Carbonyl Group Structure:

• C=O carbon is sp2, planar, trigonal (120° angles)
• Strongly polar: $C^{delta+}$ (electrophilic), $O^{delta-}$ (nucleophilic oxygen)
• Reactivity in nucleophilic addition: HCHO > RCHO > R2CO (ketones)
• Both steric and electronic (+I) effects of alkyl groups reduce ketone reactivity

Nucleophilic Addition Mechanism:

• Nu- attacks $C^{delta+}$ → tetrahedral sp3 alkoxide (C-O- intermediate) → protonation → product
• Grignard: HCHO → 1° alcohol; RCHO → 2° alcohol; R2CO → 3° alcohol
• HCN addition gives cyanohydrins (useful carbon chain extension reactions)

NH3 Derivative Reactions (all condensations — loss of H2O):

• NH2OH → oxime (C=NOH)
• C6H5NHNH2 → phenylhydrazone (C=NNHPh)
• 2,4-DNP → orange/yellow precipitate (detects ALL C=O)
• H2NCONHNH2 → semicarbazone (C=NNHCONH2)

Distinction Tests — Three Rules to Memorize:

1. 2,4-DNP positive → C=O present (aldehyde OR ketone)
2. Tollens' positive → aldehyde (silver mirror); Fehling's positive → aldehyde (red Cu2O)
3. Ketones give NEGATIVE Tollens' and Fehling's tests

Reduction Rules:

• NaBH4 / LiAlH4 → C-OH (alcohol; oxygen retained) — mild
• Clemmensen (Zn-Hg/HCl) → CH2 (no oxygen; acidic conditions)
• Wolff-Kishner $\frac{NH2NH2}{KOH}$ → CH2 (no oxygen; basic conditions)
• Choose Clemmensen if molecule is acid-tolerant; Wolff-Kishner if base-tolerant

The Alpha-Hydrogen Rule (most important rule in OC-06):

• Alpha-H present + dilute NaOH → Aldol condensation → beta-hydroxy carbonyl → (heat) → alpha,beta-unsaturated carbonyl
• No alpha-H + concentrated NaOH → Cannizzaro → RCOO-Na+ + RCH2OH (disproportionation)
• No exceptions: this rule determines the reaction type

Haloform/Iodoform Test:

• CH3CO-R + 3I2 + 3NaOH → CHI3 (yellow ppt) + RCOONa
• Positive for: CH3CHO, all CH3CO-R, ethanol, isopropanol, acetophenone
• Negative for: HCHO, CH3OH, C6H5CHO, diethyl ketone, propan-1-ol

Cannizzaro Substrates (no alpha-H):

• Formaldehyde (HCHO): no alpha-carbon
• Benzaldehyde (C6H5CHO): alpha-carbon is aromatic ring (no aliphatic C-H)
• Pivaldehyde ((CH3)3CCHO): alpha-carbon is quaternary (no H)