Organic Chemistry MCQs

Correct: B — Hydrogen bonding through O–H group. Alcohols can act as both hydrogen bond donors (O–H) and acceptors (lone pair on O), forming strong intermolecular H-bonds. This requires extra energy to break → high bp. Ethers have no O–H, so they cannot form H-bonds with themselves (only accept H-bonds from other molecules). Example: ethanol (bp 78°C) vs dimethyl ether (bp −23°C), both C₂H₆O. Alcohols are also soluble in water due to H-bonding. Lower alcohols (up to C₄) are miscible with water in all proportions. Solubility decreases as alkyl chain length increases.

Correct: C — Tertiary alcohol gives immediate turbidity. Lucas reagent = anhydrous ZnCl₂ + conc. HCl. The reaction is SN1 (ZnCl₂ coordinates to OH, making it a better leaving group, then carbocation forms). Tertiary alcohol → most stable tertiary carbocation → fastest reaction → immediate turbidity (insoluble alkyl chloride). Secondary alcohol → 5 min delay. Primary alcohol → no reaction at room temperature (no stable carbocation; requires heating). Methanol and ethanol do not react. This is a qualitative test to classify alcohols up to C₅ (higher alcohols are already insoluble).

Correct: A — 3° > 2° > 1°. Dehydration of alcohols (E1 mechanism via carbocation): tertiary carbocations are most stable → formed most easily → 3° alcohols dehydrate at lowest temperature. Example: tert-butyl alcohol dehydrates at ~85°C; ethanol requires ~170°C. With conc. H₂SO₄ at 170°C, ethanol gives ethene; at 140°C it gives diethyl ether (intermolecular dehydration). For unsymmetrical alcohols, the major alkene product follows Zaitsev's rule (more substituted, more stable alkene is the major product).

Correct: D — Resonance stabilisation of phenoxide ion. When phenol loses H⁺, the phenoxide anion C₆H₅O⁻ has its negative charge delocalised over the oxygen AND the ortho/para positions of the ring (5 resonance structures). This stabilises the anion, making it easier to form → phenol is more acidic. Ethoxide (C₂H₅O⁻) has no such delocalisation. Electron-withdrawing groups (–NO₂) on the ring increase acidity; electron-donating groups (–CH₃) decrease it. Phenol is more acidic than alcohols but less acidic than carboxylic acids (pKa ~5) and carbonic acid (pKa ~6.4).

Correct: B — Ethers via Williamson synthesis. Sodium alkoxide (R–O⁻Na⁺) + R'X → R–O–R' + NaX. Mechanism: SN2 (the alkoxide attacks the alkyl halide). Used to prepare both symmetrical and unsymmetrical ethers. For unsymmetrical ethers, the alkoxide should be derived from the more hindered alcohol, and the alkyl halide should be primary (to avoid elimination). Example: CH₃ONa + C₂H₅Cl → CH₃OC₂H₅ (methyl ethyl ether). Phenyl ethers (ArOR) can be made from sodium phenoxide + alkyl halide: C₆H₅ONa + RX → C₆H₅OR. This is an SN2 reaction, so 3° alkyl halides give mainly elimination products.

Correct: C — Aldehyde. Primary alcohol oxidation: (1) Mild oxidising agent (PCC, MnO₂) → stops at aldehyde stage; (2) Strong oxidising agent (K₂Cr₂O₇/H₂SO₄, KMnO₄) → goes to carboxylic acid. Secondary alcohol → ketone (with any oxidising agent; ketones resist further oxidation). Tertiary alcohol → no oxidation (no α-H on the OH carbon). Methanol oxidation: CH₃OH → HCHO (formaldehyde) → HCOOH (formic acid). Ethanol (breathalyser): CH₃CH₂OH → CH₃CHO → CH₃COOH (acetic acid). The Fehling's/Tollens' test distinguishes aldehydes (positive) from ketones (negative).

Correct: A — Phenol reacts with NaOH (a base) to form sodium phenoxide + water. Phenol is acidic enough (pKa ~10) to react with NaOH: C₆H₅OH + NaOH → C₆H₅ONa + H₂O. Alcohols (pKa ~16–18) do not react with NaOH. Both react with Na metal to liberate H₂. Phenol gives a distinctive violet/purple colour with neutral FeCl₃ (not alcohols). Phenol undergoes electrophilic aromatic substitution at o/p positions easily (–OH is strongly activating). Phenol reacts with Br₂ water to give 2,4,6-tribromophenol (white precipitate) — used as a test for phenol.

Correct: D — 2,4,6-tribromophenol as white precipitate. The –OH group is a powerful activating and ortho/para-directing group in electrophilic aromatic substitution. With Br₂ water (aqueous bromine, no catalyst needed), all three available o/p positions are brominated instantly → 2,4,6-tribromophenol (white precipitate). This is used as a qualitative test for phenol. Contrast: bromination of benzene requires a Lewis acid catalyst (FeBr₃) and gives only monobromination. With CS₂ (non-polar solvent) at low temperature, bromination of phenol can be controlled to give mono products (ortho + para mixture).

Correct: B — Salicylic acid. Kolbe-Schmitt reaction: C₆H₅ONa + CO₂ (300°C, 5 atm) → sodium salicylate → acid workup → salicylic acid (2-hydroxybenzoic acid). Used industrially to make aspirin (acetylsalicylic acid = salicylic acid + acetic anhydride). Related reactions on phenol: Reimer-Tiemann reaction (CHCl₃ + NaOH) → salicylaldehyde (2-hydroxybenzaldehyde). Fries rearrangement: phenyl ester → (AlCl₃, heat) → o- and p-hydroxyacetophenone. Claisen rearrangement: allyl phenyl ether → (heat, no catalyst) → o-allylphenol.

Correct: C — Ether lacks O–H for self-association through H-bonds. Both have formula C₄H₁₀O. Butanol has –OH group → forms strong intermolecular H-bonds (O–H···O), requiring much more energy to vaporise → bp 117°C. Ether has C–O–C (no O–H) → cannot donate H-bonds → only weak van der Waals forces → bp 35°C. Ethers can act as H-bond acceptors (lone pairs on O) but not donors. This is why ethers are good solvents for Grignard reactions (dissolve polar Mg compounds but don't react with them). Ether is also less dense than water and immiscible.

Correct: A — Chloroethane. Alcohols react with phosphorus halides to give alkyl halides: 3ROH + PCl₃ → 3RCl + H₃PO₃; ROH + PCl₅ → RCl + POCl₃ + HCl; ROH + SOCl₂ → RCl + SO₂ + HCl (best method — volatile SO₂ and HCl gas easily removed). The PCl₅ method gives inversion of configuration at a chiral centre (SN2-like). The SOCl₂ method can give retention or inversion depending on solvent. All three methods convert –OH to –Cl. Similarly, PBr₃ gives alkyl bromides. These are better than using HX for sensitive substrates.

Correct: D — The explosive made from glycerol is glyceryl trinitrate (nitroglycerin, GTN), NOT TNT. TNT (trinitrotoluene) is made from toluene, not glycerol. Glycerol + conc. HNO₃/H₂SO₄ → glyceryl trinitrate (nitroglycerin), an explosive used in dynamite (absorbed on kieselguhr by Alfred Nobel). Glycerol (HOCH₂CHOHCH₂OH) is a byproduct of saponification (fats + NaOH → soap + glycerol). It is hygroscopic, viscous, and sweet — used in cosmetics, food, and pharmaceuticals as a humectant. It has three –OH groups → extensive H-bonding → very high bp (290°C) and high viscosity.

Correct: B — Alkyl iodide + alcohol first; with excess HI, both become alkyl iodides. Ethers resist most chemical reactions (strong C–O bonds + no acidic H + no leaving group). Strong acid HI (or HBr) can cleave ethers: R–O–R' + HI → R–I + R'–OH (first cleavage); R'–OH + HI → R'–I + H₂O (second cleavage with excess HI). The C–O bond of the more substituted alkyl group breaks (SN2 or SN1 depending on substitution). HF is weakest acid and least effective. HI > HBr > HCl in reactivity for ether cleavage. Phenyl ethers (ArOR): only the alkyl–O bond cleaves (ArO–R), never the Ar–O bond.

Correct: C — Acetic anhydride. Salicylic acid (2-hydroxybenzoic acid) + (CH₃CO)₂O → acetylsalicylic acid (aspirin) + CH₃COOH. The phenolic –OH is acetylated. Acetic anhydride is preferred over acetic acid (reaction is faster, goes to completion) and acetyl chloride (too reactive, may give side products). Aspirin is an analgesic, antipyretic, and anti-inflammatory drug (NSAID — inhibits cyclooxygenase enzyme, reducing prostaglandin synthesis). Methyl salicylate (oil of wintergreen) is the methyl ester of salicylic acid — used as a topical analgesic.

Correct: A — Renewable, high octane, clean burning. Ethanol (C₂H₅OH): high octane number (~109 research octane number) → anti-knock properties. Produced by fermentation of sugars (renewable). Burns more completely than petrol → fewer unburnt hydrocarbon emissions. Blended with petrol (E10, E85) to extend supply and reduce net CO₂ (plants absorb CO₂ during growth). Denatured alcohol (industrial spirit) = ethanol + small amounts of methanol or pyridine (to make it undrinkable, tax-free). Methanol is more toxic (causes blindness/death even in small amounts) — oxidised to formic acid in body which damages optic nerve.

Correct: D — Primary → red, secondary → blue, tertiary → colourless. Victor Meyer test: alcohol → alkyl iodide (PI₃ or HI) → silver nitrite → nitroalkane → treat with HNO₂ → treat with KOH/NaOH: Primary alcohol → nitrous acid with –CH₂NO₂ → pseudo-nitrolic acid → red colour with KOH. Secondary → nitroso compound → blue colour. Tertiary → no α-H → no reaction with HNO₂ → colourless. This test is based on the reactivity of nitroalkanes with nitrous acid. Also useful: chromic acid test (Jones reagent) — primary and secondary alcohols give colour change (Cr⁶⁺ orange → Cr³⁺ green); tertiary alcohols give no colour change.

Correct: B — 2,4,6-trinitrophenol (picric acid). Phenol with dilute HNO₃ → mixture of 2-nitrophenol (major, steam-distillable, intramolecular H-bond) and 4-nitrophenol (minor, higher bp, intermolecular H-bond). With excess conc. HNO₃/H₂SO₄ → 2,4,6-trinitrophenol (picric acid, pale yellow crystals, mp 122°C). Picric acid is a strong acid (pKa 0.38), stronger than carboxylic acids, because three –NO₂ groups strongly stabilise the phenoxide anion. It is an explosive (used in Lyddite). Also used to form picrates (crystalline derivatives) for characterisation of organic compounds.

Correct: C — Zymase. C₆H₁₂O₆ → (zymase, yeast, anaerobic) → 2C₂H₅OH + 2CO₂. Zymase is a mixture of enzymes present in yeast (Saccharomyces cerevisiae). Invertase converts sucrose → glucose + fructose. Amylase (diastase) converts starch → maltose. Maltase converts maltose → glucose. Maximum ethanol concentration achievable by fermentation is ~14–15% (higher concentrations kill the yeast). Industrial ethanol (absolute/anhydrous) is obtained by azeotropic distillation using benzene (removes water; forms benzene-water-ethanol ternary azeotrope which distils off, leaving anhydrous ethanol).

Correct: A — p-nitrophenol > phenol > p-cresol > water > ethanol. –NO₂ (electron-withdrawing by resonance and induction) at para position further stabilises phenoxide → more acidic. –CH₃ (electron-donating by hyperconjugation) destabilises phenoxide → less acidic than phenol. Water (pKa 15.7) is more acidic than ethanol (pKa 16–18) because the ethyl group pushes electrons onto oxygen (inductive effect), making O–H bond harder to break. Among phenols: EWG (–NO₂, –CN, –COOH, halogens) increase acidity; EDG (–CH₃, –OCH₃, –NH₂) decrease acidity.

Correct: D — 1-propanol (anti-Markovnikov). Hydroboration-oxidation (H. C. Brown, Nobel 1979): BH₃ adds to alkene syn (cis) with B going to less substituted carbon (anti-Markovnikov, bulky BH₃ goes to less hindered end) → trialkylborane → H₂O₂/NaOH → alcohol with OH at less substituted carbon. Net result: syn addition of water across double bond, anti-Markovnikov regiochemistry. Contrast: acid-catalysed hydration (H₃O⁺) gives Markovnikov product (OH at more substituted carbon) = 2-propanol from propene. Hydroboration-oxidation gives 1-propanol from propene — useful for primary alcohol synthesis.

Correct: A — Aldehydes only. Tollens' reagent = [Ag(NH₃)₂]⁺ (silver diammine complex). Aldehydes (R–CHO) are reducing agents — they reduce Ag⁺ to Ag° (silver mirror on test tube walls): RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag↓ + 4NH₃ + H₂O. Ketones (R–CO–R') do NOT reduce Tollens' reagent (no H on carbonyl C). Exception: formaldehyde (HCHO) reduces very strongly. Fehling's test (blue Cu²⁺ → red Cu₂O precipitate) also works with aldehydes but NOT aromatic aldehydes (C₆H₅CHO is not oxidised by Fehling's). Schiff's reagent turns pink/magenta with aldehydes.

Correct: C — Aliphatic aldehydes only. Fehling's solution = Fehling A (CuSO₄) + Fehling B (NaOH + sodium potassium tartrate). Aliphatic aldehydes reduce Cu²⁺ (blue) to Cu₂O (brick-red precipitate). Aromatic aldehydes (C₆H₅CHO) do NOT reduce Fehling's. Ketones do NOT reduce Fehling's. Tollens' reagent works for both aliphatic AND aromatic aldehydes. Schiff's reagent (decolourised rosaniline) gives pink/red with aldehydes — more sensitive than Tollens' or Fehling's. Benedict's solution is similar to Fehling's but more stable (used in urine glucose test).

Correct: D — Carbonyl compounds with α-H + dilute base/acid. Aldol condensation: two molecules of acetaldehyde (CH₃CHO) + dil. NaOH → 3-hydroxybutanal (aldol = ald + ol: has both aldehyde and alcohol). On heating, aldol → α,β-unsaturated carbonyl compound (crotonaldehyde) by dehydration. Conditions: requires α-H (no α-H → Cannizzaro reaction); dilute NaOH (base) or dilute H⁺ (acid). Cross-aldol: two different carbonyl compounds → mixture of products (less useful synthetically unless one has no α-H). Acetone undergoes aldol: CH₃COCH₃ → diacetone alcohol (4-methyl-4-hydroxypent-2-one).

Correct: B — Aldehydes with NO α-H + conc. NaOH. Cannizzaro reaction is a disproportionation: one aldehyde molecule is oxidised (→ carboxylate) while another is reduced (→ alcohol). Example: 2HCHO + NaOH → CH₃OH + HCOONa (formaldehyde → methanol + sodium formate). Benzaldehyde: 2C₆H₅CHO + NaOH → C₆H₅CH₂OH + C₆H₅COONa. Aldehydes with α-H prefer aldol condensation. Cross-Cannizzaro: HCHO + another aldehyde (no α-H) → HCOOH + other alcohol (HCHO is always the one oxidised — strongest reductant). Tilden's reaction: trichloroacetaldehyde (CCl₃CHO) undergoes Cannizzaro.

Correct: A — Lactonitrile (cyanohydrin). CH₃CHO + HCN → CH₃CH(OH)CN (lactonitrile). Mechanism: CN⁻ (nucleophile) attacks the carbonyl carbon (electrophilic C in C=O) → tetrahedral alkoxide intermediate → protonation → cyanohydrin. Cyanohydrins can be hydrolysed to α-hydroxy acids: CH₃CH(OH)CN + H₂O/H⁺ → lactic acid (CH₃CH(OH)COOH). This is important: it is a method to increase the carbon chain by one carbon. Ketones also form cyanohydrins but reaction is slower (more hindered carbonyl). Acetone + HCN → acetone cyanohydrin → methacrylonitrile (basis of Perspex/Lucite synthesis).

Correct: C — Aldehyde. Rosenmund reduction: RCOCl + H₂ → (Pd/BaSO₄, poisoned catalyst, xylene) → RCHO + HCl. The Pd catalyst is "poisoned" (partially deactivated with BaSO₄ or thiourea/quinoline) to prevent over-reduction to primary alcohol. Useful for preparing aromatic aldehydes from aromatic acyl chlorides. Other aldehyde preparations: (1) Stephen reaction: RCN + SnCl₂/HCl → imine → hydrolysis → RCHO (aliphatic and aromatic); (2) Etard reaction: ArCH₃ + CrO₂Cl₂ → ArCHO; (3) Gattermann-Koch: ArH + CO/HCl (AlCl₃, CuCl catalyst) → ArCHO (limited to benzene derivatives).

Correct: D — Oxygen is more electronegative (δ−), carbon is δ+. In C=O, oxygen (EN = 3.5) pulls the electron density toward itself: C^δ+–O^δ−. This makes the carbonyl carbon susceptible to nucleophilic attack (nucleophilic addition). After nucleophile adds to C, the C=O π bond breaks, oxygen gets the electron pair (forms alkoxide). In ketones, two alkyl groups donate electrons to carbonyl C (inductive effect) → less electrophilic than aldehydes. This is why aldehydes are more reactive towards nucleophilic addition than ketones. Formaldehyde is most reactive (no alkyl groups). Also steric: aldehydes have one H (less hindered) vs ketones (two R groups).

Correct: B — α-halogenation of carboxylic acids. CH₃COOH + Br₂ (red P) → BrCH₂COOH (bromoacetic acid) + HBr. Mechanism: (1) P + Br₂ → PBr₃; (2) RCH₂COOH + PBr₃ → RCH₂COBr (acyl bromide, more reactive); (3) Br₂ adds at α-C to give α-bromo acyl bromide; (4) Hydrolysis → α-bromo acid. Red phosphorus acts as a catalyst. HVZ works only with carboxylic acids (not esters/ketones) because the acyl halide intermediate is key. The α-halo acid can be further converted to α-amino acid (by NH₃ displacement) or α-hydroxy acid (by NaOH hydrolysis).

Correct: A — Dimer formation through two H-bonds. Carboxylic acids exist as cyclic dimers (8-membered ring) in non-polar solvents and in the gas phase: two –COOH groups form two O–H···O hydrogen bonds simultaneously. This doubles the effective molecular mass → much higher bp. Example: acetic acid (CH₃COOH, MW 60, bp 118°C) vs ethanol (C₂H₅OH, MW 46, bp 78°C). The dimer of acetic acid has MW 120. Lower fatty acids (up to C₄) are liquids with pungent odour (rancid smell); butanoic acid (butyric acid) smells like rancid butter. Higher fatty acids (>C₁₀) are waxy solids.

Correct: C — Ester (Fischer esterification). RCOOH + R'OH ⇌ RCOOR' + H₂O (conc. H₂SO₄ or dry HCl as catalyst, heat). Reversible reaction — equilibrium shifted right by removing water (azeotropic distillation, molecular sieves) or using excess alcohol. Mechanism: nucleophilic acyl substitution — alcohol O attacks the protonated carbonyl carbon. Acid derivatives in decreasing order of reactivity: acyl halide > anhydride > ester > amide > carboxylate. Hydrolysis of ester gives back acid + alcohol: RCOOR' + H₂O → RCOOH + R'OH (acid/base catalysis). Saponification (base hydrolysis of ester) is irreversible: RCOOR' + NaOH → RCOONa + R'OH.

Correct: D — Soda lime decarboxylation of carboxylate salts. RCOONa + NaOH (CaO) → R–H + Na₂CO₃. This is a dry distillation. Example: CH₃COONa + NaOH (soda lime) → CH₄ + Na₂CO₃. Useful in lab to prepare lower alkanes. Simple carboxylic acids do not decarboxylate easily on gentle heating (they require very high temperatures or special conditions). Exceptions that decarboxylate easily: β-keto acids (have carbonyl at β-position — cyclic 6-membered TS); malonic acid and oxalic acid on gentle heating. Kolbe's electrolysis: 2RCOONa + 2H₂O → R–R + 2CO₂ + H₂ (electrolysis; anode oxidation).

Correct: B — Acetic acid. RMgX + CO₂ (dry ice) → RCOOMgX → H₃O⁺ → RCOOH. CH₃MgBr + CO₂ → CH₃COOMgBr → H₂O/H⁺ → CH₃COOH (acetic acid) + Mg(OH)Br. This reaction increases chain length by one carbon (C1 → C2 acid from methyl Grignard). Grignard reagent + HCHO → 1° alcohol (chain +1); + RCHO → 2° alcohol (chain +1); + R₂CO → 3° alcohol (chain +1); + CO₂ → carboxylic acid (chain +1); + ester → 3° alcohol (chain +1); + nitrile → ketone after hydrolysis. These represent the synthetic versatility of Grignard reagents.

Correct: A — C=O to CH₂ (Clemmensen reduction). Zn-Hg amalgam + conc. HCl → direct reduction of C=O to CH₂ (bypasses alcohol). Used in acidic conditions — suitable for acid-stable compounds. Example: PhCOCH₃ → PhCH₂CH₃. Wolff-Kishner reduction (NH₂NH₂ + KOH, ethylene glycol, 200°C) also achieves C=O → CH₂ but under basic conditions — suitable for base-stable (acid-labile) compounds. Meerwein-Ponndorf-Verley (MPV): C=O → CHOH using Al(OiPr)₃ (chemoselective — reduces C=O, not C=C). LiAlH₄ reduces C=O → CHOH (primary or secondary alcohol), RCOOH → RCH₂OH, RCOOR' → 2 alcohols. NaBH₄ reduces only C=O, not ester/acid (milder).

Correct: C — Resonance stabilisation of carboxylate anion. RCOO⁻ has two equivalent resonance structures with the negative charge equally distributed over both oxygens. Both C–O bonds in carboxylate are equivalent (intermediate between single and double bond, bond order = 1.5, length ~127 pm). This delocalisation greatly stabilises the anion → carboxylic acid readily donates H⁺. Alkoxide (RO⁻) has no such resonance. Electron-withdrawing groups (–Cl, –NO₂) near the COOH increase acidity (stabilise anion further by induction). Trichloroacetic acid (Cl₃CCOOH, pKa = 0.66) is far more acidic than acetic acid (pKa = 4.76).

Correct: D — Unique in many ways. Formaldehyde (HCHO): (1) Gas at RT (bp −21°C); formalin = 40% aq. solution (preservative). (2) No α-H → undergoes Cannizzaro reaction (2HCHO + NaOH → CH₃OH + HCOONa). (3) Most reactive carbonyl compound (no steric/electronic hindrance). (4) Reduces Tollens' (very strong reducing agent — actually reduces ammoniacal silver so well that it's used for silvering mirrors). (5) Does not show tautomerism (no α-H). (6) Forms polyoxymethylene (paraformaldehyde) on standing. (7) Reacts with ammonia to give hexamethylenetetramine (urotropine, C₆H₁₂N₄) — antiseptic. (8) Carcinogen at high concentrations.

Correct: B — Acetaldehyde (CH₃CHO). Acetaldehyde has: CH₃CO– group → positive iodoform test (gives CHI₃); R–CHO (aldehyde group) → positive Tollens' test (silver mirror). Acetone: positive iodoform (CH₃CO– present), but negative Tollens' (ketone — no H on carbonyl C). Benzaldehyde: positive Tollens' (aldehyde), but negative iodoform (C₆H₅CO– group — phenyl, not methyl attached to C=O). Propanal (C₂H₅CHO): positive Tollens', but negative iodoform (no CH₃CO– group, has C₂H₅CO–). So only acetaldehyde satisfies both conditions simultaneously.

Correct: C — Amide. Beckmann rearrangement: ketoxime (R₂C=NOH) + H₂SO₄ (or PCl₅) → amide (RCONHR'). For cyclohexanone oxime → ε-caprolactam (cyclic amide) + H₂SO₄ → lactam (industrially used to make Nylon-6 polymer). Mechanism: OH of oxime is protonated → leaves as water → the group anti (trans) to OH migrates to nitrogen with its electrons → nitrilium ion → water adds → amide. The group that migrates is the one trans to OH (stereospecific). This is used in industrial synthesis of polyamides. Named after Ernst Otto Beckmann (1886).

Correct: A — Trichloroacetic acid (pKa = 0.66), strongest of these. Three Cl atoms are strong electron-withdrawing groups (–I effect via induction). They withdraw electron density from the carboxylate anion, stabilising the negative charge → easier to donate H⁺ → more acidic. pKa values: CCl₃COOH (0.66) < CHCl₂COOH (1.48) < CH₂ClCOOH (2.86) < CH₃COOH (4.76) < (CH₃)₃CCOOH (5.05). Electron-donating alkyl groups destabilise carboxylate anion → less acidic. Benzoic acid (pKa 4.20) is slightly more acidic than acetic acid due to inductive withdrawal by the ring (not resonance donation at COOH position).

Correct: D — Acetyl chloride (acyl chloride) + POCl₃ + HCl. CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl. Also: 3CH₃COOH + PCl₃ → 3CH₃COCl + H₃PO₃; CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl. Acyl chlorides are the most reactive acid derivatives (good for making other derivatives). Reactivity order of acid derivatives toward nucleophilic acyl substitution: RCOCl > (RCO)₂O > RCOOR' > RCONH₂. Acyl chlorides hydrolyse vigorously with water, react with alcohol (→ ester), with NH₃ (→ amide), with RNH₂ (→ N-substituted amide), with RCOO⁻ Na⁺ (→ anhydride). Example: CH₃COCl + NH₃ → CH₃CONH₂ + HCl.

Correct: B — Aromatic aldehydes (ArCHO) via Gattermann-Koch. ArH + CO + HCl → (AlCl₃, CuCl, high pressure) → ArCHO. Effective for benzene and its derivatives. Cannot be used for phenols/phenol ethers (deactivated by Lewis acid). Gattermann reaction (modified): ArH + HCN/HCl → (AlCl₃) → ArCH=NH (imine) → hydrolysis → ArCHO (works for phenols too). Kolbe-Schmitt: sodium phenoxide + CO₂ → sodium salicylate → salicylaldehyde (Reimer-Tiemann reaction uses CHCl₃/NaOH). Friedel-Crafts acylation (RCOCl/AlCl₃) gives ketone ArCOR, not aldehyde (since formyl chloride HCOCl is unstable). Hence Gattermann-Koch is the standard method for ArCHO.

Correct: C — CH₃NH₂ > NH₃ > Aniline. Basicity: the availability of the lone pair on N determines base strength. CH₃NH₂: methyl group donates electrons (inductive effect) → lone pair more available → stronger base than NH₃. Aniline (C₆H₅NH₂): lone pair on N delocalised into the aromatic ring (resonance) → lone pair less available → weakest base. pKb values (lower = stronger base): CH₃NH₂ (3.38) < NH₃ (4.74) < Aniline (9.38). In gas phase: CH₃NH₂ > (CH₃)₂NH > (CH₃)₃N > NH₃. In aqueous solution: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃ (solvation effect modifies order for tertiary amine).

Correct: A — Primary amine with one fewer C. RCONH₂ + Br₂ + NaOH → RNH₂ + CO₂ + NaBr + H₂O. Mechanism: amide → N-bromo amide → rearrangement (Lossen-type: N migrates to carbonyl C) → isocyanate (R–N=C=O) → hydrolysis → RNH₂ + CO₂. Key: the carbonyl C is lost as CO₂ → product amine has one C fewer. Example: CH₃CH₂CONH₂ (propanamide) → CH₃CH₂NH₂ (ethylamine). This is an important method to prepare primary amines (compare with Gabriel synthesis which also gives 1° amines). Also: reduction of nitro compounds → primary amines; reduction of nitriles → primary amines (LiAlH₄).

Correct: D — Benzene diazonium chloride. C₆H₅NH₂ + NaNO₂ + HCl (0–5°C) → C₆H₅N₂⁺Cl⁻ + NaCl + H₂O. Diazonium salts are formed only below 5°C (unstable at higher temperatures). Above 5°C: diazonium hydrolyses → phenol + N₂. Diazonium salts are key intermediates in aromatic synthesis: Sandmeyer → ArCl, ArBr, ArCN; Balz-Schiemann → ArF; with H₃PO₂ → ArH (deamination); with aq. NaOH → ArOH; coupling with phenol/amine → azo dye (Ar–N=N–Ar'). Azo coupling: C₆H₅N₂⁺ + C₆H₅OH → p-hydroxyazobenzene (orange dye). Azo dyes are used as textile dyes, pH indicators.

Correct: B — N,N-disubstituted sulfonamide, insoluble in NaOH. Hinsberg test results: Primary amine (RNH₂) + C₆H₅SO₂Cl → N-monosubstituted sulfonamide (R–NH–SO₂C₆H₅) → has acidic N–H → dissolves in NaOH → forms N-sulfonamide sodium salt. Secondary amine (R₂NH) + C₆H₅SO₂Cl → N,N-disubstituted sulfonamide (R₂N–SO₂C₆H₅) → NO acidic N–H → insoluble in NaOH (precipitate). Tertiary amine (R₃N) → does NOT react with C₆H₅SO₂Cl (no N–H) → no sulfonamide → amine dissolves in HCl (amine + HCl → ammonium salt, water-soluble) but not in Hinsberg reagent directly.

Correct: A — Unstable aliphatic diazonium salt decomposes at RT. 1° aliphatic amine + HNO₂ → aliphatic diazonium ion (R–N₂⁺) — highly unstable, decomposes immediately at room temperature (unlike aromatic diazonium) → N₂ gas + carbocation → products: alcohol (+ H₂O), alkene (+ H⁺), alkyl chloride (+ Cl⁻). Useful as: N₂ gas evolution confirms 1° aliphatic amine. Contrast: 1° aromatic amine → stable diazonium salt at 0–5°C (aromaticity stabilises). 2° amine + HNO₂ → N-nitrosamine (R₂N–NO, yellow oily liquid, possible carcinogen). 3° aliphatic amine + HNO₂ → forms salt (R₃N·HNO₂) — no N–NO formation since no N–H.

Correct: C — Lone pair delocalisation into ring. In aniline, the N lone pair overlaps with the π system of the ring (conjugation). This is evidenced by: (1) N is nearly sp² hybridised (not sp³); (2) C–N bond length in aniline (140 pm) is shorter than in alkylamines (147 pm) — partial double bond character; (3) Aniline is planar or nearly planar. This delocalisation reduces electron density on N → weak base (Kb much smaller than alkylamines). Consequence: aniline is far less nucleophilic than alkylamines → requires more vigorous conditions for N-alkylation. pKb: aniline = 9.38; NH₃ = 4.74; methylamine = 3.38.

Correct: D — Aniline. C₆H₅NO₂ + 6[H] → (Fe/HCl or Sn/HCl, acidic) → C₆H₅NH₂ + 2H₂O. Industrial preparation: catalytic hydrogenation of nitrobenzene (Fe catalyst, 300°C) or chemical reduction. Partial reduction intermediates: NO₂ → NO → NHOH → NH₂. With Zn/NH₄Cl (neutral medium) → C₆H₅NHOH (phenylhydroxylamine). With Zn/NaOH → azoxybenzene (C₆H₅N(O)=NC₆H₅). With LiAlH₄ → aniline. Aniline is the starting material for many synthetic dyes (mauveine — first synthetic dye by W.H. Perkin 1856), pharmaceuticals (sulfa drugs), rubber chemicals, and polyurethanes.

Correct: B — Azo compound (orange/yellow dye). Diazonium coupling: C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ (slightly alkaline/neutral) → C₆H₅–N=N–C₆H₄–NH₂ (p-aminoazobenzene, an orange dye). The electrophilic diazonium cation (weak electrophile) attacks the activated ring (phenol or aromatic amine) at the para position (preferentially). At pH < 5 (acid): couples with aniline to give diazoamino compound (ArN=N–NHAr). At pH 5–9 (slightly alkaline): couples at p-position of aniline → azo compound. With phenol: in alkaline medium (phenoxide form is more reactive) → orange azo dye. Orange II, Congo Red, methyl orange are important azo dyes/indicators.

Correct: A — 2,4,6-tribromoaniline. The –NH₂ group is strongly activating (ortho/para director) → all three o/p positions brominated rapidly by Br₂ water (no catalyst needed). Similar to phenol + Br₂ water. To get monobromo product, the –NH₂ group must be deactivated: acetylation of aniline (CH₃CO)₂O → acetanilide (–NHCOCH₃), then brominate (–NHCOCH₃ is still activating but less so) → 4-bromoacetanilide → hydrolysis → 4-bromoaniline. This is the Acetylation-bromination-hydrolysis sequence for preparing para-substituted aniline derivatives. Acetylation also protects the amine from strong oxidising conditions.

Correct: C — Primary amines only. Carbylamine test (isocyanide test): 1° amine + CHCl₃ + alc. KOH → isocyanide (R–N≡C, carbylamines) with extremely offensive smell. Mechanism: CHCl₃ + KOH → :CCl₂ (dichlorocarbene) → reacts with R–NH₂ → R–N=CCl₂ → KOH removes 2HCl → R–N≡C (isocyanide). Both aliphatic and aromatic 1° amines give this test. 2° and 3° amines → no reaction (no N–H for initial step). This test is specific for 1° amines. Isocyanides are poisonous. Mnemonic: Primary → Positive isocyanide; Secondary and Tertiary → No carbylamine.

Correct: D — Methyl orange is an azo dye. Methyl orange = p-(dimethylaminoazobenzene)sulfonic acid sodium salt. Structure: (CH₃)₂N–C₆H₄–N=N–C₆H₄–SO₃Na. pH range: 3.1 (red) to 4.4 (yellow/orange). Prepared by: diazotisation of sulphanilic acid → coupling with dimethylaniline. In acidic form: –N=N– protonated → quinonoid structure (red). In basic form: azo form (yellow). Methyl orange is used as an indicator in strong acid–strong base and strong acid–weak base titrations (pH change is in acidic range). Examples of azo dyes: Congo Red (pH 3–5, blue-violet to red), Orange II, malachite green (triphenylmethane dye, not azo).

Correct: B — p-dimethylaminoazobenzene. C₆H₅N₂⁺ + C₆H₅N(CH₃)₂ → p-CH₃)₂N–C₆H₄–N=N–C₆H₅ (para coupling). N,N-dimethylaniline has no N–H, so coupling occurs on the ring at the para position (N is still activating: lone pair conjugated into ring makes p-position highly electron-rich). This dye was once used as a food colourant ("butter yellow") but is now banned (carcinogen). The azo linkage (–N=N–) is the chromophore responsible for colour. Extended conjugation (aromatic rings connected by –N=N–) absorbs visible light. Auxochromic groups (–OH, –NH₂, –N(CH₃)₂) modify colour/intensity. Bathochromic shift: absorption moves to longer wavelength (red shift).

Correct: A — Acetanilide. C₆H₅NH₂ + (CH₃CO)₂O → C₆H₅NHCOCH₃ (acetanilide) + CH₃COOH. This is nucleophilic acyl substitution — the amine N attacks the carbonyl C of acetic anhydride. Acetanilide was the first synthetic pharmaceutical drug (analgesic/antipyretic, introduced 1886 as "Antifebrin"). Acetylation of aniline is used in organic synthesis to: (1) Protect the –NH₂ group from oxidation; (2) Reduce its activating power for selective halogenation at para position; (3) The acetamido group (–NHCOCH₃) is still ortho/para directing but less activating than –NH₂. After the desired reaction, hydrolysis removes the acetyl group to regenerate the amine.

Correct: C — (CH₃)₂NH (7°C) > CH₃NH₂ (−6°C) > (CH₃)₃N (3°C). Boiling points of amines depend on: (1) H-bonding capacity (N–H bonds) — 1° and 2° amines form H-bonds (N–H···N); 3° amines cannot (no N–H). (2) Molecular mass. CH₃NH₂: 1°, 2 N–H, bp −6°C. (CH₃)₂NH: 2°, 1 N–H, but higher MW → bp 7°C. (CH₃)₃N: 3°, no N–H → only van der Waals → despite higher MW, bp is 3°C (lower than (CH₃)₂NH). So trimethylamine has surprisingly low bp due to lack of H-bonding. N–H···N H-bonds are weaker than O–H···O (N is less electronegative than O), so amines have lower bp than corresponding alcohols.

Correct: D — Competitive inhibitor of PABA. Sulfonamides are structurally similar to para-aminobenzoic acid (PABA, H₂N–C₆H₄–COOH), which bacteria use to synthesise folic acid (needed for nucleotide synthesis). Sulfonamides competitively inhibit the enzyme that uses PABA → bacteria cannot make folic acid → cannot grow/divide. Humans get folic acid from diet (no de novo synthesis) → sulfonamides selectively toxic to bacteria. Sulfanilamide (para-aminobenzenesulfonamide): H₂N–C₆H₄–SO₂NH₂. First sulfa drug (Prontosil, 1932, Gerhard Domagk, Nobel Prize 1939). Later replaced largely by penicillin antibiotics. Used in urinary tract infections and some other bacterial infections still.

Correct: B — Free aldehyde group; glucose is a reducing sugar. Glucose (aldohexose) has a free –CHO group in its open-chain form. This reduces Tollens' (Ag⁺ → Ag) and Fehling's (Cu²⁺ → Cu₂O). All monosaccharides are reducing sugars. Among disaccharides: maltose (1α→4 glycosidic bond, free reducing end) and lactose are reducing; sucrose (1α→2β linkage between both anomeric carbons) is non-reducing. Reducing sugars reduce Benedict's/Fehling's/Tollens'. This is the basis of Benedict's test for blood glucose. Glucose is also known as dextrose (D-glucose, dextrorotatory). D-glucose: all –OH groups in R configuration except C2 (in D-fructose, C2 has different config).

Correct: C — Glucopyranose (6-membered ring), ~99% in solution. Glucose C1 –CHO reacts intramolecularly with C5 –OH to form a hemiacetal (ring): this creates a new chiral centre at C1 (anomeric carbon). α-D-glucopyranose: –OH at C1 is axial (on the same side as the ring oxygen in Haworth); β-D-glucopyranose: –OH at C1 is equatorial (trans to C6 –CH₂OH in Haworth). Mutarotation: α-D-glucose ([α]D = +112.2°) ↔ open chain ↔ β-D-glucose ([α]D = +18.7°), equilibrium at +52.5°. β-D-glucose is more stable (all equatorial groups in chair conformation). Fructose forms furanose (5-membered) ring more commonly.

Correct: A — Both anomeric carbons engaged in glycosidic bond. Sucrose = α-D-glucopyranose + β-D-fructofuranose linked by α(1→2)β glycosidic bond between C1 of glucose (anomeric) and C2 of fructose (anomeric). Both reducing ends are blocked → no free –CHO or hemiketal → cannot open chain → non-reducing. Hydrolysis of sucrose (invertase enzyme or dilute HCl) → glucose + fructose (this mixture is called invert sugar; optical rotation inverts from +66.5° for sucrose to −19.9° for the mixture because fructose, −92.3°, is more strongly levorotatory). Table sugar is sucrose (C₁₂H₂₂O₁₁). Lactose: glucose + galactose, C1 of galactose linked to C4 of glucose (galactose C1 is free → reducing).

Correct: D — α vs β glycosidic linkage is the key difference. Starch = amylose (unbranched, α-1,4 links, helical) + amylopectin (branched, α-1,4 main chain + α-1,6 at branch points, every 24–30 glucose units). Human enzyme (amylase, amyloglucosidase) can cleave α(1→4) and α(1→6) bonds. Cellulose: β(1→4) links → linear, hydrogen-bonded chains → fibrous, insoluble, structural role in plant cell walls. Humans lack cellulase enzyme → cannot digest cellulose (dietary fibre). Glycogen: like amylopectin but more branched (α-1,6 branch every 8–10 units) — animal storage polysaccharide. Iodine test: starch gives blue-black (amylose helix traps I₂ molecules); glycogen gives brownish-red; cellulose gives no colour.

Correct: B — –COOH + –NH₂ → –CO–NH– + H₂O (condensation). The peptide bond (amide bond): –CO–NH– is formed by condensation (loss of water) between the carboxyl group of one amino acid and the amino group of the next. Dipeptide has 1 peptide bond; tripeptide has 2; protein has n−1 peptide bonds for n amino acids. The peptide bond has partial double bond character (resonance: –C(=O)–NH– ↔ –C(–O⁻)=NH–) → planar, slightly rigid (~6 atoms coplanar). Protein backbone forms regular structures: α-helix (secondary structure, intrachain H-bonds between C=O and N–H four residues apart); β-sheet (interchain H-bonds, pleated sheet). R groups determine the tertiary structure.

Correct: C — Loss of 3D structure, not primary structure. Denaturation disrupts non-covalent interactions (H-bonds, hydrophobic interactions, van der Waals, ionic bonds) and some covalent interactions (disulfide bonds with reducing agents) that maintain secondary, tertiary, quaternary structure. Causes: heat, extreme pH, urea, organic solvents, heavy metal ions. The primary structure (sequence of amino acids, peptide bonds) is NOT destroyed. Protein loses biological activity because active site shape is disrupted. Often irreversible (e.g., boiling an egg — egg white albumin denatures). Renaturation (refolding) is possible in some cases (e.g., RNase experiment by Anfinsen — denatured with urea, renatured by removing urea, regained activity; proved primary structure determines protein folding).

Correct: A — Must be obtained from diet (cannot be synthesised in body). There are 20 standard amino acids in proteins. Essential amino acids (8–9 for adults): valine, leucine, isoleucine, phenylalanine, tryptophan, methionine, threonine, lysine (and histidine for children/infants). Non-essential: can be synthesised by the body (e.g., glycine, alanine, aspartate, glutamate). All amino acids (except proline) are α-amino acids with the structure: H₂N–CH(R)–COOH. At physiological pH (7.4), amino acids exist as zwitterions: ⁺H₃N–CH(R)–COO⁻. Isoelectric point (pI): pH at which zwitterion form predominates (net charge = 0); electrophoresis — at pI, amino acid/protein does not migrate.

Correct: D — A=T (2 H-bonds) and G≡C (3 H-bonds). Watson-Crick double helix (1953): two antiparallel strands held together by H-bonds between complementary bases: Adenine–Thymine: 2 hydrogen bonds; Guanine–Cytosine: 3 hydrogen bonds. G≡C pairs are stronger → DNA with higher G+C content has higher melting temperature (Tm). Chargaff's rules: [A] = [T] and [G] = [C] in double-stranded DNA (derived from this pairing). In RNA, Thymine is replaced by Uracil (U) — RNA has A–U base pairs (also 2 H-bonds). The helix makes one full turn every 10 base pairs (3.4 nm pitch, 0.34 nm per base pair, 2 nm diameter). The two strands are antiparallel: one is 5'→3', other is 3'→5'.

Correct: B — Base + sugar + phosphate. Nucleoside = nitrogenous base + pentose sugar (no phosphate). Nucleotide = nucleoside + phosphate group. DNA: deoxyribose (2'-deoxyribose) + bases A/G/C/T + phosphate. RNA: ribose + bases A/G/C/U + phosphate. Purine bases: Adenine (A) and Guanine (G) — two-ring (bicyclic). Pyrimidine bases: Cytosine (C), Thymine (T), Uracil (U) — one ring. Sugar is attached to base via N-glycosidic bond. Phosphodiester bond links 3'-OH of one nucleotide to 5'-phosphate of the next (backbone of nucleic acid). ATP (adenosine triphosphate) = nucleotide with 3 phosphates — energy currency of the cell.

Correct: C — DNA: deoxyribose + T; RNA: ribose + U. Key differences: (1) Sugar: DNA = 2'-deoxyribose (no –OH at C2'); RNA = ribose (–OH at C2'). (2) Base: DNA has Thymine (T, 5-methyluracil); RNA has Uracil (U). Both have A, G, C. (3) Strands: DNA = typically double-stranded helix; RNA = typically single-stranded (but can form secondary structures). (4) Location: DNA mainly in nucleus + mitochondria; RNA in nucleus + cytoplasm + ribosome. Types of RNA: mRNA (messenger, carries genetic code from DNA to ribosome); tRNA (transfer, brings amino acids); rRNA (ribosomal, structural/catalytic in ribosome). (5) Function: DNA = storage of genetic information; RNA = expression of genetic information.

Correct: A — Mostly proteins; some ribozymes (RNA). Enzymes are biological catalysts that: (1) Lower activation energy (Ea) of reactions. (2) Are highly specific (lock-and-key model: Emil Fischer; induced-fit model: Koshland). (3) Not consumed in reactions. (4) Work at mild conditions (body temperature, physiological pH). (5) Can be regulated (allosteric, competitive/non-competitive inhibition). The active site is a specific pocket/groove where substrate binds. Coenzymes (non-protein organic cofactors, often vitamins like NAD⁺/NADH derived from Niacin/B₃) assist enzymes. Metalloenzymes have metal ions (Zn²⁺ in carbonic anhydrase, Fe in haemoglobin, Mg²⁺ in chlorophyll). Ribozymes: RNA with catalytic activity (e.g., ribonuclease P, ribosomal peptidyl transferase activity).

Correct: D — Vitamins A, D, E, K are fat-soluble (stored in fatty tissues/liver). Fat-soluble (ADEK): A (retinol, vision, night blindness if deficient); D (calciferol, calcium/bone metabolism, rickets if deficient); E (tocopherol, antioxidant); K (phylloquinone, blood clotting, synthesised by gut bacteria). Water-soluble: Vitamin C (ascorbic acid, collagen synthesis, scurvy if deficient) and all B vitamins (B₁ thiamine, B₂ riboflavin, B₃ niacin/nicotinamide, B₅ pantothenic acid, B₆ pyridoxine, B₇ biotin, B₉ folic acid, B₁₂ cobalamin). Water-soluble vitamins are not stored → must be consumed regularly. Fat-soluble can accumulate → toxicity possible with over-supplementation (hypervitaminosis A, D).

Correct: B — Fructose is a ketohexose (keto at C2), sweetest natural sugar. Fructose (C₆H₁₂O₆) = levulose = fruit sugar. Keto group at C2, 4 –OH groups, –CH₂OH at C6. Despite having no aldehyde, fructose gives a positive Tollens' and Fehling's test because in basic conditions, fructose (ketose) undergoes tautomerism (enolisation) to form glucose and mannose (aldoses) via 1,2-enediol intermediate, and the aldose reduces the reagent. Fructose is levorotatory ([α]D = −92.3°). Relative sweetness: Fructose (1.73) > sucrose (1.0) > glucose (0.74) > maltose (0.32) > lactose (0.16). In the ring form, fructose exists primarily as furanose (5-membered ring: C2 keto + C5 –OH).

Correct: C — Glycerol (propane-1,2,3-triol) + 3 fatty acids = triglyceride. Fats: triglycerides where fatty acids are saturated (no C=C double bonds) — solid at RT (e.g., butter, lard). Oils: unsaturated fatty acids (one or more C=C double bonds) — liquid at RT (e.g., olive oil, sunflower oil). Unsaturated fats → lower melting point (cis double bonds prevent close packing). Hydrogenation of oils (Ni catalyst) → margarine (saturated fat). Saponification: fat + NaOH → soap (sodium salt of fatty acid) + glycerol. Soap works by forming micelles (hydrophobic tail → oil, hydrophilic head → water). Phospholipids: glycerol + 2 fatty acids + phosphate + organic group → cell membrane bilayer. Waxes: long-chain alcohol + long-chain fatty acid ester (not glycerol-based).

Correct: A — Primary structure = amino acid sequence. Levels of protein structure: (1) Primary: sequence of amino acids linked by peptide bonds — determined by DNA sequence. (2) Secondary: local regular structures — α-helix (right-handed coil, intrachain H-bonds between C=O of residue n and N–H of residue n+4); β-sheet (extended, interchain H-bonds, parallel or antiparallel). (3) Tertiary: overall 3D shape of single polypeptide — maintained by H-bonds, disulfide bridges (–S–S–, Cys residues), hydrophobic interactions, salt bridges. (4) Quaternary: arrangement of multiple polypeptide subunits (e.g., haemoglobin has 2α + 2β subunits). Insulin: first protein to have primary structure determined (Sanger, 1955, Nobel Prize). Myoglobin: first 3D structure solved (Kendrew, 1958, Nobel 1962).

Correct: D — Galactose–glucose with β(1→4) bond; reducing sugar. Lactose = β-D-galactopyranose + D-glucopyranose linked by β(1→4) glycosidic bond. The C1 of glucose is free → reducing sugar (gives positive Tollens' and Fehling's). Hydrolysed by enzyme lactase (β-galactosidase) in the small intestine; lactose intolerance = deficiency of lactase. Maltose = glucose + glucose (α-1,4 bond) — reducing sugar (one free C1 anomeric OH); formed from starch hydrolysis (amylase). Sucrose = glucose + fructose (α-1,2β bond) — non-reducing (both anomeric carbons blocked). Cellobiose = glucose + glucose (β-1,4) — from cellulose hydrolysis; reducing sugar.

Correct: B — I₂ trapped in amylose helix (charge-transfer complex). Amylose (unbranched component of starch, α-1,4 linkages) adopts a helical coil structure. I₂ molecules fit snugly inside the helix, forming a charge-transfer complex (starch-iodine) that absorbs light at ~620 nm → blue-black colour. On heating, helix unwinds → I₂ released → colour disappears; on cooling → colour returns. Amylopectin (branched, α-1,4 and α-1,6) gives reddish-brown colour (shorter helical segments). Glycogen (most branched) gives brownish-red to no colour. This is a very sensitive test for starch; used in titrations as an indicator (starch-I₂ endpoint turns blue-black). Vitamin C (ascorbic acid) decolourises iodine (used in iodometric assay).

Correct: C — Quaternary structure: 2α + 2β subunits + haem groups. Haemoglobin (Hb): MW ~64,000; four subunits (2α-chains + 2β-chains). Each subunit contains one haem group (iron-porphyrin complex with Fe²⁺ at centre). Fe²⁺ binds O₂ reversibly → carries O₂ in blood. Cooperative binding: binding of O₂ to one subunit increases affinity of other subunits (sigmoidal O₂ binding curve). CO binds haem ~240× more strongly than O₂ → CO poisoning. Myoglobin: single polypeptide (tertiary structure) with one haem → O₂ storage in muscles. Sickle cell anaemia: single amino acid mutation (Glu6→Val in β-chain) → HbS forms fibres when deoxygenated → sickle-shaped RBCs. Insulin: primary structure was first determined (Sanger); two chains (A and B) linked by disulfide bridges.

Correct: A — Adenosine (base + sugar, no phosphate) is a nucleoside. Nucleoside = nitrogenous base + pentose sugar. Nucleotide = nucleoside + phosphate group(s). Nucleosides: adenosine (A + ribose), guanosine (G + ribose), cytidine (C + ribose), uridine (U + ribose), thymidine (T + deoxyribose). Deoxynucleosides have deoxyribose (d-prefix). Nucleotides: AMP, ADP, ATP (1, 2, 3 phosphates). ATP hydrolysis → ADP + Pᵢ releases ~30 kJ/mol (energy currency). cAMP (cyclic AMP): formed from ATP by adenylyl cyclase → second messenger in hormonal signalling. Drugs targeting nucleoside/nucleotide metabolism: zidovudine (AZT, HIV treatment — nucleoside reverse transcriptase inhibitor).

Correct: D — More branched than starch (amylopectin), α-1,4 + α-1,6 bonds. Glycogen: polysaccharide of glucose with α-1,4 main chain and α-1,6 branch points every 8–10 residues (vs amylopectin's every 24–30). More branching → more ends available for rapid glucose release (during exercise, stress) by glycogen phosphorylase. Stored in liver (regulates blood glucose) and muscle (local energy for contraction). Starvation → glycogen depleted in ~24h. Glycogen storage diseases (e.g., Pompe, McArdle) result from enzyme defects. Gives reddish-brown colour with iodine (not blue-black like starch, because shorter helical segments). Molecular weight of glycogen can be very large (10⁷–10⁸ Da). Insulin promotes glycogen synthesis (glycogenesis); glucagon promotes glycogen breakdown (glycogenolysis).