Paper II — Q2
2.(a)(i) Based on Hückel rule, predict the above compounds as aromatic, antiaromatic, and nonaromatic. [(A) cyclopentadiene, (B)…
2.(a)(i) Based on Hückel rule, predict the above compounds as aromatic, antiaromatic, and nonaromatic. [(A) cyclopentadiene, (B) 1-methylborole, (C) 1-methylphosphole] 5 marks 2.(a)(ii) Identify the above reactions that show primary kinetic isotope effect and secondary kinetic isotope effect. [(A) PhCH2–H* + Br• → PhCH2• + *H–Br; (B) p-methoxybenzaldehyde (CHO carrying H*) + HCN → cyanohydrin; (C) retro-Diels–Alder of the 9,10-dihydro-9,10-ethanoanthracene carrying H* on the bridge, giving anthracene + ethene (H*); (D) Hofmann elimination of the (2-H*-cyclopentyl)trimethylammonium hydroxide giving cyclopentene + Me3N + H*–OH] 5 marks 2.(a)(iii) Identify (A) and (B) in the above reactions and explain the mechanism. [butan-2-one with NaOMe/MeOH gives (A); with LDA, THF, −78°C gives (B)] 5 marks 2.(b)(i) The rate of hydrolysis (k1 and k2) of the reaction (A) is much faster than that of (B). Explain. [(A) ClCH2CH2–S–CH2CH2Cl + H2O (k1) → HOCH2CH2–S–CH2CH2OH; (B) Cl(CH2)5Cl + H2O (k2) → HO(CH2)5OH] 5 marks 2.(b)(ii) The reaction of the compounds (A) and (B) with AgClO4 in MeOH gives the same product (C). Explain. [(A) and (B) are the two anomeric 2,3,4,6-tetra-O-acetylglucosyl bromides; (C) is the 1,2-trans methyl glycoside] 5 marks 2.(b)(iii) Write the major product of the above reaction showing proper stereochemistry and explain the mechanism. [H2C=C=O; (i) hν, (ii) cis-but-2-ene] 10 marks 2.(c)(i) Write the structure of the product of the above reaction and provide suitable mechanism. [2-(cyclopent-3-en-1-yl)ethyl tosylate, AcOH, heat (solvolysis)] 10 marks 2.(c)(ii) Write the preferred position (C-2 or C-3) in electrophilic substitution of indole. Explain your observation with the help of resonance structures. 5 marks
हिंदी में प्रश्न पढ़ें
2.(क)(i) हकल नियम के अनुप्रयोग से निम्न यौगिकों का उनके ऐरोमैटिक, एंटी-ऐरोमैटिक, तथा नॉन-ऐरोमैटिक आधार पर पहचान करें । [(A) साइक्लोपेन्टाडाइन, (B) 1-मेथिलबोरोल, (C) 1-मेथिलफ़ॉस्फ़ोल] 5 2.(क)(ii) निम्नलिखित अभिक्रियाओं की प्राथमिक गतिक समस्थानिक प्रभाव व द्वितीयक गतिक समस्थानिक प्रभाव दर्शाने के आधार पर पहचान करें । [(A) PhCH2–H* + Br• → PhCH2• + *H–Br; (B) p-मेथॉक्सीबेन्ज़ैल्डिहाइड (CHO पर H*) + HCN; (C) सेतु पर H* वाले एथेनोऐन्थ्रासीन का प्रतीप डील्स–ऐल्डर → ऐन्थ्रासीन + एथीन (H*); (D) (2-H*-साइक्लोपेन्टिल)ट्राइमेथिलअमोनियम हाइड्रॉक्साइड का हॉफ़मैन विलोपन → साइक्लोपेन्टीन + Me3N + H*–OH] 5 2.(क)(iii) निम्नलिखित अभिक्रियाओं में उत्पाद (A) तथा (B) को पहचानिए एवं सम्मिलित क्रियाविधि की व्याख्या कीजिए । [ब्यूटेन-2-ऑन + NaOMe/MeOH → (A); LDA, THF, −78°C → (B)] 5 2.(ख)(i) अभिक्रिया (A) की जल अपघटन दर (k1 व k2) अभिक्रिया (B) से बहुत द्रुत है । व्याख्या कीजिए । [(A) ClCH2CH2–S–CH2CH2Cl + H2O; (B) Cl(CH2)5Cl + H2O] 5 2.(ख)(ii) MeOH में AgClO4 के साथ यौगिक (A) और (B) की अभिक्रियाएं समान उत्पाद (C) देती हैं । व्याख्या कीजिए । [(A), (B) दो ऐनोमरी टेट्रा-O-ऐसीटिलग्लूकोसिल ब्रोमाइड; (C) 1,2-ट्रांस मेथिल ग्लाइकोसाइड] 5 2.(ख)(iii) त्रिविम रसायन दर्शाते हुए, निम्नलिखित अभिक्रिया के मुख्य उत्पाद की संरचना लिखिए एवं क्रियाविधि की व्याख्या कीजिए । [H2C=C=O; (i) hν, (ii) cis-ब्यूट-2-ईन] 10 2.(ग)(i) निम्नलिखित अभिक्रिया के उत्पाद की संरचना लिखिए व क्रियाविधि की व्याख्या कीजिए । [2-(साइक्लोपेन्ट-3-ईन-1-इल)एथिल टोसिलेट, AcOH, गर्म (विलायक अपघटन)] 10 2.(ग)(ii) इंडोल के इलेक्ट्रॉनरागी प्रतिस्थापन अभिक्रिया में अधिमान्य स्थान (C-2 या C-3) की पहचान कीजिए । अनुनादी संरचनाओं की सहायता से अपने प्रेक्षण की व्याख्या कीजिए । 5
The figure this question refers to, in words
The question paper is a scan and the diagram did not survive as text. This is the figure as read from the original page — every component, value and label — so the question can be worked from the text below.
(a(i)) Three chemical structures labeled (A), (B), and (C). Structure (A) is a five-membered ring with two double bonds (cyclopentadiene). Structure (B) is a five-membered ring with two double bonds and a boron atom at the bottom vertex bonded to a methyl group (CH3). Structure (C) is a five-membered ring with two double bonds and a phosphorus atom at the bottom vertex bonded to a methyl group (CH3).
(a(ii)) Four reaction schemes labeled (A), (B), (C), and (D). Reaction (A) shows PhCH2-H* reacting with Br- to form PhCH2- and H-Br. Reaction (B) shows 4-methoxybenzaldehyde with a labeled aldehydic hydrogen (H) reacting with HCN to form a cyanohydrin product where the original aldehydic hydrogen is now attached to the chiral center. Reaction (C) shows a bicyclic compound (a norbornene derivative) with a labeled hydrogen (H*) on the bridge carbon undergoing a reaction to form anthracene and ethene (H2C=CH2) with one labeled hydrogen (H*). Reaction (D) shows a cyclopentane ring with a labeled hydrogen (H*) on one carbon and a trimethylammonium group (N+Me3) on the adjacent carbon reacting with hydroxide ion (OH-) to form cyclopentene, trimethylamine (Me3N), and water with a labeled hydrogen (*H-OH).
(a(iii)) A reaction scheme starting with 2-butanone (methyl ethyl ketone). An arrow points to the left labeled (A) with reagents NaOMe and MeOH. An arrow points to the right labeled (B) with reagents LDA and THF, -78 degrees C.
(b) A reaction scheme showing ketene, represented with H2C=C=O (two single bonds from H to C, which is double-bonded to C, which is double-bonded to O), reacting over an arrow with conditions: 'i) hv' and 'ii) cis-but-2-ene', leading to a question mark ('?').
(b(i)) Two chemical reactions are shown: Reaction (A): Bis(2-chloroethyl) sulfide, Cl-CH2-CH2-S-CH2-CH2-Cl, reacts with H2O over an arrow labelled 'k1' to form bis(2-hydroxyethyl) sulfide, HO-CH2-CH2-S-CH2-CH2-OH. Reaction (B): 1,5-Dichloropentane, Cl-CH2-CH2-CH2-CH2-CH2-Cl, reacts with H2O over an arrow labelled 'k2' to form 1,5-pentanediol, HO-CH2-CH2-CH2-CH2-CH2-OH.
(b(ii)) A reaction scheme showing two stereoisomers (A) and (B) both reacting with AgClO4 in MeOH to give the same product (C): Compound (A): A pyranose sugar derivative in chair conformation with ring oxygen in the back-right. Substituents are: C5 has an equatorial -CH2OAc group; C4 has an equatorial -OAc group; C3 has an equatorial -OAc group; C2 has an axial -OC(=O)CH3 group pointing downwards; C1 (anomeric position) has an equatorial -Br group. An arrow with reagents 'AgClO4, MeOH' points to Product (C). Compound (B): The same pyranose chair conformation and substituents as (A) at C2, C3, C4, and C5, but at C1 the -Br group is axial (pointing straight down). An arrow labelled 'AgClO4, MeOH' points upwards-right to the same Product (C). Product (C): The same pyranose derivative in chair conformation, with C5 equatorial -CH2OAc, C4 equatorial -OAc, C3 equatorial -OAc, C2 axial -OC(=O)CH3, and at C1 an equatorial -OMe group.
(b(i)) Two chemical reaction schemes labeled (A) and (B). Reaction (A) shows 1,4-bis(chloromethyl)thiane (a six-membered ring containing a sulfur atom, with a -CH2-Cl group attached to the carbon adjacent to the sulfur on both sides) reacting with H2O to form 1,4-bis(hydroxymethyl)thiane (the corresponding diol), with the rate constant labeled k1. Reaction (B) shows 1,5-dichloropentane (a linear chain of 5 carbons with a Cl atom at each end) reacting with H2O to form 1,5-pentanediol (a linear chain of 5 carbons with an OH group at each end), with the rate constant labeled k2.
(b(ii)) A reaction scheme showing two reactants (A) and (B) converting to a single product (C). Reactant (A) is a pyranose derivative (specifically a 2-deoxy-2-bromopyranose) with an acetyl group (AcO) at the C-3 and C-4 positions, a CH2-OAc group at C-5, and an acetoxy group (O-C(=O)CH3) at C-1. The C-2 position has a Br atom. Reactant (B) is the epimer of (A) at the C-2 position, where the Br atom is in the opposite orientation (axial vs equatorial). Both reactants are shown reacting with AgClO4 in MeOH (methanol) to form product (C). Product (C) is the corresponding 2-methoxy derivative, where the Br atom at C-2 has been replaced by an OMe (methoxy) group. The stereochemistry of the other substituents (AcO groups and ring oxygen) remains unchanged.
(b(iii)) A reaction scheme showing the starting material formaldehyde (H2C=O) reacting with cis-but-2-ene. The reaction conditions are listed as i) hv (light) and ii) cis-but-2-ene. An arrow points to a question mark indicating the product is to be determined.
(b(i)) Two chemical reaction schemes labeled (A) and (B). Reaction (A) shows 1,4-bis(chloromethyl)thioether (Cl-CH2-CH2-S-CH2-CH2-Cl) reacting with H2O to form 1,4-bis(hydroxymethyl)thioether (HO-CH2-CH2-S-CH2-CH2-OH). The rate constant for this reaction is labeled k1. Reaction (B) shows 1,6-dichlorohexane (Cl-CH2-CH2-CH2-CH2-CH2-CH2-Cl) reacting with H2O to form 1,6-hexanediol (HO-CH2-CH2-CH2-CH2-CH2-CH2-OH). The rate constant for this reaction is labeled k2.
(b(ii)) A reaction scheme involving two reactants (A) and (B) and one product (C). Reactant (A) is a pyranose derivative (specifically a 2-deoxy-2-bromopyranose derivative) with an acetyl group (AcO) at C-3, C-4, and C-5, and an acetoxy group (OAc) at C-6. The C-2 position has a bromine atom (Br) in an axial orientation. Reactant (B) is the epimer of (A) at C-2, where the bromine atom is in an equatorial orientation. Both reactants react with AgClO4 in MeOH to form the same product (C). Product (C) is the corresponding 2-methoxy derivative, where the bromine atom at C-2 has been replaced by a methoxy group (OMe) in an equatorial orientation.
(b(iii)) A reaction scheme showing the starting material formaldehyde (H2C=O) reacting with cis-but-2-ene. The reaction conditions are listed as i) hv (light) and ii) cis-but-2-ene. An arrow points to a question mark, indicating the product is to be determined.
(c) A reaction scheme showing a cyclopent-3-en-1-yl ring attached at position 1 to a -CH2CH2OTs group (2-(cyclopent-3-en-1-yl)ethyl tosylate). The reaction arrow is labeled above with 'AcOH' and below with 'heat / (Solvolysis) / (विलायक अपघटन)', leading to a question mark ('?').
(c(i)) A reaction scheme showing a starting material: a cyclopentene ring substituted at the 3-position with a 2-tosylethyl group (-CH2CH2OTs). The reaction conditions are AcOH (acetic acid) and heat, with the label '(Solvolytic)'. An arrow points to a question mark indicating the product is to be determined.
(c(i)) A reaction scheme showing a starting material: a cyclopentene ring substituted at the 3-position with a -CH2CH2OTs group (where OTs is a tosylate leaving group). The reaction conditions are AcOH (acetic acid) and heat, with the label '(Solvolytic)'. An arrow points to a question mark, indicating the product is to be determined.
Model answer
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(a)(i) Hückel classification
(A) Cyclopentadiene has an sp³ CH₂, so the ring is not fully conjugated: it is non-aromatic.
(B) 1-Methylborole is planar and cyclic with four π electrons (two C=C) and an empty p orbital on boron that is conjugated with them. 4π (4n) in a cyclic, planar, fully conjugated ring: antiaromatic.
(C) 1-Methylphosphole: the phosphorus lone pair could give 6π, but P is pyramidal and its lone pair is poorly delocalised, so the ring is non-aromatic (at most very weakly aromatic), unlike pyrrole.
(a)(ii) Kinetic isotope effects
Primary KIE: the C–H(D) bond is broken or formed in the rate-determining step. (A) H* is transferred from the benzylic carbon to Br•, and (D) in Hofmann elimination the hydroxide removes H* as the C–H bond breaks.
Secondary KIE: the labelled C–H bond is not broken, but its carbon changes hybridisation or environment. (B) The aldehyde carbon bearing H* goes from sp² to sp³ on cyanohydrin formation. (C) In the retro-Diels–Alder reaction the bridge carbon bearing H* goes from sp³ to sp² in ethene, without breaking its C–H bond.
(a)(iii) Enolates of butan-2-one
(A): NaOMe/MeOH, reversible deprotonation, equilibrium control. The more substituted, more stable enolate, CH₃C(O⁻)=CHCH₃, is the thermodynamic enolate.
(B): LDA (bulky, strong base), THF, −78 °C, irreversible. The base removes the more accessible, less hindered hydrogens of the CH₃ group, giving H₂C=C(O⁻)CH₂CH₃, the kinetic enolate.
(b)(i) Faster hydrolysis of the mustard (A)
The sulfur of (A) is a neighbouring nucleophile. It displaces the chloride from the β-carbon intramolecularly (anchimeric assistance) to give a cyclic episulfonium (thiiranium) ion. Water then opens this strained ion rapidly. The rate-determining step is unimolecular, so the rate is raised by a large factor. In (B) there is no participating group, so water has to attack each C–Cl directly in a slow ordinary SN2 reaction, hence k₁ ≫ k₂.
(b)(ii) Same product from two anomeric bromides
Ag⁺ abstracts bromide from either anomer. In each case the C-2 acetate carbonyl oxygen attacks C-1 from the face opposite the departing bromide and forms the same bridged acetoxonium (1,3-dioxolan-2-ylium) ion. Since both anomers give this one intermediate, the original C-1 configuration is lost. Methanol then attacks C-1 from the face opposite the bridge, giving the 1,2-trans methyl glycoside (C) as the single product. A free oxocarbenium ion would have given an anomeric mixture. This is neighbouring-group participation (Koenigs–Knorr).
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(b)(iii) Ketene, hν, then cis-but-2-ene
Photolysis of ketene (H₂C=C=O) expels CO and gives singlet methylene (:CH₂). Singlet carbene adds to an alkene in one concerted step, so the geometry of the alkene is retained (stereospecific syn addition). The two methyl groups of cis-but-2-ene stay on the same side of the new ring. The major product is cis-1,2-dimethylcyclopropane.
Triplet methylene, by contrast, adds stepwise through a diradical and would give both cis and trans isomers.
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(c)(i) Solvolysis of 2-(cyclopent-3-enyl)ethyl tosylate in AcOH
The substrate is a homoallylic tosylate, and the ring π bond takes part as the tosylate leaves (π-participation, anchimeric assistance). The alkene carbon attacks the carbon bearing OTs and forms a new five-membered ring. The product cation is a bicyclo[2.2.1] (2-norbornyl) cation, drawn classical or nonclassical, which acetic acid captures from the less hindered exo face.
The product is exo-2-norbornyl acetate (exo-bicyclo[2.2.1]hept-2-yl acetate), not the simple unrearranged primary acetate. The carbon skeleton is read here from the drawing.
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(c)(ii) Electrophilic substitution of indole
Indole substitutes preferentially at C-3. Attack at C-3 gives a cation in which the positive charge is on C-2 and is stabilised by the nitrogen lone pair (an iminium-type structure), while the benzene ring keeps its full aromatic sextet. Attack at C-2 places the positive charge on C-3, where it can be stabilised only by delocalising into the benzene ring, which disrupts benzene aromaticity. The C-3 intermediate is therefore lower in energy and forms faster.
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How this answer will be evaluated
Framework: Hückel Rule, Kinetic Isotope Effect, Neighboring Group Participation
What earns full marks: Correct identification of all species and mechanisms with clear reasoning.
Evaluation rubric
Each sub-part is marked on its own, against the marks and word limit printed on the paper.
- (a)(i) Classify the three rings as aromatic, antiaromatic or non-aromatic. 5 marks
derive— given → assumptions → stepwise derivation → result → check
Must cover
- A cyclopentadiene non-aromatic (sp3 CH2)
- B 1-methylborole antiaromatic (4π, empty p on B)
- C 1-methylphosphole non-aromatic (pyramidal P)
- Hückel 4n+2 versus 4n applied
Loses marks
- Calling borole aromatic.
Earns more
- Partial credit for two of three correct.
Extra mark
- Contrasts phosphole with pyrrole.
- (a)(ii) Classify reactions A–D as primary or secondary KIE. 5 marks
enumerate— list the items in order → one line each → no commentary
Must cover
- A primary (H* transferred)
- D primary (E2, H* removed)
- B secondary (sp2 to sp3)
- C secondary (hybridisation change, C–H not broken)
Loses marks
- Calling D secondary.
Earns more
- Partial credit for two correct.
Extra mark
- States the rate-determining-step criterion.
- (a)(iii) Identify A and B and explain. 5 marks
enumerate— list the items in order → one line each → no commentary
Must cover
- A thermodynamic enolate
- B kinetic enolate
- A: NaOMe/MeOH reversible, more substituted
- B: LDA, −78 °C, irreversible, less hindered H
Loses marks
- Swapping kinetic and thermodynamic.
Earns more
- Partial credit for structures without control explanation.
Extra mark
- Gives both enolate structures.
- (b)(i) Explain why the mustard hydrolyses faster. 5 marks
explain— definition/context → points in order → small example → short close
Must cover
- Sulfur acts as internal nucleophile
- Episulfonium (thiiranium) ion
- Anchimeric assistance; faster unimolecular step
- B has no participating group; slow SN2
Loses marks
- Attributing the rate to leaving-group ability.
Earns more
- Partial credit for naming neighbouring-group participation only.
Extra mark
- Notes mustard-gas alkylation link.
- (b)(ii) Explain why both anomers give the same glycoside. 5 marks
explain— definition/context → points in order → small example → short close
Must cover
- Ag+ removes bromide
- C-2 acetate participates
- Common acetoxonium (dioxolanylium) ion
- MeOH attacks anti to give 1,2-trans product
Loses marks
- Invoking a free oxocarbenium ion.
Earns more
- Partial credit for participation without the common ion.
Extra mark
- Koenigs–Knorr named.
- (b)(iii) Give the product of ketene photolysis with cis-but-2-ene, with stereochemistry. 10 marks
explain— definition/context → points in order → small example → short close
Must cover
- Photolysis gives CO + singlet :CH2
- Concerted stereospecific syn addition
- Geometry retained
- cis-1,2-dimethylcyclopropane
Loses marks
- Giving trans or a mixture.
Earns more
- Partial credit for the carbene without stereochemistry.
Extra mark
- Contrasts triplet carbene (mixture).
- (c)(i) Give the solvolysis product and mechanism. 10 marks
explain— definition/context → points in order → small example → short close
Must cover
- Homoallylic tosylate; π participation
- New five-membered ring; 2-norbornyl cation
- Acetate captured from exo face
- exo-2-norbornyl acetate
Loses marks
- Giving the unrearranged primary acetate.
Earns more
- Partial credit for participation without the norbornyl product.
Extra mark
- Mentions nonclassical ion.
- (c)(ii) State the preferred position and explain using resonance. 5 marks
explain— definition/context → points in order → small example → short close
Must cover
- C-3 is preferred
- C-3 attack: charge on C-2, stabilised by N lone pair
- Benzene sextet retained
- C-2 attack disrupts benzene aromaticity
Loses marks
- Choosing C-2.
Earns more
- Partial credit for C-3 without resonance argument.
Extra mark
- Draws both Wheland intermediates.
What "Explain" is asking you to do
Make the working of something clear — what sets it off, what follows from what, and what it produces. Explain is the Commission's mechanism word: it dominates the technical papers and the “explain why” stems, where the marks sit in the causal chain and not in the label.
Structure that answers it
State what it is → the initiating condition → the chain of cause, step by step → an instance where it plays out → what the chain produces
Where marks are lost
Describing what something looks like instead of why it works that way. Naming the stages without linking them reads as description too.
Practice this exact question
Write your answer and it is marked point by point against the model answer above — what you covered, what you missed, what you got wrong.
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