Q.Describe the following :

Four named carbonyl-chemistry reactions — ester formation, base-induced disproportionation of an α-H-less aldehyde, crossed aldol condensation, and loss of CO₂ from a carboxylate — each described with its defining example.

1. Esterification: The acid-catalysed reaction of a carboxylic acid with an alcohol to form an ester and water. Concentrated H₂SO₄ or dry HCl gas is used as catalyst, and the reaction is reversible: RCOOH + R'OH ⇌(H⁺, Δ) RCOOR' + H₂O e.g. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl acetate).
2. Cannizzaro reaction: Aldehydes that do NOT have an α-hydrogen atom (e.g. HCHO, C₆H₅CHO — benzaldehyde) cannot undergo aldol condensation. Instead, when treated with concentrated alkali (NaOH/KOH), they undergo a self oxidation–reduction (disproportionation): one molecule of the aldehyde is reduced to the corresponding alcohol while a second molecule is simultaneously oxidised to the carboxylate salt. e.g. 2HCHO + conc. NaOH → CH₃OH + HCOONa
3. Cross aldol condensation: An aldol condensation carried out between two DIFFERENT aldehydes and/or ketones (at least one of which has an α-hydrogen), in the presence of dilute base (or acid). Because either partner can act as the nucleophilic enolate (donor) or as the electrophilic carbonyl (acceptor), a cross aldol condensation typically gives a mixture of up to four different aldol products, unlike a simple (self) aldol condensation of one carbonyl compound, which gives just one product. e.g. CH₃CHO + CH₃COCH₃ --dil.NaOH--> a mixture of aldol products.
4. Decarboxylation: The loss of a molecule of carbon dioxide (CO₂) from a carboxylic acid (usually from its sodium salt), giving an alkane with one carbon atom fewer than the original acid. It is carried out by heating the sodium salt of the carboxylic acid with soda lime (NaOH/CaO): RCOONa + NaOH --CaO, Δ--> RH + Na₂CO₃ e.g. CH₃COONa + NaOH --CaO, Δ--> CH₄ + Na₂CO₃ …