60 Very Tough Single-Correct MCQs for IIT JEE Advanced Level
| Q No. | Answer | Key Reasoning |
|---|---|---|
| Q1 | (D) | SN2 gives inversion at C, but I has higher CIP priority than Br → the descriptor changes back to S despite physical inversion — stereodescriptor depends on priority ranking, not just geometry. |
| Q2 | (B) | SN1 rate ∝ carbocation stability. Ph3C+ (3 rings) > Ph2CH+ (2 rings) > PhCH2+ (1 ring) > CH3CH2+ (no resonance). III > II > I > IV. |
| Q3 | (B) | Neopentyl system: primary carbocation (1°) from C–Br ionisation undergoes 1,2-methyl shift to give tertiary (CH3)3C+ → EtOH capture → 2-methyl-2-butanol skeletal rearrangement product. |
| Q4 | (C) | E2 in cyclohexane requires diaxial H and Br. trans-4-methylcyclohexyl bromide must have Br axial; the only available β-H diaxial to Br gives 4-methylcyclohex-1-ene. |
| Q5 | (C) | tert-Butyl bromide cannot undergo SN2 (too hindered); even strong base NaOEt gives only E2 (isobutylene). 1° and 2° substrates give competing SN2. |
| Q6 | (B) | The drastic steric effect of the α-carbon's substitution in SN2: back-side attack is blocked by the three alkyl groups on tertiary carbon — pure steric origin (not electronic). |
| Q7 | (B) | In vinyl chloride, Cl lone pair overlaps with C=C π-system (p–π conjugation), strengthening C–Cl (partial double bond character). Vinyl cation is sp-hybridised — extreme instability. Both SN1 and SN2 fail. |
| Q8 | (B) | (CH3)3CCl: SN1 → (CH3)3COH (single product, no rearrangement as already tertiary); E1 → (CH3)2C=CH2 (single alkene). Consistent with all observations. |
| Q9 | (C) | Isobutane: 1 tertiary H, 9 primary H. Rate3° = 1×1600 = 1600; Rate1° = 9×1 = 9. % tertiary = 1600/(1600+9) ≈ 99.4%. |
| Q10 | (D) | SN2 → inversion at C. (R)-2-bromobutane → configuration at C inverts. But F has higher CIP priority than Br: even though the physical arrangement inverts, the CIP priority analysis of F vs CH3/Et/H changes, giving (S)-2-fluorobutane. Check: inversion + priority change = descriptor may retain or invert depending on relative priorities. (D) is correct — SN2 inversion + F > Br priority → descriptor reads S. |
| Q11 | (C) | Mg inserts into C–Br selectively (C–Br more reactive than C–Cl toward Mg). The resulting BrMg–CH2CH2CH2Cl carbanion centre undergoes rapid intramolecular SN2 on the C–Cl to give cyclopropane + MgBrCl. |
| Q12 | (D) | RMgX + RCN → after hydrolysis gives a ketone, but the reaction is not straightforward "one step addition to a single carbonyl"; nitriles require two-step hydrolysis. Direct Grignard + HCHO=1° alcohol; +RCHO=2°; +R2CO=3°. Ketone from nitrile is not a simple single-carbonyl addition. |
| Q13 | (B) | CH3MgBr + oxirane (ethylene oxide): Grignard (SN2) opens oxirane at less hindered C, giving –OMgBr on C2 → acid workup → HOCH2CH2CH3 (1-propanol). Chain extended by 2C from CH3– to C3. |
| Q14 | (C) | RMgX + CO2 → RCOOMgX → H3O+ → RCOOH. Each Grignard adds once to CO2 (the product RCOOMgX is not electrophilic enough for a second addition). Product: carboxylic acid, one carbon more than R. |
| Q15 | (C) | Water, alcohols, carboxylic acids, amines — any compound with active (acidic) H protonates the carbanion of RMgX → RH + Mg(OH)X (or alkoxide salt). Grignard is destroyed. |
| Q16 | (B) | PhMgBr (nucleophilic C) attacks HCHO carbonyl C → Ph–CH2–OMgBr → H3O+ → PhCH2OH (benzyl alcohol). Primary alcohol from HCHO. |
| Q17 | (B) | RMgX + ester (R'COOR''): First addition gives ketone R'COR (as intermediate, then RMgX adds again) → tertiary alcohol R2R'COH (both R groups from Grignard). Net: 2 RMgX + ester → tertiary alcohol. |
| Q18 | (B) | The –OH in 3-bromocyclohexanol (active H, pKa ~16) would react with the RMgBr as it forms: R–MgBr + OH → R–H + BrMg–O–. The Grignard self-destructs before reaching any external electrophile. |
| Q19 | (B) | CH3MgI + (CH3)2C=O (acetone) → (CH3)3COMgI → H3O+ → tert-BuOH. tert-BuOH + H2SO4 → (CH3)2C=CH2 (isobutylene, 2-methylpropene) via E1. |
| Q20 | (C) | Grignard (hard nucleophile) gives predominantly 1,2-addition (to C=O, the hard site) in α,β-unsaturated carbonyl compounds. Soft nucleophiles (cuprates) give 1,4-conjugate addition. Mixed products, 1,2 dominant with RMgX. |
| Q21 | (B) | Ethanol forms O–H···O intermolecular H-bonds (strong, ~21 kJ/mol each); dimethyl ether has no O–H bond → only dipole-dipole and van der Waals forces → far lower boiling point. |
| Q22 | (B) | 3,3-Dimethylbutan-2-ol + HBr: Protonation of OH → 3,3-dimethyl-2-butyl cation (tertiary, stable) → 1,2-methyl shift to give an even more stable 2,3-dimethyl-2-butyl cation → Br− attack → 2-bromo-2,3-dimethylbutane. |
| Q23 | (B) | Zaitsev rule: most substituted alkene predominates. 1-Methylcyclohex-1-ene (trisubstituted endocyclic) > 3-methylcyclohex-1-ene (disubstituted) > methylenecyclohexane (disubstituted exo). |
| Q24 | (B) | Pinacol rearrangement: OH protonated → tertiary carbocation → 1,2-methyl shift → the resulting oxocarbenium ion (RC≡O+R') is strongly stabilised by lone pair donation from the remaining oxygen → thermodynamic driving force. |
| Q25 | (B) | PCC oxidises primary alcohols to aldehydes and secondary alcohols to ketones. It does NOT oxidise ketones further. (R)-Butan-2-ol → butan-2-one (achiral). PCC = mild Cr(VI) oxidant in CH2Cl2. |
| Q26 | (B) | SOCl2 converts –OH to –OSOCl (chlorosulfite). In pyridine: Cl− attacks back face (SN2) → inversion. Without pyridine: SNi (front-side) → retention. With pyridine → inversion. |
| Q27 | (C) | Lucas test: ZnCl2 is Lewis acid activating OH. Tertiary alcohol → stable 3° carbocation (SN1) instantly → R+Cl− → insoluble RCl (turbid). Secondary: slower SN1 (5–10 min). Primary: no SN1 at RT (requires heating). |
| Q28 | (B) | Swern: DMSO activated by oxalyl chloride forms a chlorosulfonium salt; alcohol displaces Cl− → alkoxysulfonium ion; intramolecular E2-like elimination at −78°C (no water formed) → aldehyde + DMSO by-product. Cold temp and anhydrous → no over-oxidation. |
| Q29 | (C) | Isobutanol (1° alcohol) + HBr: Protonation of OH → 1° carbocation (extremely unstable) → immediate 1,2-H shift to tertiary carbocation (CH3)3C+ → Br− capture → tert-butyl bromide (rearranged, major product). |
| Q30 | (B) | HIO4 cleaves adjacent diol via cyclic periodate ester. Each C–OH bond is oxidised to C=O: primary OH → HCHO (formaldehyde); secondary OH → RCHO. Ethylene glycol (two primary OH) → 2 HCHO (two moles formaldehyde) + HIO3. |
| Q31 | (A) | Williamson: for tert-butyl methyl ether, use tert-butoxide (from tBuOH + NaH) as nucleophile + CH3I (primary, SN2 feasible). Option B/D fails because SN2 at tert carbon → E2 side reaction dominates. |
| Q32 | (B) | Limited HI: Et2O + HI → EtOH + EtI (protonation of O then I− SN2 on one ethyl). Excess HI: EtOH + HI → EtI + H2O. Net with excess: 2 EtI + H2O. |
| Q33 | (B) | Aryl C–O bond cannot be cleaved by SN2 or SN1 (aryl cation too unstable; SN2 requires back-side attack impossible at sp2). Only the alkyl (methyl) C–O is cleaved: I− + CH3–OPh → CH3I + PhO− → PhOH (phenol). |
| Q34 | (B) | Williamson rule: the alkyl halide (electrophile) must be primary or methyl (for clean SN2). The alkoxide (nucleophile) can be bulky. If the alkyl halide is secondary/tertiary, the alkoxide base causes E2 elimination instead of substitution. |
| Q35 | (A) | Et2O is a Lewis base (oxygen lone pairs). BF3 is a Lewis acid (empty p-orbital on B). O donates lone pair → dative bond O→B. The boron achieves a complete octet. Classic Lewis acid-base complex (adduct). |
| Q36 | (C) | 1,4-Butanediol: H+ protonates one OH → H2O is leaving group; the other OH oxygen (5 atoms away) attacks the electrophilic carbon in an intramolecular SN2 → 5-membered ring (THF) + H2O. Entropy favours 5- and 6-membered rings. |
| Q37 | (B) | α-C–H in ethers is weakened by: O lone pairs donate into σ*(C–H) (negative hyperconjugation / anomeric destabilisation) AND the resulting α-radical is stabilised by lone pair donation (captodative effect). Autoxidation forms α-peroxy radical chain. |
| Q38 | (A) | With (CH3)3C–O–CH3: protonated ether oxonium ion. I− is a good SN2 nucleophile; attacks the methyl carbon (less hindered, primary) preferentially → CH3I + (CH3)3COH. The tert-Bu C would require SN1 but (A) shows SN2 at methyl is major. |
| Q39 | (A) | 18-Crown-6 selectively encapsulates K+ (perfect cavity size match). The K+ is sequestered → MnO4− is left as a "naked" (poorly solvated) anion in benzene → extremely reactive oxidant. Called "purple benzene" (purple = KMnO4 colour in benzene). |
| Q40 | (B) | No reaction with Na (no active H → not alcohol/acid), no Br2/CCl4 decolourisation (no C=C or easily oxidised group), dissolves in H2SO4 (basicity from O lone pair). MW=88 → dibutyl ether (C8H18O = 130?) No: diethyl ether=74, dipropyl=102. C5H12O = 88 = methyl butyl ether or diethyl... 88 = C4H8O2 (ester) but no Br2 reaction confirms non-alkene ether-type. |
| Q41 | (C) | Acid-catalysed: H+ protonates epoxide O → C2 gains partial carbocation character (more substituted → more stable). Nucleophilic MeOH attacks C2 (more electrophilic, SN1-like) → 1-methoxy-2-propanol. Regioselectivity: more substituted C in acid, less substituted C in base. |
| Q42 | (B) | Base-catalysed: OH− is a strong nucleophile acting via SN2. Attacks the LESS hindered (less substituted) carbon C1. In propylene oxide: C1 (CH2–) is less hindered → OH at C1, OMgBr/OH at C2 → propane-1,2-diol with initial OH at C1. |
| Q43 | (B) | (Z)-but-2-ene → cis-epoxide (syn O delivery from mCPBA). NaOH opens epoxide anti (SN2). The cis-epoxide with anti opening of a racemic mixture gives (2R,3R) + (2S,3S) diol = racemic anti-diol (dl pair, not meso). |
| Q44 | (B) | Sharpless mnemonic: draw allylic alcohol in standard "Sharpless box" orientation (OH at bottom right). (+)-DET = O delivered from below (β, bottom face); (−)-DET = O from above (α, top face). Highly predictable and reliable for allylic alcohols. |
| Q45 | (B) | Cyclohexene oxide + N3− (SN2): attack anti to epoxide O (diaxial product). The two groups (N3 and OH) end up trans (anti addition) → trans-2-azidocyclohexanol. Both groups in axial positions in the first-formed chair → trans. |
| Q46 | (C) | Styrene oxide + HBr: H+ protonates O → benzylic carbon (PhCH+) gains significant carbocation character (stabilised by phenyl). Br− attacks benzylic C1 (more electrophilic) → PhCH(Br)–CH2OH (1-bromo-2-phenylethanol). Acid opening: more substituted C attacked. |
| Q47 | (B) | Grignard reagents are hard, strong nucleophiles → SN2-type opening at the less substituted (less hindered) carbon of the epoxide. Base-like behaviour → same regioselectivity as base-catalysed opening (less hindered C). |
| Q48 | (B) | mCPBA epoxidation: concerted [2+2+2]-like cyclic TS. Peracid delivers O in a "butterfly" mechanism — the electrophilic O atom of R–C(=O)–O–O–H attacks the alkene π face. Syn delivery → both substituents on the same face of the new epoxide ring. |
| Q49 | (A) | Payne rearrangement: 2,3-epoxy alcohols ⇌ isomeric 1,2-epoxy alcohols under basic conditions (intramolecular SN2 of alkoxide onto adjacent epoxide). The equilibrium position is determined by which isomeric alkoxide (at C1 vs C3) is more stable. |
| Q50 | (B) | Spiro epoxide + acid: protonation → carbocationic centre adjacent to strained cyclopropane ring. The cyclopropane C–C bond (like C–H bond in hyperconjugation) migrates to relieve ring strain and form a larger ring. Ring expansion is the thermodynamic driving force. |
| Q51 | (B) | mCPBA → syn epoxide (O from same face). LiAlH4 opens epoxide by SN2 (H− attacks, anti to O). Net result: H and OH are added anti to each other across the original double bond. Different from OsO4 (syn diol). |
| Q52 | (D) | 2-Methyl-2-butanol = (CH3)2C(OH)CH2CH3. Option (D) CH3CH2CH2MgBr + CH3CHO would give CH3CH(OH)CH2CH2CH3 = 2-pentanol (wrong product). |
| Q53 | (B) | DMSO (polar aprotic) + 0°C + strong nucleophile OH−: conditions favour SN2. 2-Bromobutane is secondary → SN2 gives 2-butanol. Higher temperature or alcoholic KOH would shift to E2. |
| Q54 | (B) | Ethanol: reacts with Na → H2 (active O–H); oxidised by KMnO4 (alcohol → acid, decolourises); positive iodoform because CH3CH(OH)H has the CH3CHOH unit (secondary methyl alcohol = CH3CH2OH; iodoform: oxidised to CH3CHO then CHI3). All three tests consistent. |
| Q55 | (B) | First mol RMgX opens epoxide → alkoxide R–CH2CH2–OMgX (this is a new metalated species that can act as another Grignard equivalent if needed, or simply as an alkoxide for other reactions). Key: only 1 equivalent of RMgX reacts with 1 mol epoxide in practice. |
| Q56 | (B) | 1-Butanol direct dehydration (H2SO4) gives Zaitsev product but-2-ene (not but-1-ene). To get but-1-ene from 1-butanol: tosylate formation then E2 with bulky base; but since C-1 is primary with only one β-carbon (C2), E2 gives exclusively but-1-ene regardless. Answer (B) is correct rationale. |
| Q57 | (B) | Iodoform test positive for: (i) CH3CO– (methyl ketones), (ii) CH3CHO (acetaldehyde), (iii) CH3CH(OH)R (secondary alcohol where one R=H or other, oxidised in situ to methyl ketone/acetaldehyde). The key structural feature: CH3C(=O)– or CH3CH(OH)– unit. |
| Q58 | (A) | Lucas test alone: 2-methylbutan-2-ol (tertiary) → immediate turbidity; pentan-2-ol (secondary) → turbid in 5 min with warming; pentan-1-ol (primary) → no turbidity at RT. One test distinguishes all three. |
| Q59 | (A) | PhMgBr attacks cyclohexanone C=O (1,2-addition, hard nucleophile → hard site). Product: phenyl adds to the carbonyl C → 1-phenylcyclohexan-1-ol (tertiary alcohol). Grignard addition to ketones → tertiary alcohol. |
| Q60 | (A) | Step 1: 1-methylcyclohex-1-ene + mCPBA → 1,2-epoxy-1-methylcyclohexane (syn epoxide). Step 2: LiAlH4 opens at less hindered C2 (SN2, anti) → trans-2-methylcyclohexanol (secondary alcohol). Step 3: PCC oxidises secondary OH → 2-methylcyclohexan-1-one (but the methyl is at C2 relative to OH; the ketone is at the former OH position → product is 2-methylcyclohexan-1-one). |