Organic Chemistry
Organic Chemistry
Organic chemistry in NEB Class 12 covers haloalkanes and haloarenes, alcohols, phenols, ethers, aldehydes, ketones, carboxylic acids and amines. The exam rewards correct IUPAC names, reagent-based distinctions and mechanism arrows with the right intermediates.
Core rules
- Haloalkanes: SN1 is favoured by 3° carbocation stability and polar protic solvents; SN2 by 1° halides and strong nucleophiles, with inversion of configuration.
- Alcohols: Lucas test distinguishes 3° (immediate cloudiness), 2° (on warming) and 1° (no reaction at room temperature). Phenol is more acidic than alcohol because the phenoxide ion is resonance-stabilised.
- Carbonyls: aldehydes reduce Tollens' and Fehling's reagents; ketones do not. Iodoform test is positive for CH3CO– groups.
- Carboxylic acids: acidity rises with electron-withdrawing groups; acid derivatives hydrolyse in the order acyl chloride > anhydride > ester > amide.
- Amines: basicity depends on lone-pair availability; in aqueous solution, secondary amines often exceed primary and tertiary.
Worked example
An unknown C3H6O gives a positive iodoform test and a positive Brady's test. Identify it.
Reasoning: C3H6O has one degree of unsaturation, consistent with a carbonyl. Brady's reagent detects aldehydes and ketones, so the oxygen is a carbonyl. Iodoform requires a CH3–CO– unit. Propanone, CH3COCH3, fits both; propanal would give a positive Brady test but not iodoform.
Traps
Writing propanal as C3H6O and stopping there; using Tollens' for ketones; confusing Lucas with iodoform; forgetting that phenol does not give the alcohol tests; and drawing SN2 without showing the transition state or inversion.