Q1 50M Compulsory explain Organic chemistry reaction mechanisms
(a) Ionization of 3,4-dichloro-1,2,3,4-tetramethylcyclobutene in SbF₅-SO₂ at –75 °C produces a reaction intermediate of its own kind. Predict the intermediate and comment on its stability and aromaticity. (10 marks)
(b) Predict the structures of X and Y, and also mention the major product: (10 marks)
(c) Out of the following list of reactants along with the reagents, identify the reaction which is not a condensation reaction. Propose a suitable mechanism for the products formed in this reaction: (10 marks)
(i) Acetaldehyde is reacted with dilute KOH
(ii) Benzaldehyde is reacted with acetic anhydride in presence of sodium acetate
(iii) Benzaldehyde is reacted with malonic ester in presence of a base
(iv) Benzaldehyde is reacted with concentrated KOH
(d) Write down the structure(s) of the product(s) obtained in the following reactions. Provide suitable justification and propose the mechanisms: (10 marks)
(i) [reaction diagram not shown]
(ii) [reaction diagram not shown]
(e) Predict the product(s) formed on heating the cyclopentadiene and provide a suitable justification to your answer. (10 marks)
Answer approach & key points
Explain the reaction mechanisms and predict intermediates/products for each sub-part with clear reasoning. Allocate approximately 20% time to each sub-part (a-e) as all carry equal marks (10 each). For (a), focus on the cyclobutadienyl dication formation and Hückel's rule; for (b), identify X and Y with stereochemical considerations; for (c), distinguish condensation vs. Cannizzaro mechanism; for (d), draw structures with mechanistic arrows; for (e), analyze Diels-Alder dimerization. Use clear diagrams throughout and conclude with stability justifications.
- (a) Formation of 1,2,3,4-tetramethylcyclobutadienyl dication (2π-electron, aromatic by Hückel's rule) with square planar geometry and diamagnetic character
- (b) Identification of X and Y as reaction intermediates/products with correct stereochemistry and regioselectivity, stating the major product with reasoning
- (c) Recognition that (iv) Benzaldehyde + conc. KOH is NOT a condensation reaction but Cannizzaro reaction; detailed mechanism showing hydride transfer and disproportionation
- (d) Accurate prediction of product structures with curved-arrow mechanisms showing electron flow, stereochemical outcomes, and thermodynamic/kinetic control where applicable
- (e) Prediction of endo-dicyclopentadiene via [4+2] Diels-Alder dimerization with justification based on secondary orbital interactions and Alder endo rule
Q2 50M predict Organic synthesis and reaction mechanisms
(a) Predict the products X and Y in the following reactions: (5×3=15 marks)
(i) [compound] →(1) O₃ (2) H₂O₂→ X + Y
(ii) [compound] →PBr₃→ [intermediate] →Br₂ (57%)→ X
(iii) CH₃—CH(CH₃)—CH₃ →Br₂/hv, 127 °C→ X + Y
(b) (i) What happens when the compound A is heated with one equivalent of HI? Give the structure of the product. Justify your answer: (5 marks)
A [structure not shown]
(ii) Predict [A] and [B] in the following reaction: (5 marks)
[Starting compound] →NBS, CCl₄→ [A] →NaOEt, EtOH/Δ→ [B]
(iii) Give the structures of alkenes expected after dehydrohalogenation of 2-chloro-2,3-dimethylpentane by sodium ethoxide. (5 marks)
(c) (i) Match the following for the synthesis of class of compounds listed in Set-I with the reactants and reagents of Set-II: (10 marks)
Set-I: (A) Quinoline, (B) Isoquinoline, (C) Indole
Set-II: (1) Cinnamaldehyde, hydroxylamine, P₂O₅; (2) Acetaldehyde, phenylhydrazine, acid; (3) Glycerol, aniline, acid, nitrobenzene; (4) α-Halocarbonyl compound, thiourea
Propose the mechanism for the synthesis of indole.
(ii) Write the reaction sequence for the synthesis of L-dopa (B), a drug for the treatment of Parkinson's disease, from L-tyrosine (A), an α-amino acid: (10 marks)
Answer approach & key points
Predict products systematically across all six sub-parts, allocating approximately 30% time to part (a) [15 marks], 20% to part (b) [15 marks], and 50% to part (c) [20 marks]. Begin with clear structural drawings for each reaction, followed by brief mechanistic justification where marks demand explanation. For (c)(ii), explicitly trace the L-dopa synthesis from L-tyrosine with stereochemical retention at the α-carbon.
- (a)(i) Ozonolysis with oxidative workup: identify cleavage products as carboxylic acids/ketones based on alkene substitution pattern
- (a)(ii) PBr₃ conversion of alcohol to alkyl bromide, followed by stereospecific anti-addition of Br₂ yielding meso or racemic dibromide
- (a)(iii) Free-radical bromination selectivity: predict 2° vs 3° hydrogen abstraction ratio at 127°C using relative reactivity data
- (b)(i) HI cleavage of ethers/esters: apply软硬酸碱原理 to predict cleavage site and product structure with justification
- (b)(ii) NBS allylic bromination followed by E2 elimination: identify allylic bromide [A] and conjugated diene [B]
- (b)(iii) Zaitsev vs Hofmann elimination: predict major and minor alkene products from 2-chloro-2,3-dimethylpentane with carbocation stability analysis
- (c)(i) Matching: Quinoline-Skraup (3), Isoquinoline-Bischler-Napieralski variant (1), Indole-Fischer (2); Fischer indole mechanism with [3,3]-sigmatropic rearrangement
- (c)(ii) L-dopa synthesis: electrophilic aromatic substitution (nitration, reduction), diazotization-hydroxylation with retention of configuration at α-carbon
Q3 20M explain Organic reaction mechanism and acetal chemistry
The compound A, acetal of acetaldehyde, on reaction with B, under given reaction conditions, yields C and the final product D :
(i) Write down the structures of the products C and D. Propose a mechanism with suitable explanation.
(ii) Write down the product(s) if ortho-ester E is used instead of the compound A under the similar reaction conditions :
Answer approach & key points
The directive 'explain' demands clear reasoning with mechanistic detail. Structure your answer as: (i) Identify A as acetaldehyde dimethyl acetal (1,1-dimethoxyethane) and B as a Grignard reagent or nucleophile; propose acid-catalyzed cleavage leading to hemiacetal intermediate C and final alcohol D after nucleophilic addition (~60% effort). (ii) For ortho-ester E (triethyl orthoacetate or similar), show analogous cleavage yielding different product distribution due to three OR groups (~40% effort). Use curved arrows throughout and label all intermediates.
- For (i): Correct identification of A as acetaldehyde dimethyl acetal (CH₃CH(OCH₃)₂) and recognition that B is a Grignard reagent (RMgX) or organolithium reagent acting under acidic workup conditions
- For (i): Structure of C as the hemiacetal intermediate (CH₃CH(OH)(OCH₃)) formed after protonation and loss of one methoxy group; structure of D as the tertiary alcohol (CH₃CH(OH)R) from nucleophilic addition
- For (i): Detailed acid-catalyzed mechanism showing protonation of acetal oxygen, departure of methanol as leaving group, formation of oxonium ion, nucleophilic attack by R⁻ from Grignard reagent, and acidic workup
- For (ii): Recognition that ortho-ester E (e.g., triethyl orthoacetate CH₃C(OCH₂CH₃)₃) undergoes similar acid-catalyzed cleavage but produces ethyl ester and ethanol as products instead of hemiacetal/alcohol pathway
- For (ii): Explanation that ortho-esters have three alkoxy groups leading to different reactivity—two equivalents of ethanol eliminated to form ester rather than stable hemiacetal intermediate
- Comparative insight: Both reactions demonstrate protection/deprotection strategy in organic synthesis; acetals protect aldehydes while ortho-esters serve as acyl anion equivalents or protecting groups for carboxylic acids
Q4 50M explain Organic synthesis and reaction mechanisms
(a) (i) Complete the following reaction by showing stepwise reaction mechanism for the formation of products :
(ii) The compound A is optically active and upon treating A with alcoholic sodium ethoxide, it looses its optical activity. Justify :
(b) (i) If more than one equivalent of Br₂ at high temperature are allowed to react with cyclopentane, how many dibromocyclopentanes would you expect as products? Draw their structures and name them.
(ii) How will you convert p-nitrotoluene to m-nitrotoluene?
(c) By using appropriate reagents and conditions, how will you convert phenol into coumarin? Give suitable mechanism for this transformation.
Answer approach & key points
Begin with a brief introduction acknowledging the diverse reaction types covered. For part (a), spend ~25% time on mechanism with curved arrows and stereochemistry; for (b)(i), allocate ~20% on systematic enumeration of dibromo isomers with clear naming; for (b)(ii), devote ~15% on the diazotization route with positional control; for (c), reserve ~40% on the Pechmann condensation mechanism with all intermediates. Conclude by summarizing the synthetic strategies employed.
- Part (a)(i): Complete stepwise mechanism showing carbocation formation, rearrangement if any, and nucleophilic attack with proper curved arrow notation
- Part (a)(ii): Explanation of racemization via SN1/SN2/E2 pathway with formation of planar intermediate or elimination leading to achiral product
- Part (b)(i): Enumeration of all dibromocyclopentane stereoisomers (1,1-; 1,2-cis/trans; 1,3-cis/trans) with correct IUPAC names including stereodescriptors
- Part (b)(ii): Synthetic route via nitration of toluene, oxidation to benzoic acid, nitration at meta position, then reduction and re-oxidation OR diazotization-based approach
- Part (c): Pechmann condensation using malic acid/sulfuric acid or Knoevenagel-type condensation with β-keto ester, showing all mechanistic steps including electrophilic aromatic substitution and lactonization
- Stereochemical analysis: Identification of meso compounds, enantiomeric pairs, and optical activity changes in relevant parts
- Reagent and condition specificity: Concentrated H₂SO₄, high temperature, alcoholic NaOEt, etc. with their mechanistic implications
Q5 50M Compulsory discuss Organic chemistry and spectroscopy
(a) Write the structures of nucleosides and nucleotides, and discuss the primary structures of DNA and RNA. (10 marks)
(b) Write down the products obtained after photolysis of 2-methylcyclohexanone in solution phase. Explain the formation of products. (10 marks)
(c) (i) By using appropriate reactants, reagents and conditions and using acetylene as starting material, how will you synthesize the following compound?
(ii) Identify the products W, X, Y and Z in the following reactions: (10 marks)
(d) (i) How will you distinguish between NH stretching absorption of a primary amine and a secondary amine by using IR spectroscopy?
(ii) How will you distinguish among primary, secondary and tertiary alcohols on the basis of PMR spectroscopy? (8 marks)
(e) Given below are the IR and NMR spectral characteristics of three compounds:
(i) IR : 1750 cm⁻¹; NMR : δ 2·0 (s, 3H), 5·1 (s, 2H) and 7·3 (s, 5H)
(ii) IR : 1740 cm⁻¹; NMR : δ 3·5 (s, 3H), 3·6 (s, 2H) and 7·4 (s, 5H)
(iii) IR : 3200–2800 (various bands) and 1700 cm⁻¹; NMR : δ 2·75 (t, 2H), 2·95 (t, 2H), 7·4 (s, 5H) and 12·0 (s, 1H)
Match each of these spectral data with one of the following structures: (1), (2), (3), (4), (5) (10 marks)
Answer approach & key points
This multi-part question requires balanced coverage across 8 sub-parts totaling 48 marks. Allocate approximately 20% time to part (a) on nucleic acids, 20% to part (b) on photochemistry, 20% to part (c) on synthesis and reaction identification, 17% to part (d) on spectroscopic distinctions, and 23% to part (e) on spectral interpretation. Begin each sub-part with clear structural diagrams, follow with mechanistic or explanatory text, and conclude with specific applications or distinctions requested.
- Part (a): Correct structures of nucleoside (base + sugar) and nucleotide (base + sugar + phosphate); DNA vs RNA primary structure differences in sugar (deoxyribose vs ribose) and bases (T vs U)
- Part (b): Norrish Type I cleavage products from 2-methylcyclohexanone; biradical intermediate formation and subsequent fragmentation pathways in solution phase
- Part (c)(i): Multi-step synthesis from acetylene using appropriate reagents (NaNH₂, alkyl halides, hydration, oxidation, etc.); (c)(ii): Identification of products W, X, Y, Z with correct structures
- Part (d)(i): IR distinction of 1° vs 2° amines by NH stretching (doublet vs singlet, ~3400-3300 cm⁻¹ region); (d)(ii): PMR distinction using OH proton exchange, coupling patterns, and chemical shift differences
- Part (e): Correct matching of spectral data to structures—(i) ester with benzyl group, (ii) methyl ester with CH₂, (iii) carboxylic acid with broad OH and characteristic α-protons
Q6 50M explain Organic reactions and polymer chemistry
(a) (i) Predict the products in the following reactions: Propose suitable mechanism to justify your answer.
(ii) Giving justification, write the major and minor product(s). Comment upon the chirality of recovered reactant (if any). (5+10=15 marks)
(b) (i) How will you synthesize polypropylene (PP) by using Ziegler-Natta catalysis? Discuss the mechanism and its advantages over conventional polymerization. (10 marks)
(ii) How is Perlon synthesized from ε-Caprolactam? Give the mechanism of the reaction. (5 marks)
(c) (i) Write the structures of the products X, Y and Z in the following reactions and indicate the mechanism for the formation of X: (1) OsO₄, Pyridine (2) H⁺, H₂O; X →[HIO₄] Y + Z (10 marks)
(ii) Write the major and minor products in the following reaction. Discuss the stereochemistry along with reaction mechanism for the formation of the major product: Ph-C(=O)-CH₃ →[m-CPBA] ? + ? (Major product + Minor product) (10 marks)
Answer approach & key points
Explain the reaction mechanisms and product formations across all four sub-parts, allocating approximately 30% time to (a)(i)-(ii) combined (15 marks), 30% to (b)(i)-(ii) combined (15 marks), and 40% to (c)(i)-(ii) combined (20 marks). Begin with clear product predictions and mechanisms for each transformation, using curved-arrow notation and stereochemical representations throughout. Conclude with comparative remarks on catalytic systems where relevant.
- (a)(i) Correct prediction of products with detailed mechanistic pathway showing electron flow and intermediates
- (a)(ii) Identification of major vs minor products with regio/stereochemical justification; analysis of chirality in recovered reactant
- (b)(i) Ziegler-Natta synthesis of polypropylene: TiCl₄/AlEt₃ system, coordination-insertion mechanism, isotactic control, advantages over free-radical polymerization
- (b)(ii) Perlon (Nylon-6) synthesis: ε-caprolactam ring-opening polymerization, acid/base-catalyzed mechanism with initiation and propagation steps
- (c)(i) OsO₄ dihydroxylation product X, HIO₄ cleavage to carbonyl fragments Y and Z; syn-addition mechanism with cyclic osmate ester intermediate
- (c)(ii) Baeyer-Villiger oxidation: regioselectivity based on migratory aptitude (Ph > CH₃), retention of configuration, stereochemical outcome with mechanism
Q7 50M discuss Protein structures and spectroscopic analysis
(a) Discuss the different types of secondary structures of proteins and compare these structures with tertiary structure of proteins. 15 marks
(b) Complete the following reactions and give suitable mechanisms for the formation of products:
(i) CH₃—CH₂—CH₂—C(=O)—Cl (1) 2NaBH₄ → ?
(2) H₃O⁺
(ii) CH₃—C≡C—CH₃ Na in liq. NH₃ → ?
10+5=15 marks
(c) (i) A compound (A) containing C, H and O has molecular weight 102 and displays two signals in the ¹H NMR spectrum at δ 1·1 (d) and 3·55 (septet) in the integral ratio of 6:1. Treatment of A with 1 mole of HI gives rise to B and C. In the IR spectrum, B gives a strong absorption band at 3330 cm⁻¹, whereas its ¹H NMR spectrum shows signals at δ 1·05(d, 6H), 3·6 (septet, 1H) and 4·4 (s, 1H, disappeared with D₂O). The ¹H NMR spectrum of C gives signals at δ 1·9(d, 6H) and 4·25 (septet, 1H). Reaction of A with excess of HI gives only C. Identify A, B and C. Write all the reactions involved. 10 marks
(ii) Three isomeric compounds having MF C₅H₁₀O give positive 2,4-DNP test and display the following NMR spectral characteristics. Identify the compounds. Among them, one isomeric compound on treatment with KOH (concentrated) gives two products. Write their structures also:
(1) A triplet at δ1.05 and a quartet at δ2.47
(2) Two singlets
(3) A doublet at δ1.0, a singlet at δ2.1 and a septet at δ2.2
10 marks
Answer approach & key points
The directive 'discuss' for part (a) requires comprehensive coverage with critical comparison, while parts (b) and (c) demand mechanistic and analytical problem-solving. Allocate approximately 30% time/words to part (a) (15 marks), 30% to part (b) (15 marks), and 40% to part (c) (20 marks). Structure: begin with protein secondary structures and their comparison with tertiary structure; proceed to complete reaction mechanisms with clear electron-flow arrows; conclude with systematic spectral analysis for structural elucidation of unknowns.
- Part (a): Description of α-helix (right-handed, 3.6 residues/turn, 0.54 nm pitch, intrachain H-bonds) and β-pleated sheet (parallel and antiparallel, interchain H-bonds); comparison with tertiary structure (3D folding, hydrophobic interactions, disulfide bridges, ionic bonds, hydrogen bonds)
- Part (b)(i): Recognition that NaBH₄ reduces acid chloride to aldehyde then to primary alcohol; mechanism involves hydride attack on carbonyl carbon followed by protonation; final product CH₃CH₂CH₂CH₂OH
- Part (b)(ii): Birch reduction of internal alkyne; trans-alkene formation via radical anion mechanism; product is trans-2-butene
- Part (c)(i): Structure determination of A as isopropyl isopropyl ether (diisopropyl ether), B as isopropanol, C as isopropyl iodide; interpretation of NMR splitting patterns (d and septet indicating isopropyl group), IR OH stretch at 3330 cm⁻¹, and D₂O exchange
- Part (c)(ii): Identification of three isomers—(1) pentan-2-one, (2) pentan-3-one, (3) 3-methylbutan-2-one; recognition that pentan-3-one undergoes self-condensation with concentrated KOH to give diacetone alcohol and mesityl oxide
Q8 50M explain Spectroscopic techniques in organic chemistry
(a) (i) Among the following compounds I and II, which has more carbonyl stretching frequency in IR spectra? Explain:
I
II
5 marks
(ii) Complete the following reaction. Write the structures of A, B and C, and explain how IR spectroscopy is helpful to distinguish among them:
H₃C OH CH₃
H⁺
A + B + C
10 marks
(b) (i) Calculate the λ_max values of the following compounds:
(1)
(2)
10 marks
(ii) Explain why 1,3-butadiene exhibits a lower λ_max for π → π* transitions compared to that of 1,3,5-hexatriene.
5 marks
(c) (i) What is McLafferty rearrangement? Discuss the mass spectral fragmentation of butyl butyrate with the following given data of ions:
m/z 101, m/z 73, m/z 71 and m/z 56
Write the structures of fragment ions.
10 marks
(ii) (1) A compound with MF C₂H₂BrCl exhibits two doublets (J = 16 Hz) in its PMR spectrum. Suggest a suitable structure along with other possible structures.
(2) How can the structures of A and B be decided based on their UV spectral data?
[ λ_max = 296 nm (ε_max = 10700) and
λ_max = 281 nm (ε_max = 20800) ]
10 marks
Answer approach & key points
Begin with a concise introduction on spectroscopic techniques as structural tools. For (a), allocate ~15 marks worth: explain carbonyl frequency trends using conjugation/hybridization effects, then complete the dehydration reaction identifying A (alkene), B (ether), C (alkene isomer), with IR distinguishing C=C vs C=O regions. For (b), spend ~15 marks: apply Woodward-Fieser rules for λ_max calculations, then explain conjugation length effect on HOMO-LUMO gap. For (c), allocate ~20 marks: define McLafferty rearrangement with radical mechanism, draw fragmentation pathway for butyl butyrate showing m/z 101 (acylium), 73, 71, 56; for (c)(ii) deduce trans-1-bromo-2-chloroethene from J=16 Hz coupling, and assign λ_max 296 nm to trans-stilbene (higher ε, planar) vs 281 nm to cis-stilbene using steric hindrance arguments. Conclude with integration of spectroscopic data for structural elucidation.
- (a)(i) Correct identification of higher ν(C=O) in compound with less conjugation/stronger C=O bond; explanation via resonance and bond order effects
- (a)(ii) Acid-catalyzed dehydration of 2-butanol giving 1-butene (A), 2-butene mixture (B=trans, C=cis or B=ether if intermolecular); IR distinguishes O-H (3300 cm⁻¹), C=C (1650 cm⁻¹), C-O (1100 cm⁻¹) regions
- (b)(i) Accurate Woodward-Fieser rule application: base value + substituent increments + solvent corrections for both diene/triene systems
- (b)(ii) Explanation of bathochromic shift with extended conjugation: decreased HOMO-LUMO energy gap in hexatriene vs butadiene
- (c)(i) McLafferty rearrangement definition: γ-hydrogen transfer via six-membered cyclic TS with radical cation cleavage; specific fragment structures for butyl butyrate (m/z 101: CH₃CH₂CH₂CO⁺, m/z 73: CH₃CH₂CO⁺, m/z 71: C₄H₉⁺, m/z 56: C₄H₈⁺·)
- (c)(ii)(1) trans-1-bromo-2-chloroethene from J=16 Hz (trans vicinal coupling); cis isomer excluded by large J value
- (c)(ii)(2) UV assignment: higher λ_max with lower ε (296 nm, ε=10700) for trans-stilbene vs lower λ_max with higher ε (281 nm, ε=20800) for cis-stilbene due to steric hindrance reducing planarity and conjugation efficiency in cis form