Section 1: Structure and Electronic Properties of the Carbonyl Group
The C=O group is the defining feature of aldehydes (R-CHO) and ketones (R-CO-R'). The carbonyl carbon is sp2-hybridized: three sigma bonds form a trigonal planar arrangement, and the remaining p orbital forms a pi bond with oxygen. Because oxygen has a much higher electronegativity than carbon, the C=O bond is polarized so the carbon bears a partial positive charge (). This electrophilicity is the driving force for nucleophilic addition. Reactivity in nucleophilic addition decreases from HCHO (most reactive) to ketones (least reactive) due to increasing electron donation and steric effects of alkyl substituents.
Section 2: Nucleophilic Addition Reactions
The mechanism of nucleophilic addition involves: (a) nucleophile attacks , (b) tetrahedral sp3 alkoxide intermediate forms (C=O pi bond breaks, charge goes to O), (c) protonation gives addition product. Important nucleophiles tested at NEET: HCN (gives cyanohydrin), Grignard reagents RMgX (give alcohols: primary/secondary/tertiary depending on HCHO/RCHO/R2CO), and hydride reagents (NaBH4, LiAlH4 → give alcohols).
Section 3: Reactions with NH3 Derivatives
Four NH3 derivatives undergo condensation (nucleophilic addition then elimination of water) with C=O: hydroxylamine → oximes (C=NOH); phenylhydrazine → phenylhydrazones (C=NNHPh); 2,4-DNP → orange/yellow 2,4-dinitrophenylhydrazone precipitate (used to detect C=O in both aldehydes and ketones); semicarbazide → semicarbazones (C=NNHCONH2). All form a new C=N bond as water is eliminated.
Section 4: Distinction Tests for Aldehydes vs Ketones
Four tests are critical: (1) 2,4-DNP test — positive for both; provides initial carbonyl detection. (2) Tollens' test (ammoniacal AgNO3) — silver mirror with aldehydes only; ketones give no reaction. (3) Fehling's test (alkaline Cu2+) — brick-red Cu2O precipitate with aldehydes only; ketones do not react. (4) Iodoform test — yellow CHI3 precipitate with methyl ketones, acetaldehyde, ethanol, and isopropanol. The testing hierarchy: 2,4-DNP first (detects C=O), then Tollens'/Fehling's (distinguishes aldehyde from ketone), then iodoform (identifies methyl ketone).
Section 5: Reduction of Carbonyl Compounds
Two categories of reduction: (A) to alcohol — NaBH4 (mild, selective for C=O, protic solvents) and LiAlH4 (stronger, dry ether); both give C-OH, retaining oxygen. (B) to methylene — Clemmensen (Zn-Hg/conc. HCl, acidic) and Wolff-Kishner (NH2NH2/KOH, basic, ethylene glycol); both completely remove oxygen. The choice of B method depends on the pH sensitivity of other functional groups in the molecule.
Section 6: Aldol Condensation
Requirement: alpha-hydrogen must be present. Mechanism: base (dilute NaOH) forms enolate from alpha-H; enolate attacks another carbonyl molecule; new C-C bond forms; beta-hydroxy carbonyl compound (aldol) produced. On heating: dehydration to alpha,beta-unsaturated carbonyl compound (conjugated system). Crossed aldol between non-enolizable (no alpha-H) and enolizable aldehydes gives directed product (enolate attacks the non-enolizable one).
Section 7: Cannizzaro Reaction
Requirement: NO alpha-hydrogen. Mechanism: concentrated OH- adds to one aldehyde molecule forming tetrahedral intermediate; hydride transferred to second aldehyde; one becomes carboxylate (oxidized), one becomes alcohol (reduced). Key examples: HCHO, C6H5CHO, (CH3)3CCHO. The crossed Cannizzaro (HCHO + C6H5CHO) preferentially oxidizes HCHO (more reactive) and reduces C6H5CHO to benzyl alcohol.
Section 8: Haloform Reaction
Methyl ketones and their oxidizable precursors react with I2/NaOH: CH3CO-R + 3I2 + 3NaOH → CHI3 + RCOONa. CHI3 is the yellow iodoform precipitate. Positive compounds: any CH3CO-R, CH3CHO, ethanol (→ CH3CHO in situ), isopropanol (→ acetone in situ). Negative: HCHO, methanol, diethyl ketone (CH3CH2COCH2CH3), propan-1-ol.