1. Aldol Condensation & Cross-Aldol Reaction

Eligibility: Aldehydes or ketones possessing at least one α-hydrogen atom (e.g., CH3CHO, CH3COCH3). Reagent: Dilute alkali (dilute NaOH or Ba(OH)2). Reaction: Two molecules of aldehyde/ketone condense via enolate ion attack to form a β-hydroxyaldehyde (aldol) or β-hydroxyketone (ketol). Upon gentle heating (Δ) with acid/base, dehydration occurs yielding an α,β-unsaturated carbonyl compound (e.g., 2 CH3CHO + dil. NaOH -> CH3-CH(OH)-CH2-CHO -> [Δ] CH3-CH=CH-CHO, but-2-enal). Cross-Aldol: Condensation between two different carbonyl compounds yields a mixture of up to 4 distinct products unless one reactant lacks α-hydrogens (e.g., Benzaldehyde C6H5CHO + Acetaldehyde CH3CHO yields cinnamaldehyde C6H5-CH=CH-CHO as major product).

2. Cannizzaro Reaction & Cross-Cannizzaro

Eligibility: Aldehydes having NO α-hydrogen atoms (e.g., Formaldehyde HCHO, Benzaldehyde C6H5CHO, Trimethylacetaldehyde (CH3)3C-CHO). Reagent: Concentrated alkali (50% NaOH or KOH). Reaction: Disproportionation (self-redox) reaction where one molecule is reduced to a primary alcohol while the other is oxidized to a carboxylic acid salt. Example: 2 HCHO + 50% NaOH -> CH3OH (Methanol) + HCOONa (Sodium formate); 2 C6H5CHO + conc. NaOH -> C6H5CH2OH (Benzyl alcohol) + C6H5COONa (Sodium benzoate). Cross-Cannizzaro: When reacted with Formaldehyde (HCHO + C6H5CHO), formaldehyde is preferentially oxidized to formate due to faster nucleophilic attack of OH- on its unhindered carbonyl carbon, leaving the other aldehyde reduced to alcohol.

3. Hydrocarbon Syntheses: Wurtz, Fittig & Wurtz-Fittig Reactions

Alkyl and aryl halide coupling mediated by sodium metal in dry ether: (1) Wurtz Reaction: 2 R-X + 2 Na (dry ether) -> R-R + 2 NaX. Best suited for preparing symmetrical alkanes with an even number of carbon atoms (e.g., 2 CH3CH2Br -> n-butane). Using mixed alkyl halides gives an inseparable mixture of three alkanes. (2) Fittig Reaction: 2 Ar-X + 2 Na (dry ether) -> Ar-Ar (Biphenyl / Diphenyl) + 2 NaX. (3) Wurtz-Fittig Reaction: Ar-X + R-X + 2 Na (dry ether) -> Ar-R (Alkylarene, e.g., Bromobenzene + Methyl bromide -> Toluene C6H5CH3) + 2 NaX.

4. Phenol Reactions: Reimer-Tiemann & Kolbe's Synthesis

Electrophilic aromatic substitution on activated phenoxide rings: (1) Reimer-Tiemann Reaction: Phenol treated with chloroform (CHCl3) in the presence of aqueous NaOH at 340 K, followed by acid hydrolysis, introduces an aldehyde group (-CHO) ortho to the phenolic -OH group, producing Salicylaldehyde (2-hydroxybenzaldehyde). The electrophile is dichlorocarbene (:CCl2). If CCl4 is used instead of CHCl3, Salicylic acid is formed. (2) Kolbe's Reaction (Kolbe-Schmitt): Phenol is treated with NaOH to form sodium phenoxide, which reacts with carbon dioxide (CO2, a weak electrophile) under 4-7 atm pressure at 400 K, followed by acidification to produce Salicylic acid (2-hydroxybenzoic acid), the key precursor for Aspirin (acetylsalicylic acid).

5. Carbonyl Deoxygenation: Clemmensen vs Wolff-Kishner Reduction

Direct reduction of carbonyl groups (>C=O) into methylene groups (>CH2): (1) Clemmensen Reduction: Reagents: Zinc amalgam and concentrated hydrochloric acid (Zn-Hg / conc. HCl). Aldehydes/ketones are converted directly into corresponding alkanes (e.g., CH3COCH3 -> CH3CH2CH3). Caution: Unsuitable for substrates containing acid-sensitive functional groups (e.g., acetals, -OH, C=C double bonds). (2) Wolff-Kishner Reduction: Reagents: Hydrazine (NH2NH2) followed by heating with KOH in a high-boiling solvent like ethylene glycol at 453-473 K. Caution: Unsuitable for base-sensitive groups (e.g., esters, halides). Together, these complementary reactions ensure conversion regardless of acid/base sensitivity.

6. Nitrogen Compounds: Hoffmann Bromamide & Gabriel Phthalimide

High-yield primary amine preparations: (1) Hoffmann Bromamide Degradation: Primary amide (R-CONH2) heated with bromine (Br2) and aqueous or ethanolic NaOH/KOH: R-CONH2 + Br2 + 4 NaOH -> R-NH2 + Na2CO3 + 2 NaBr + 2 H2O. Crucial exam feature: The resulting primary amine contains EXACTLY ONE LESS CARBON ATOM than the parent amide (step-down reaction). Intermediate involves an isocyanate (R-N=C=O). (2) Gabriel Phthalimide Synthesis: Potassium phthalimide reacted with alkyl halide (R-X) followed by alkaline hydrolysis produces pure primary aliphatic amines (R-NH2). Cannot prepare aromatic primary amines (aniline) because aryl halides do not undergo nucleophilic substitution with phthalimide anion.