Q1 50M Compulsory explain Organic chemistry reaction mechanisms and stereochemistry
1.(a)(i) pKₐ value of cyclopentadiene is almost similar to water. Explain. 5 marks
1.(a)(ii) Rate of hydrogen exchange reaction in the above compound (A) is 6000 times faster than that of (B). Explain. 5 marks
1.(b)(i) Write the IUPAC nomenclature of the above compound by assigning the stereochemistry. 5 marks
1.(b)(ii) Arrange the above radicals in ascending order of their dimerisation ability. 5 marks
1.(c) The reaction of methyl iodide with sodium azide is faster in N,N-dimethyl formamide (DMF) than in methanol. Explain. 10 marks
1.(d) The above compounds both undergo photo-induced electrocyclic reactions. What are the structures and stereochemistry of the products? 10 marks
1.(e)(i) Identify the major product of the above reaction. 5 marks
1.(e)(ii) Identify the name reaction which produces nitrogen as a byproduct.
(A) Fischer Indole synthesis
(B) von Richter reaction
(C) Stobbe reaction
(D) Bischler-Napieralski reaction 5 marks
Answer approach & key points
Explain the underlying chemical principles for each sub-part, allocating time proportionally: ~15% on (a)(i)-(ii) aromaticity and kinetic acidity, ~15% on (b)(i)-(ii) stereochemical nomenclature and radical stability, ~20% on (c) solvent effects in SN2 reactions, ~25% on (d) Woodward-Hoffmann rules for electrocyclic reactions, and ~25% on (e)(i)-(ii) reaction identification and named reactions. Begin with clear structural representations, develop mechanistic reasoning with orbital diagrams where relevant, and conclude with comparative summaries.
- For 1(a)(i): Explanation of cyclopentadiene's enhanced acidity (pKa ~16) due to aromatic stabilization of cyclopentadienyl anion (6π-electron Hückel system) making it comparable to water (pKa ~15.7)
- For 1(a)(ii): Kinetic vs thermodynamic acidity distinction; compound A (cyclopentadiene) undergoes rapid H/D exchange via aromatic transition state, while compound B (e.g., acetone or similar non-aromatic enolizable compound) lacks this stabilization
- For 1(b)(i): Correct IUPAC name with E/Z or R/S stereochemical descriptors based on Cahn-Ingold-Prelog priority rules; proper numbering and identification of principal functional group
- For 1(b)(ii): Radical dimerization ability correlates inversely with stability; order reflects degree of conjugation, steric hindrance, and resonance stabilization (tertiary < secondary < primary < methyl, or specific order based on given structures)
- For 1(c): Explanation of SN2 rate enhancement in polar aprotic solvents (DMF) vs polar protic (methanol); hard-soft acid-base considerations, nucleophile solvation effects, and transition state stabilization
- For 1(d): Application of Woodward-Hoffmann rules for 4n and 4n+2 π-electron systems; conrotatory vs disrotatory ring closure under photochemical conditions with stereochemical outcome prediction
- For 1(e)(i): Identification of major product based on named reaction mechanism (likely Fischer indole synthesis or related transformation)
- For 1(e)(ii): Recognition that Fischer indole synthesis produces N2 as byproduct via hydrazone intermediate and [3,3]-sigmatropic rearrangement with subsequent elimination
Q2 50M explain Organic chemistry aromaticity and reaction mechanisms
2.(a)(i) Based on Hückel rule, predict the above compounds as aromatic, antiaromatic, and nonaromatic. 5 marks
2.(a)(ii) Identify the above reactions that show primary kinetic isotope effect and secondary kinetic isotope effect. 5 marks
2.(a)(iii) Identify (A) and (B) in the above reactions and explain the mechanism. 5 marks
2.(b)(i) The rate of hydrolysis (k₁ and k₂) of the reaction (A) is much faster than that of (B). Explain. 5 marks
2.(b)(ii) The reaction of the compounds (A) and (B) with AgClO₄ in MeOH gives the same product (C). Explain. 5 marks
2.(b)(iii) Write the major product of the above reaction showing proper stereochemistry and explain the mechanism. 10 marks
2.(c)(i) Write the structure of the product of the above reaction and provide suitable mechanism. 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
Answer approach & key points
Explain requires systematic demonstration of reasoning with evidence. Structure: brief introduction stating Hückel's rule and kinetic isotope effect principles; body addressing each sub-part sequentially with clear mechanistic diagrams, rate comparisons, and stereochemical analysis; conclusion summarizing key electronic effects governing aromaticity and reaction outcomes.
- Correct application of Hückel (4n+2)π rule to classify rings as aromatic, antiaromatic (4nπ, planar), or nonaromatic (non-planar/insufficient conjugation)
- Distinction between primary KIE (C-H/D bond cleavage in rate-determining step) and secondary KIE (hybridization change at adjacent carbon)
- Identification of intermediates (A) and (B) with arrow-pushing mechanisms showing carbocation, carbanion, or radical pathways
- Explanation of rate differences via anchimeric assistance, neighboring group participation, or ring strain effects in hydrolysis
- Stereochemical analysis using chair conformations, anti-periplanar geometry, and orbital overlap requirements for elimination/substitution
- Resonance stabilization of indole electrophilic substitution intermediates at C-2 vs C-3 positions
Q3 50M explain Organic chemistry reactions and mechanisms
3.(a) Would you expect the above conversion to require heat or light? Explain using molecular orbital diagram. (15 marks)
3.(b) 3-Phenyl-4-pentenal is synthesised in the following manner:
(i) Identify the type of pericyclic reaction involved in the formation of unsaturated intermediate.
(ii) What elements are lost when the intermediate is converted to 3-phenyl-4-pentenal?
(iii) Identify the carbon atom in the starting acrylic acid that becomes the aldehyde carbon in the pentenal.
(iv) Write the steps involved for this transformation. (15 marks)
3.(c) Explain the regioselectivity of a Hoffmann elimination reaction with the help of Newman projection formula. (10 marks)
3.(d) Write down the structure(s) of the product(s) obtained in the above reactions. Provide suitable justification and propose the mechanisms.
(i) H₃CO + △
(ii) OCH₃ + △ (10 marks)
Answer approach & key points
Explain the photochemical/thermal requirements for each pericyclic transformation using orbital symmetry principles. For part (a), construct HOMO-LUMO diagrams to justify heat vs light conditions. For part (b), identify the [3,3]-sigmatropic rearrangement (Claisen/Cope type), trace carbon atom mapping from acrylic acid to aldehyde, and show retro-ene or oxidation steps. For part (c), use Newman projections along Cα-Cβ bond to demonstrate anti-periplanar geometry preference for less substituted alkene formation. For part (d), analyze thermal [2+2] cycloadditions or electrocyclic reactions of methoxy-substituted systems. Allocate approximately 30% time to (a), 30% to (b), 20% to (c), and 20% to (d), with diagrams constituting roughly 40% of total response.
- Part (a): Correct identification of conrotatory/disrotatory mode based on 4n/4n+2 π-electron system; construction of HOMO under thermal vs photochemical conditions showing symmetry-allowed pathway
- Part (b)(i): Recognition of [3,3]-sigmatropic rearrangement (oxy-Cope or Claisen variant) as the pericyclic step forming the unsaturated intermediate
- Part (b)(ii)-(iii): Identification of CO₂ or formaldehyde loss; precise carbon mapping from C-1 or C-3 of acrylic acid derivative to aldehyde carbon via isotopic labeling logic
- Part (c): Newman projection showing anti-periplanar β-hydrogen from less substituted carbon (Hofmann rule); explanation of steric vs electronic factors favoring terminal alkene
- Part (d): Structure determination of cycloaddition/electrocyclic products; thermal allowedness based on orbital symmetry; methoxy substituent effects on regioselectivity
Q4 50M solve Organic synthesis and reaction mechanisms
4.(a)(i) Complete the above reaction and write the steps involved in the reaction. (5 marks)
4.(a)(ii) Arrange the above alkenes in ascending order of their relative rate of bromination in methanol. (5 marks)
4.(a)(iii) Complete the above reactions and give suitable mechanism. (10 marks)
4.(b) Complete the above reaction and give suitable mechanism. (10 marks)
4.(c)(i) How many β-hydroxy carbonyl compounds may be theoretically formed in the above reaction? Write the structure of the products formed. (10 marks)
4.(c)(ii) Write the synthesis of benzonitrile starting from benzene (Hint: may involve more than one step). (10 marks)
Answer approach & key points
Solve each sub-part systematically, beginning with reaction completion and mechanism elucidation for 4(a)(i), (iii) and 4(b), followed by rate analysis for 4(a)(ii), stereochemical enumeration for 4(c)(i), and multi-step synthesis for 4(c)(ii). Structure the answer with clear sub-headings, balanced chemical equations with electron-pushing arrows, and concise structural diagrams.
- Correct identification of reaction types: electrophilic addition, nucleophilic substitution, aldol condensation, and Sandmeyer/Gattermann-type transformations
- Accurate depiction of carbocation stability and Markovnikov/anti-Markovnikov regioselectivity in alkene bromination
- Proper arrow-pushing mechanisms showing intermediates: cyclic bromonium ions, enolates, tetrahedral intermediates
- Stereochemical analysis for β-hydroxy carbonyl formation: syn/anti aldol products, racemic mixtures, and crossed aldol possibilities
- Logical synthetic sequence for benzonitrile: nitration → reduction → diazotization → Sandmeyer cyanation or Rosenmund-von Braun alternative
Q5 50M Compulsory explain Organic spectroscopy and reaction mechanisms
(a) How many signals would you expect in the ¹H NMR spectrum of above compounds ? Mark these protons. 10 marks
(b) Compare the C=C stretching vibrations in the above compounds and give a suitable explanation for your answer. 10 marks
(c) (i) Identify A and B. The polymerisation of the rearranged product of B, unaided by any catalyst, gives rise to a 'synthetic rubber'. Name this rubber along with its structural formula.
2 CH≡CH →[Cu₂Cl₂/NH₄Cl] A →[HCl] B
(ii) When rubber balls and other objects made of rubber are exposed to the air for long periods of time, they turn brittle and crack. This does not happen to objects made of polyethylene. Explain. 10 marks
(d) Reduction of camphor with LiAlH₄ leads to 90% of the isomer in which the OH group is cis to the bridge. Give a suitable explanation of this observation. 10 marks
(e) Identify the products in the above reactions with plausible mechanism.
(i) CH₃—C(=O)—CH₃ →[hν/Vapour phase]
(ii) CH₃—C(=O)—CH₃ →[hν/Room temperature] 10 marks
Answer approach & key points
Explain the spectroscopic, mechanistic, and synthetic aspects across all seven sub-parts with balanced coverage: allocate ~15% each to (a) NMR signals and (b) IR vibrations; ~25% to (c) acetylene chemistry and rubber degradation; ~20% to (d) camphor stereochemistry; and ~25% to (e) photochemical mechanisms. Begin with clear structural diagrams, proceed with systematic analysis using chemical principles, and conclude with real-world applications where relevant.
- For (a): Correct number of ¹H NMR signals with proton labeling on given compounds, explaining chemical equivalence and splitting patterns
- For (b): Comparison of C=C stretching frequencies with explanation based on conjugation, ring strain, and substituent effects on bond strength
- For (c)(i): Identification of A as vinylacetylene (CH₂=CH-C≡CH) and B as chloroprene (2-chloro-1,3-butadiene), with neoprene rubber structure and polymerization
- For (c)(ii): Explanation of rubber oxidation via allylic hydrogen abstraction and peroxide formation versus polyethylene stability due to saturated backbone
- For (d): Stereochemical outcome of LiAlH₄ reduction of camphor favoring endo alcohol (isoborneol) via steric approach control and Cieplak model
- For (e)(i): Vapor phase photolysis of acetone giving biacetyl and methane via Norrish Type II cleavage
- For (e)(ii): Room temperature photolysis giving pinacol via radical coupling in liquid phase with different cage effects
Q6 50M solve Spectroscopy, polymer chemistry and organic structure determination
(a)(i) The frequencies of vibration of the following molecules in their v = 0 states are HCl : 2885 cm⁻¹; D₂ : 2990 cm⁻¹; DCl : 1990 cm⁻¹ and HD : 3627 cm⁻¹. Calculate the energy change of the following reaction : HCl + D₂ → DCl + HD. Determine whether energy is liberated or absorbed.
[Given : h = 6·626 × 10⁻³⁴ Js, c = 2·998 × 10⁸ ms⁻¹, Nₐ = 6·022 × 10²³ mol⁻¹] 10 marks
(a)(ii) The IR spectra of butyric acid and ethyl butyrate show sharp strong singlet absorption at 1725 cm⁻¹ and 1740 cm⁻¹, respectively. By contrast, the IR spectrum of butyric anhydride shows a broad, sharp doublet at 1750 cm⁻¹ and 1825 cm⁻¹. Why are these so different ? 5 marks
(b)(i) Write the structure of product(s) in the above reactions : 10 marks
(b)(ii) What is meant by 'Tacticity' of a polymer ? Distinguish among isotactic, syndiotactic and atactic polymers. 5 marks
(c)(i) An organic compound having molecular formula C₁₆H₂₅NO gave following IR and ¹H NMR data : IR(cm⁻¹) = 1690; ¹H NMR(CDCl₃, 400 MHz) : δ 1·11(t, J = 7Hz, 6H), 1·29(d, J = 7Hz 6H), 2·40 (q, J = 7Hz, 4H), 2·55(t, J = 7Hz, 2H), 2·65(t, J = 7 Hz, 2H), 3·12(septet, 1H), 7·21(d, J = 8Hz, 2H), 7·81(d, J = 8Hz, 2H). Determine the structure of the compound. 10 marks
(c)(ii) Assign and arrange the lettered protons in the increasing order of their chemical shift value in ¹H NMR spectrum. 10 marks
Answer approach & key points
Solve this multi-part spectroscopy and polymer chemistry problem by allocating approximately 35% time to part (a) covering vibrational energy calculations and IR interpretation, 25% to part (b) on reaction products and tacticity definitions, and 40% to part (c) involving complete structure elucidation from spectral data. Begin each sub-part with clear identification of the chemical principle involved, show all calculations with proper units, draw unambiguous structures with stereochemistry where relevant, and conclude with explicit answers to each directive.
- For (a)(i): Apply zero-point energy formula E = ½hcν̃ for each molecule, calculate ΔE = [E(DCl) + E(HD)] - [E(HCl) + E(D₂)], convert to kJ/mol using Avogadro's number, and correctly identify energy liberation (exothermic)
- For (a)(ii): Explain Fermi resonance in anhydrides (coupling of C=O stretch with overtone of C-O stretch), symmetric/asymmetric stretching modes, and contrast with isolated C=O in esters/acids
- For (b)(i): Draw correct product structures for unspecified reactions (typically Grignard, reduction, or substitution sequences common in UPSC papers) with proper stereochemistry
- For (b)(ii): Define tacticity as stereochemical arrangement of substituents; distinguish isotactic (same side), syndiotactic (alternating), and atactic (random) with 3D representations
- For (c)(i): Deduce structure as N,N-diisopropyl-4-ethylbutyrylbenzamide or similar amide from IR (1690 cm⁻¹, amide C=O), molecular formula, and complete NMR analysis including coupling patterns and integration
- For (c)(ii): Assign all lettered protons and arrange in order: methyl/methylene (δ 0.9-2.8) < methine (δ 3.1) < aromatic (δ 7.2-7.8), citing shielding/deshielding effects
Q7 50M draw Nucleic acids, photochemistry and organic spectroscopy
(a) (i) Draw the structure of 2'-deoxycytidine-3'-monophosphate.
(ii) Why nucleotides and nucleic acids are acidic ? Mention the factor(s) that stabilize the DNA duplex.
(iii) Show the hydrogen bonding in between cytosine and guanine base pair. 15
(b)(i) Identify all the possible products in the above reaction showing the sequential pathways of their formation : 10
(b)(ii) 1,3-Butadiene solution on irradiation with UV light follows path I. However, irradiating 1,3-butadiene in the presence of benzophenone at 366 nm follows path II. Give a suitable explanation. 5
(c)(i) Write the structure of the compounds (A), (B), (C), (D) and (E). 10
(c)(ii) From the reaction sequence below, answer the following questions :
(I) Write structure of (A) and (B)
(II) Write the oxidation state of Osmium in the intermediate (A) and its geometry. 5
(c)(iii) Write the structure of (A) and (B) in the above reactions. 5
Answer approach & key points
Begin with (a) parts (i)-(iii) on nucleic acids (15 marks), spending ~30% time on accurate structural drawings and explanations of acidity/stabilization. For (b)(i)-(ii) photochemistry (15 marks), allocate ~30% time to identify products with mechanistic pathways and explain photosensitization by benzophenone. Devote ~40% time to (c) spectroscopy/organometallic problems (20 marks), solving all five structures in (c)(i) and the osmium complex in (c)(ii)-(iii) with correct oxidation states and geometries. Use clear sequential numbering for all structures.
- For (a)(i): Correct Haworth projection of 2'-deoxycytidine-3'-monophosphate showing β-configuration at C-1', 2'-deoxy (no OH), phosphate at 3'-position, and cytosine base
- For (a)(ii): Explanation that phosphate groups (pKa ~1-2) make nucleotides acidic; DNA duplex stabilization via Watson-Crick H-bonding, base stacking (van der Waals), and hydrophobic effects in aqueous medium
- For (a)(iii): Accurate depiction of three hydrogen bonds between cytosine (N-3, O-2, N-4) and guanine (N-1, N-2, O-6) with correct donor-acceptor geometry
- For (b)(i)-(ii): Identification of electrocyclic ring closure products (4π conrotatory thermal vs photochemical); explanation of benzophenone as triplet sensitizer enabling intersystem crossing to T1 state of butadiene for disrotatory closure
- For (c)(i): Spectroscopic deduction of five compounds using IR, NMR, and MS data with correct functional group identification and structural elucidation
- For (c)(ii)-(iii): Osmium tetroxide dihydroxylation mechanism showing cyclic osmate ester intermediate with Os(VI) oxidation state and trigonal bipyramidal/octahedral geometry; correct diol products
Q8 50M solve Spectroscopy: mass, UV, rotational, NMR and IR
(a)(i) For the following compound :
(i) Identify the site of initial ionization under EI conditions.
(ii) Draw the structure of ion having m/z = 58 value.
(iii) Calculate the m/z value of metastable ion formed during the formation of above ion. 10
(a)(ii) Draw the structural formula of a ketone with MF C₇H₁₂O that absorbs in the UV with λₘₐₓ = 249 nm. 5
(b)(i) (A) Which of the above molecule(s) will show a microwave rotational spectrum ? Justify your answer.
H₂, HCl, BrF, CH₄ and H₂O
(B) The first rotational absorption of ¹²C¹⁶O was observed at 3·84235 cm⁻¹ while that of ¹³C¹⁶O was observed at 3·67337 cm⁻¹. Find their rotational constants. 10
(b)(ii) Estimate the expected splitting (J in Hz) for the lettered protons in the above compounds : 5
(c)(i) In the ¹H NMR spectrum of acetyl acetone the height of the integration curve at methylene (–CH₂) signal was found to be 10 mm and that of methine (=CH) signal was 22 mm. Calculate the percentage of keto and enol form in the sample. 10
(c)(ii) How could the above pair of isomeric compounds be differentiated by mass spectrometry ? 5
(c)(iii) How many fundamental vibrational frequencies would be expected to be observed in the IR spectrum of CO₂ ? Sketch them. Identify the IR-active vibrations with justification. 5
Answer approach & key points
Solve this multi-part spectroscopy problem by allocating time proportionally to marks: spend ~40% on part (a) [15 marks], ~30% on part (b) [15 marks], and ~30% on part (c) [20 marks]. Begin with clear identification of the compound in (a)(i), then systematically work through each sub-part showing all calculations and structures. For rotational and NMR problems, state relevant formulas before substituting values. Conclude each section with boxed final answers.
- (a)(i) Correct identification of ionization site under EI (lone pair on oxygen), accurate structure of m/z=58 fragment (McLafferty rearrangement product), and correct metastable ion calculation using m* = (m₂)²/m₁
- (a)(ii) Correct structural formula of C₇H₁₂O ketone with extended conjugation matching λₘₐₓ=249 nm (Woodward-Fieser rules application)
- (b)(i)(A) Correct selection of microwave-active molecules (HCl, BrF, H₂O) with justification based on permanent dipole moment requirement for pure rotational spectra
- (b)(i)(B) Accurate calculation of rotational constants B for both isotopologues using ṽ = 2B(J+1) with J=0→1 transition
- (b)(ii) Correct estimation of spin-spin coupling constants (J values) for lettered protons using typical vicinal, geminal, and long-range coupling constants
- (c)(i) Correct calculation of keto-enol equilibrium percentages using integration ratio and the 2:1 proton count relationship
- (c)(ii) Appropriate differentiation strategy using mass spectrometry (fragmentation patterns, McLafferty rearrangement, or molecular ion stability differences)
- (c)(iii) Correct determination of 3N-5 = 4 fundamental vibrations for linear CO₂, with proper sketching and IR activity assignment (asymmetric stretch active, symmetric stretch and bends inactive in IR)