Organic Chemistry Class 12 Notes

IUPAC Nomenclature

The parent chain (longest continuous carbon chain) is numbered to give the lowest locant to the principal functional group. The suffix reflects the functional group; substituents are listed alphabetically with their locants.

• Priority order for suffixes:
• $-COOH > -SO_3H > -COOR > -COCl > -CONH_2 > -CN > -CHO > -CO- > -OH > -NH_2$
• .
• For multiple bonds use -ene, -yne; for functional groups use the appropriate suffix.
• Numbering: principal functional group gets the lowest possible number.

Reaction Mechanisms and Intermediates

• $\text{Carbocation:}$
• sp², planar, electron-deficient — stability order
• $3^\circ > 2^\circ > 1^\circ > \text{CH}_3^+$
• .
• $\text{Carbanion:}$
• electron-rich, pyramidal — stability order
• $\text{CH}_3^- > 1^\circ > 2^\circ > 3^\circ$
• .
• $\text{Free radical:}$
• stability
• $3^\circ > 2^\circ > 1^\circ$
• .
• SN1 favours tertiary substrates (carbocation intermediate); SN2 favours primary (concerted backside attack).

Haloalkanes and Haloarenes

Haloalkanes are formed by the replacement of H by halogen. Reactivity for SN2: $\text{R-I} > \text{R-Br} > \text{R-Cl} > \text{R-F}$. Haloarenes are much less reactive toward nucleophilic substitution than haloalkanes.

• Finkelstein (NaI in acetone) converts R-Cl to R-I.
• Swarts reaction (AgF/Hg₂F₂) converts R-Cl/R-Br to R-F.
• Sandmeyer reaction: aryl diazonium + CuCl/CuBr gives halobenzenes.
• Grignard reagent (RMgX) — a strong nucleophile and base.

Alcohols, Phenols and Ethers

Alcohols are distinguished by their boiling points (H-bonding) and acidity. Phenols are acidic because the phenoxide ion is resonance-stabilised; electron-withdrawing groups increase phenol acidity.

Aldehydes and Ketones

• Carbonyl carbon is attacked by nucleophiles; aldehydes are more reactive than ketones (less steric hindrance, more polar C=O).
• Tollens' reagent (ammoniacal AgNO₃) — aldehydes give a silver mirror, ketones do not.
• Fehling's test — aldehydes give a red Cu₂O precipitate; aromatic aldehydes do not.
• Iodoform test — positive for CH₃-CO- group and secondary alcohols that oxidise to it.
• Clemmensen (Zn-Hg/HCl) and Wolff-Kishner (NH₂NH₂/KOH) reduce a carbonyl to a methylene group.

Carboxylic Acids

Carboxylic acids are stronger acids than phenols and alcohols because the carboxylate anion is resonance stabilised. Electron-withdrawing groups (like -NO₂, -Cl) increase acidity; electron-donating groups decrease it.

• Decarboxylation: heating a sodium salt with soda-lime gives an alkane.
• Reduction with LiAlH₄ gives a primary alcohol.
• Acid chloride formation via PCl₅, PCl₃ or SOCl₂.
• Hell-Volhard-Zelinsky (HVZ) reaction: α-halogenation of acids with halogen + red phosphorus.

Amines

Amines are classified as primary, secondary or tertiary. Aliphatic amines are more basic than ammonia because alkyl groups donate electron density to nitrogen; aromatic amines (aniline) are much less basic.

• Basicity in gas phase:
• $3^\circ > 2^\circ > 1^\circ > \text{NH}_3$
• ; in aqueous phase solvation effects change the order.
• Hinsberg test distinguishes 1°, 2° and 3° amines using benzenesulphonyl chloride.
• Carbylamine reaction: a primary amine + CHCl₃ + KOH gives a foul-smelling isocyanide.
• Diazotisation of aromatic primary amines with NaNO₂ + HCl gives diazonium salts.

Solved Examples

Example: Arrange in order of basicity: aniline, methylamine, dimethylamine, trimethylamine (aqueous).

Solution: In aqueous medium the order is $2^\circ > 1^\circ > 3^\circ > \text{NH}_3 > \text{aniline}$. Dimethylamine (2°) is most basic; aniline is least basic due to resonance delocalisation of the N lone pair into the ring.