₂O with aliphatic aldehydes but not with ketones or aromatic aldehydes. Schiff's reagent gives pink colour with aldehydes.
6. Strong oxidants (KMnO₄, K₂Cr₂O₇) oxidise aldehydes to acids and break ketones into acids with fewer carbons.