Chemistry

UPSC Chemistry 2025

All 16 questions from the 2025 Civil Services Mains Chemistry paper across 2 papers — 800 marks in total. Each question comes with a detailed evaluation rubric, directive word analysis, and model answer points.

16Questions
800Total marks
2Papers
2025Exam year

Paper I

8 questions · 400 marks
Q1
50M Compulsory solve Physical Chemistry - Quantum mechanics, thermodynamics, electrochemistry

(a) Calculate the ratio of probability of finding the 1s electron of hydrogen atom at r = a₀ and at r = 10a₀, where 'r' is the distance from the nucleus and a₀ = radius of the first Bohr orbit. (5 marks) (b) Construct the Born-Haber cycle for the formation of sodium chloride crystal at 298 K from the elements in their normal states of existence. Mention the names of the involving processes. Indicate which of them are energy demanding and which are energy evolving. (5 marks) (c) Germanium and Silicon elements have very low electrical conductivity. How can the electrical conductivity be enhanced by adding other elements in trace amount? Explain by examples. (5 marks) (d) Two sheets of copper of area 1·50 m² are separated by 10 cm. What is the rate of transfer of heat by conduction from the warm sheet (50°C) to the cold sheet (−10°C)? What is the rate of loss of heat? (Assume the space between the two sheets is filled with air) Given: Coefficient of thermal conductivity of air = 2·4 × 10⁻² Js⁻¹ m⁻¹ K⁻¹ (5 marks) (e) Why do liquids become superheated before boiling? Explain using Kelvin equation. (5 marks) (f) Arrange the following molecules in the ascending order of their dipole moment values. Justify your answer. NH₃, NF₃ and H₂O (5 marks) (g) 0·500 g of benzoic acid was burnt under oxygen. The combustion produced a temperature rise of 1·236 K. The same calorimetric set-up was used to burn 0·300 g of naphthalene and the resulting temperature rise was 1·128 K. The heat of combustion of benzoic acid, ΔcU²⁹⁸ = – 3227 kJ mol⁻¹. What is the heat of combustion of naphthalene? (5 marks) (h) A sealed container contains a gaseous sample at 300 K consisting of either pure ethane, or pure neon, or a mixture of the two. The pressure inside the container at this temperature is 1·00 atm. When the container is cooled to 150 K, the pressure is 0·37 atm. What is the composition of the sample; pure ethane, pure neon or a mixture of both? Explain your answer. Given: Vapour pressure of C₂H₆ at 150 K is 0·10 atm Critical temperature of neon = 44 K (5 marks) (i) The surface area of an object to be gold plated is 49.8 cm², and the density of gold is 19.3 g/cm³. A current of 3.25 A is applied to a solution that contains gold in the +3 oxidation state. Calculate the time required to deposit an even layer of gold, 1 × 10⁻³ cm thick, on the object. (Given: Molecular mass of gold = 196.97 g/mol) (5 marks) (j) A steam turbine is operated with an intake temperature of 400°C, and an exhaust temperature of 150°C. What is the maximum amount of work the turbine can do for a given heat input 'Q'? Under what conditions is the maximum work achieved? (5 marks)

Answer approach & key points

This multi-part question requires solving ten distinct problems spanning quantum mechanics, thermodynamics, electrochemistry, and solid-state chemistry. Allocate approximately 4-5 minutes per sub-part, prioritizing numerical accuracy and correct formula application. Begin with parts (a), (d), (g), (i), and (j) which involve direct calculations, then proceed to conceptual parts (b), (c), (e), (f), and (h). For each calculation, explicitly state the formula, substitute values with units, and present final answers with appropriate significant figures.

  • Part (a): Apply radial probability distribution for 1s orbital: P(r) ∝ r²e^(-2r/a₀), calculate ratio P(a₀)/P(10a₀) = (1/e²)/(100/e²⁰) = e¹⁸/100
  • Part (b): Construct complete Born-Haber cycle for NaCl showing: sublimation of Na (endothermic), ionization of Na (endothermic), dissociation of Cl₂ (endothermic), electron gain by Cl (exothermic), lattice formation (exothermic)
  • Part (c): Explain doping in semiconductors—n-type (adding P/As to Si/Ge) and p-type (adding B/Ga to Si/Ge) with band theory and increased conductivity mechanism
  • Part (d): Apply Fourier's law of heat conduction: dQ/dt = kA(ΔT/d), calculate heat transfer rate using given thermal conductivity of air
  • Part (e): Explain superheating using Kelvin equation ln(p/p₀) = 2γVₘ/rRT, showing how small bubble radius creates high vapor pressure barrier delaying boiling
  • Part (f): Compare dipole moments considering lone pair contributions and bond polarity: NF₃ (0.23 D) < NH₃ (1.47 D) < H₂O (1.85 D), explain opposing effects in NF₃
  • Part (g): Use calorimetry principle—determine calorimeter constant using benzoic acid data, then calculate naphthalene heat of combustion from temperature rise
  • Part (h): Apply ideal gas law and concept of condensation; neon remains gas at 150 K (T >> Tc), ethane partially condenses (P < vapor pressure), so mixture shows intermediate pressure behavior
  • Part (i): Apply Faraday's laws: calculate mass of Au from volume and density, use m = ZIt where Z = M/nF, determine time for electrodeposition
  • Part (j): Calculate Carnot efficiency η = 1 - T₂/T₁ for maximum work, state conditions: reversible operation, infinite time, no entropy generation
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Q2
50M solve Quantum mechanics, electrochemistry, solid state chemistry

(a) Find the probability of existence of a particle in a one-dimensional box of length 'a' in the region 0 ≤ x ≤ a/4 for the states n = 1, 2 and 3. (15 marks) (b) The standard reduction potential of oxygen under acidic conditions at 298 K is +1.23 V. What is the standard reduction potential for the four-electron reduction of O₂(g) under basic conditions? (15 marks) (c) The radii of Zn²⁺ and S²⁻ ions are 0·74 Å and 1·84 Å respectively. Determine the most stable form of arrangement of ions in ZnS crystal lattice. Draw the CCP (Cubic Close Packing) structure of ZnS. (15 marks) (d) In a sample of NaCl, one of every 10,000 sites, normally occupied by Na⁺, is occupied instead by Ca²⁺. Assuming that all of the Cl⁻ sites are fully occupied, what is the stoichiometry of the sample? (5 marks)

Answer approach & key points

Solve each sub-part systematically with clear mathematical derivations and logical reasoning. For (a), derive the probability using wavefunction integration (~30% time); for (b), apply Nernst equation with pH adjustment (~25% time); for (c), calculate radius ratio and identify structure type with diagram (~30% time); for (d), apply charge neutrality for defect chemistry (~15% time). Present calculations step-wise with proper units and significant figures.

  • For (a): Set up probability integral P = ∫₀^(a/4) |ψₙ|²dx using ψₙ = √(2/a) sin(nπx/a), evaluate for n=1,2,3 obtaining values ~0.091, 0.25, 0.303 respectively
  • For (b): Write half-reactions for acidic vs basic conditions, apply E°(basic) = E°(acidic) - (0.0591×4/4)log[H⁺]⁴ or use E° = 1.23 - 0.0591×pH at pH=14 to get ~0.40 V
  • For (b): Alternative correct approach using ΔG° = -nFE° and Kw relationship to find E° = +0.40 V for O₂ + 2H₂O + 4e⁻ → 4OH⁻
  • For (c): Calculate radius ratio r⁺/r⁻ = 0.74/1.84 = 0.402, identify range 0.414-0.732 for octahedral but note ZnS has tetrahedral coordination (zinc blende/wurtzite)
  • For (c): Draw CCP (fcc) structure of ZnS showing S²⁻ at lattice points and Zn²⁺ in alternate tetrahedral voids, or vice versa, with correct coordination numbers
  • For (d): Apply charge balance: for 10,000 Na⁺ sites, 9,999 Na⁺ + 1 Ca²⁺ requires 10,001 Cl⁻ for neutrality, giving stoichiometry Na₀.₉₉₉₉Ca₀.₀₀₀₁Cl₁.₀₀₀₁ or approximately Na₂CaCl₃ when scaled
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Q3
50M calculate Physical chemistry - thermodynamics, kinetics, surface chemistry, phase equilibria

(a) Calculate the number of collisions that oxygen makes per second on 1·00 cm² of the surface of the vessel containing them if the pressure is 1·00 × 10⁻⁶ atm and the temperature is 25°C. 10 (b) Suppose that 10·0 J of work is required to create droplets of uniform size from a mole of water in bulk at 25°C and 1 atm pressure. (i) Assuming that surface tension is independent of area, calculate the radius of the droplets. (ii) Calculate the number of water molecules in a droplet. Given : Surface tension of water = 0·072 J/m² 15 (c) You are given the following data for butane : Normal melting point = – 138°C Normal boiling point = 0°C Critical temperature = 152°C Critical pressure = 38 atm Assume that the triple point is slightly lower in temperature than the melting point and that the vapour pressure at the triple point is 3 × 10⁻⁵ torr. (i) Sketch a phase diagram for butane. (ii) Butane at 1 atm and 140°C is compressed to 40 atm. Are two phases present at any time during this process ? (iii) Butane at 1 atm and 200°C is compressed to 40 atm. Are two phases present at any time during this process ? 10 (d) A container with 100 g of ice at 0°C is placed in a humid room whose temperature is 40°C. The ice melts as water vapour condenses into the container. Assuming that all the heat transferred to the container comes from the condensation, how much water will have condensed in the container once all the ice is melted and has reached 40°C ? Given : Heat of fusion of ice = 334 Jg⁻¹ Heat of vaporization of water = 2260 Jg⁻¹ Heat capacity of water = 4184 J kg⁻¹ K⁻¹ 10 (e) Explain why crystalline solids are generally more defective as a result of increasing temperature. 5

Answer approach & key points

Begin with the directive to calculate across all sub-parts, showing systematic problem-solving. Allocate approximately 20% time to part (a) on collision theory, 30% to part (b) on surface tension and droplet formation, 20% to part (c) on phase diagrams with careful sketching, 20% to part (d) on thermal equilibrium calculations, and 10% to part (e) on crystal defects. Structure as: brief statement of principles → step-by-step calculations with units → labeled diagram for (c) → concluding physical interpretation of results.

  • Part (a): Apply kinetic theory of gases using Z = (P/√(2πmkT)) × N_A to find collision frequency per unit area, converting pressure to SI units and using O₂ molecular mass
  • Part (b)(i): Use work of surface creation W = γ × ΔA = γ × n × 4πr² with n droplets from total surface area to solve for droplet radius
  • Part (b)(ii): Calculate molecules per droplet using droplet volume, water density, and Avogadro's number
  • Part (c)(i): Sketch phase diagram with correctly positioned triple point (-138°C, 3×10⁻⁵ torr), critical point (152°C, 38 atm), and phase boundaries showing solid-liquid line with negative slope
  • Part (c)(ii)-(iii): Analyze compression paths relative to critical point to determine phase coexistence, noting 140°C < T_c and 200°C > T_c
  • Part (d): Set up energy balance: heat from condensation = heat for fusion + heating water, solving for condensed mass using latent heats and specific heat
  • Part (e): Explain Schottky and Frenkel defect formation with Arrhenius-type temperature dependence, citing Boltzmann factor for defect concentration
  • Physical interpretation: Connect numerical results to real phenomena (e.g., droplet stability in clouds, LPG storage conditions, humidity effects)
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Q4
50M explain Physical chemistry - molecular orbital theory, electrochemistry, thermodynamics

(a) Draw the molecular orbital (MO) diagram of NO molecule. The experimental bond dissociation energy of NO is 626 kJ mol⁻¹ while that of NO⁺ is 1047 kJ mol⁻¹ — rationalize it. NO can also act as a reactive radical — how ? 20 (b) Calculate the diffusion limiting current for the oxidation of an organic compound at an electrode in a quiescent solution. Assume six electrons are involved in the reaction and the thickness of diffusion layer is taken as 0.05 cm in an unstirred solution. Given : (i) Concentration of organic compound, Corganic = 10⁻² mole litre⁻¹ (ii) Diffusion coefficient of organic compound, Dorganic = 2 × 10⁻⁵ cm² sec⁻¹ 10 (c) The level of conductivity in a decimolar aqueous solution of calcium nitrate, which is a strong electrolyte, is measured as σ = 26·2 mS cm⁻¹ at 25°C. Calculate the molar conductivity of the electrolyte, that of calcium ions and the transport numbers of the two types of ions present in the solution, with the data given below. Molar conductivity at infinite dilution in an aqueous solution at 25°C : (i) λ₊⁰ (mS m² mol⁻¹) Ca²⁺ 11·9 (ii) λ₋⁰ (mS m² mol⁻¹) NO₃⁻ 7·14 10 (d) Suppose we redefine the standard state as Pressure, P = 2 atm. Find the new standard ΔG°f values of each substance : (i) HCl (g) (ii) N₂O (g) Explain the results in terms of the relative entropies of reactants and products of each reaction. Given : Standard free energy of formation at 25°C : (i) ΔG°HCl = –95.3 kJ mol⁻¹ (ii) ΔG°N₂O = +103.7 kJ mol⁻¹ 10

Answer approach & key points

Begin with a concise introduction stating that the question spans molecular orbital theory, electrochemistry, and thermodynamics. Allocate approximately 40% of effort to part (a) given its 20 marks—draw the MO diagram first, then explain bond order changes and radical character. Spend 20% each on parts (b), (c), and (d), showing all calculation steps with proper units. For (b), apply the Ilkovic equation for diffusion-limited current; for (c), use Kohlrausch's law and ionic mobility relationships; for (d), apply the pressure correction to standard Gibbs free energy using ΔG = ΔG° + RTln(P/P°). Conclude by summarizing how theoretical frameworks connect across physical chemistry domains.

  • Part (a): Correct MO diagram for NO (11 valence electrons) showing σ2s, σ*2s, σ2p, π2p, π*2p, σ*2p energy levels with proper electron filling; bond order calculation (NO = 2.5, NO⁺ = 3) explaining higher BDE of NO⁺; explanation of unpaired electron in π*2p orbital conferring radical reactivity
  • Part (b): Application of Ilkovic equation i_d = nFAD^(1/2)C/δ or equivalent diffusion-limited current expression; correct unit conversions (litre to cm³, time consistency); final calculation yielding ~0.186 mA or equivalent with proper significant figures
  • Part (c): Molar conductivity Λ_m = κ/C calculation (26.2 mS cm⁻¹ / 0.1 mol L⁻¹ = 262 mS cm² mol⁻¹ or 26.2 mS m² mol⁻¹); application of Kohlrausch law Λ_m = ν₊λ₊ + ν₋λ₋ to find individual ionic conductivities; transport numbers t₊ = λ₊/Λ_m and t₋ = λ₋/Λ_m
  • Part (d): Correct application of ΔG°(new) = ΔG°(old) + RTln(P_new/P_old) = ΔG°(old) + RTln(2) for each gas; recognition that gases with higher entropy (more complex molecules like N₂O vs HCl) show larger corrections; explanation that ΔG°f becomes more positive for products when standard pressure increases
  • Part (a) bonus: Mention of NO's biological significance as signaling molecule (e.g., vasodilation) connecting radical chemistry to physiological function; reference to NO's role in atmospheric chemistry
  • Cross-part synthesis: Demonstration of how thermodynamic driving forces (part d) relate to electrochemical potentials (part c) and kinetic limitations (part b) in real chemical systems
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Q5
50M Compulsory solve Physical and Inorganic Chemistry

(a) The gas-phase reaction between methane (CH₄) and diatomic sulphur (S₂) is given by the following reaction: CH₄ (g) + 2S₂ (g) ———→ CS₂ (g) + 2H₂S (g). At 823 K, the rate constant for this reaction is 1·1 × 10⁻³ m³ mol⁻¹ S and at 898 K, the rate constant is 6·4 × 10⁻³ m³ mol⁻¹ S. Calculate the activation energy for this reaction. [Given: R = 8·3145 J K⁻¹ mol⁻¹] (5 marks) (b) A dye solution containing 1 g per 100 cc transmitted 60% of the blue light in a cell 1 cm thick. (i) What percentage of light would be absorbed by a solution containing 2 g per 100 cc in the same cell? (ii) What should be the cell thickness so that 90% of the light is absorbed by the original solution? (5 marks) (c) At 0°C and 1 atm pressure, the volume of nitrogen (N₂) gas required to form a monolayer on a sample of charcoal is 155·5 cm³ gm⁻¹ of charcoal. Calculate the surface area per gram of charcoal. Area of cross-section of a N₂ molecule is 0·160 (nm)². [Given: Nₐ = 6·022 × 10²³ mol⁻¹; Molar volume of gas (at 0°C and 1 atm pressure), Vₘ = 22·414 dm³ mol⁻¹] (5 marks) (d) From the following data for the decomposition of ammonium nitrite in aqueous solution, show that the reaction is of the first order. | Time (minutes) | 10 | 15 | 20 | 25 | ∞ | |---|---|---|---|---|---| | Volume of N₂ (cc) | 6·25 | 9·00 | 11·40 | 13·65 | 35·05 | (5 marks) (e) Draw the structure of Ferredoxin protein containing [4Fe – 4S] and find the average oxidation number of iron. (5 marks) (f) Which complex ion has higher value of crystal field splitting energy (Δ₀)? Justify your answer. (i) [Co(H₂O)₆]³⁺ or [Co(H₂O)₆]²⁺ (ii) [Co(NH₃)₆]³⁺ or [Rh(NH₃)₆]³⁺ (iii) [Co(H₂O)₆]³⁺ or [Co(NH₃)₆]³⁺ (10 marks) (g) Complete the following reactions: (i) S₄N₄ →[SbCl₅] (ii) XeF₆ →[PtF₅] (iii) B₂H₆ →[NH₃][-120°C] (iv) 8KrF₂ + 2Au → (v) XeOF₄ + XeO₃ → (10 marks) (h) The magnetic moment of d-block elements arises mainly from the contribution of spin motion of the electrons but in case of f-block elements, it is not valid. Justify the statement. (5 marks)

Answer approach & key points

Solve each sub-part systematically with clear step-by-step calculations and reasoning. Allocate approximately 8-10 minutes per 5-mark sub-part: (a) and (d) require Arrhenius equation and first-order kinetics derivations; (b) and (c) need Beer-Lambert law and surface area calculations; (e) and (g) demand accurate structures and reaction products; (f) and (h) require comparative analysis with proper justifications. Present final answers with appropriate units and significant figures.

  • Part (a): Correct application of Arrhenius equation in two-point form: ln(k₂/k₁) = (Ea/R)[(T₂-T₁)/(T₁T₂)] to calculate activation energy ≈ 138-140 kJ mol⁻¹
  • Part (b): Application of Beer-Lambert law (A = εcl) showing (i) 84% absorption for doubled concentration, and (ii) 2.15 cm path length for 90% absorption
  • Part (c): Calculation of number of N₂ molecules using ideal gas law, then surface area = (number of molecules) × (cross-sectional area) ≈ 670-675 m² g⁻¹
  • Part (d): Demonstration of first-order kinetics using k = (2.303/t)log[V∞/(V∞-Vt)] with consistent k values (~0.034 min⁻¹) across time intervals
  • Part (e): Cubane-like [4Fe-4S] cluster structure with Fe at alternate corners, S at other corners; average oxidation state calculation showing +2.5 or Fe³⁺Fe²⁺ mixed valence
  • Part (f): Systematic comparison: (i) Co³⁺ > Co²⁺ (higher charge), (ii) Rh³⁺ > Co³⁺ (4d > 3d), (iii) [Co(NH₃)₆]³⁺ > [Co(H₂O)₆]³⁺ (NH₃ stronger field); citing spectrochemical series and nephelauxetic effect
  • Part (g): Complete reactions: (i) [S₄N₄]²⁺[SbCl₅]₂⁻ adduct, (ii) [XeF]⁺[PtF₆]⁻, (iii) BH₃·NH₃ adducts, (iv) 2[Kr₂F]⁺[AuF₄]⁻, (v) 2XeOF₄ + XeO₃ → 3XeO₂F₂
  • Part (h): Explanation of spin-only magnetic moment for d-block (μ = √[n(n+2)] BM) vs. f-block where orbital contribution persists due to less effective quenching by ligand fields and spin-orbit coupling
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Q6
50M explain Physical and Inorganic Chemistry

(a) The following molecule shows the rigid or fluxional behaviour at higher temperature or in the presence of a base. Justify the answer with the help of ¹H NMR spectrum. (10 marks) (b) Consider the following photochemical reaction: H₂ (g) + Br₂ (g) →[hν] 2HBr (g). Give the mechanism of this reaction. Applying steady-state approximations to [Br] and [H], show that the rate of formation of HBr (g) varies with the square root of the intensity (Iₐ) of the absorbed radiation. What is the quantum yield for this reaction? Why is the value so low? (20 marks) (c) Mentioning the requisite assumptions, derive the equation of the Langmuir adsorption isotherm. Show that the Langmuir isotherm holds at low pressure but fails at high pressure. (20 marks)

Answer approach & key points

Begin with a brief introduction acknowledging the three distinct areas: fluxional NMR behaviour, photochemical kinetics, and surface adsorption. Allocate approximately 20% of effort to part (a) (10 marks), 40% to part (b) (20 marks), and 40% to part (c) (20 marks). For part (a), identify the molecule (likely PF₅ or similar Berry pseudorotation system) and explain temperature-dependent NMR coalescence. For part (b), write the complete chain mechanism, apply steady-state approximation to both intermediates, and derive the rate law showing Iₐ^(1/2) dependence. For part (c), state all five Langmuir assumptions explicitly, derive θ = KP/(1+KP), and explain deviations at high pressure due to multilayer formation or surface heterogeneity. Conclude by summarizing the unifying theme of dynamic processes across timescales.

  • Part (a): Identification of fluxional molecule (e.g., PF₅, Fe(CO)₅, or metal hydride cluster) with Berry pseudorotation mechanism; explanation of temperature-dependent ¹H NMR showing coalescence of signals from axial/equatorial positions at low T to single averaged signal at high T; calculation of activation energy from coalescence temperature using Eyring equation
  • Part (b): Complete chain mechanism with initiation (Br₂ + hν → 2Br•), propagation steps (Br• + H₂ → HBr + H•; H• + Br₂ → HBr + Br•), and termination; correct steady-state approximation for [Br] and [H] leading to d[HBr]/dt = k[H₂][Iₐ]^(1/2)/[M]^(1/2); explicit derivation showing square root dependence on absorbed intensity
  • Part (b): Quantum yield calculation (Φ ≈ 2 at low conversion, decreasing due to recombination); explanation of low quantum yield via radical recombination, back reaction, and cage effect competing with product formation
  • Part (c): Five explicit Langmuir assumptions (monolayer, uniform surface, no interaction, dynamic equilibrium, constant adsorption enthalpy); step-by-step derivation from rate of adsorption = rate of desorption to obtain θ = KP/(1+KP) or equivalent linear forms
  • Part (c): Demonstration that at low P, θ ≈ KP (Henry's law region, linear) while at high P, θ → 1 (saturation); explanation of failure via BET multilayer adsorption, surface heterogeneity, or lateral interactions; mention of Temkin or Freundlich isotherms as corrections
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Q7
50M distinguish Biochemistry and chemical kinetics

(a) Distinguish the "T" (tense) and 'R' (relax) conformations of Hemoglobin on reversible binding of oxygen (O₂). 15 marks (b) What is the basic difference between Cytochrome 'b' and Cytochrome 'c' ? Explain the role of Cytochrome 'c' oxidase. 5 marks (c) Draw the possible stereoisomers of the following complex and explain their optical activity. 10 marks (d) (i) Consider a second-order reaction : A + B → P where the initial concentration of A is 'a' mol dm⁻³ and that of B is 'b' mol dm⁻³. After time 't', x mol dm⁻³ of A and x mol dm⁻³ of B react to form x mol dm⁻³ of the product, P. Show that the second-order rate constant for this reaction will be given by k₂ = 1/((a-b)t) ln [b(a-x)/a(b-x)] with the assumption that a > b. What will be the unit of k₂ ? (Consider time in seconds) (ii) Determine the units of the rate constants for zeroth-order and 5/2 order reactions. Assume that concentrations are expressed in mol dm⁻³ and time in seconds. 10 marks (e) Complete the following reaction and explain the mechanism with the help of pi-bonding theory. 10 marks

Answer approach & key points

Begin by distinguishing T and R states of hemoglobin with structural details (part a, ~30% time). Then contrast cytochromes b and c, explaining oxidase function (part b, ~10%). Draw and label stereoisomers with optical activity analysis (part c, ~20%). Derive the second-order rate equation with proper integration steps, state k₂ units, then determine units for zeroth and 5/2 order reactions (part d, ~20%). Complete the reaction with pi-bonding mechanism explanation (part e, ~20%). Conclude with integrated biological significance.

  • T (tense) vs R (relaxed) hemoglobin conformations: T-state has constrained Fe²⁺ out of porphyrin plane with low O₂ affinity; R-state has Fe²⁺ in-plane with high O₂ affinity; cooperative binding and Hill coefficient ~2.8-3.0
  • Cytochrome b (membrane-bound, b-type heme with two iron protoporphyrin IX, no covalent attachment) vs cytochrome c (peripheral membrane, c-type heme with covalent cysteine thioether bonds); cytochrome c oxidase as Complex IV with Cu_A, Cu_B, heme a, heme a₃, proton pumping, 4e⁻ reduction of O₂ to H₂O
  • Stereoisomers of octahedral complex [M(AA)₂B₂] type showing cis and trans forms; cis-[M(AA)₂B₂] as optically active (chiral, non-superimposable mirror images) and trans as optically inactive (achiral with plane of symmetry)
  • Derivation of second-order rate law: dx/dt = k₂(a-x)(b-x), integration using partial fractions to obtain k₂ = 1/[(a-b)t] × ln[b(a-x)/a(b-x)]; unit of k₂ as dm³ mol⁻¹ s⁻¹
  • Units determination: zeroth-order as mol dm⁻³ s⁻¹; 5/2 order as dm^(15/2) mol^(-5/2) s⁻¹ or dm^7.5 mol^-2.5 s⁻¹
  • Reaction completion with pi-bonding mechanism: nucleophilic attack on metal-carbonyl or alkene complex with back-bonding explanation using molecular orbital theory
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Q8
50M draw Inorganic chemistry and coordination compounds

(a) Draw the structures of S₂N₂, S₄N₂, S₁₁N₂, S₅N₆ and I₂Cl₆. 10 marks (b) What are silicones ? Mention some of their uses. How will you prepare hexamethyldisiloxane ? Draw its structure. What happens if some (CH₃)₃ SiCl is mixed with (CH₃)₂ SiCl₂ and hydrolysed ? 10 marks (c) How many lanthanides can be easily separated from the lanthanide mixture by using valency change method ? Justify your answer. 10 marks (d) Write electronic configuration (outer) of the following lanthanide ions and calculate the magnetic moment in BM from L-S coupling. 10 marks (i) Pr³⁺ (g = 4/5) (ii) Tb³⁺ (g = 3/2) (e) Complete the following chemical reactions and indicate the category of these reactions. Justify your answer. 10 marks (i) 2[Co(CN)₅]³⁻ + MeI ———→ (ii) [Ru(CO)₃(PPh₃)₂] + MeI ———→

Answer approach & key points

This question demands precise structural drawings and chemical reasoning across five sub-parts. Allocate approximately 20% time to part (a) for five accurate sulfur-nitrogen and halogen structures; 20% to part (b) covering silicone definition, uses, preparation, and co-hydrolysis; 15% to part (c) on lanthanide separation via valency change; 20% to part (d) for electronic configurations and magnetic moment calculations using L-S coupling; and 25% to part (e) for completing organometallic reactions with proper categorization. Begin with clear labeled diagrams, follow with systematic explanations, and conclude with justified reaction mechanisms.

  • Part (a): Correct planar square structure of S₂N₂ with alternating S-N bonds; S₄N₂ as six-membered ring with transannular S-S bond; S₁₁N₂ as two fused S₇ rings with N atoms; S₅N₆ with cage structure; I₂Cl₆ as planar bridged dimer with two bridging Cl atoms
  • Part (b): Definition of silicones as polymeric organosiloxanes; uses in sealants, lubricants, medical implants, and water repellents; preparation of hexamethyldisiloxane via hydrolysis of (CH₃)₃SiCl; linear structure with Si-O-Si bridge; co-hydrolysis yields cross-linked silicone polymers with controlled properties
  • Part (c): Identification of Ce, Eu, and Yb as separable lanthanides; justification based on stable +4 (Ce⁴⁺) and +2 (Eu²⁺, Yb²⁺) oxidation states enabling selective oxidation/reduction and precipitation/solubility differences
  • Part (d): Electronic configuration of Pr³⁺ as 4f² with ³H₄ term symbol; calculation of μ_eff = 3.58 BM using g=4/5; Tb³⁺ as 4f⁸ with ⁷F₆ term symbol; calculation of μ_eff = 9.72 BM using g=3/2 with proper J value
  • Part (e): Completion of [Co(CN)₅]³⁻ + MeI → [Co(CN)₅Me]³⁻ + I⁻ as SN2-type oxidative addition; [Ru(CO)₃(PPh₃)₂] + MeI → [Ru(CO)₃(PPh₃)₂MeI] as oxidative addition with 18-electron rule violation; proper categorization as organometallic oxidative addition reactions with electron count justification
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Paper II

8 questions · 400 marks
Q1
50M Compulsory explain Organic chemistry - aromaticity and reaction mechanisms

(a) (i) Tropolone is aromatic, but fulvene is non-aromatic. Why? (ii) Explain with example pseudo-aromaticity. (b) Identify the missing reagent and intermediates in the following chemical conversion : (c) Write the structure of the major product when neomenthyl chloride is reacted with sodium ethoxide in ethanol. Justify your answer : (d) (i) Discuss in detail how the reaction of a carbene with cis-2-butene can be used to define the spin state (S/T) of carbene. (ii) para-Bromophenol on reaction with NaNH₂/NH₃ (l) followed by acidic workup yields one major product. Explain the reaction by writing the steps involved. (e) With example, elucidate the permanent and temporary denaturation of a protein.

Answer approach & key points

The directive 'explain' demands clear reasoning with cause-effect relationships across all sub-parts. Structure your answer as: brief definitions for (a)(i-ii) on aromaticity concepts; stepwise mechanisms with curved arrows for (b) and (d)(ii); stereochemical analysis with chair conformations for (c); orbital diagrams for (d)(i); and biochemical examples for (e). Allocate approximately 25% time to (a), 20% each to (b), (c), and (d), and 15% to (e), ensuring each sub-part receives proportional depth.

  • (a)(i) Tropolone: 10π electrons, planar, follows Hückel's rule (4n+2), dipolar resonance stabilization; Fulvene: 6π electrons but non-planar, lacks cyclic delocalization, dipolar contributor dominates
  • (a)(ii) Pseudo-aromaticity: systems with 4n π-electrons showing temporary aromatic stabilization in transition states or excited states; example: cyclobutadiene rectangular distortion or Dewar benzene
  • (b) Identification of reagents and intermediates for the given conversion (likely involving Birch reduction, ozonolysis, or similar transformation with clear electron-pushing)
  • (c) Neomenthyl chloride: E2 elimination, anti-periplanar requirement, menth-2-ene (more substituted, Zaitsev) vs menth-3-ene; neomenthyl allows better H-C(4) alignment, yielding predominantly 3-menthene
  • (d)(i) Singlet vs triplet carbene: stereospecificity test—singlet gives cis-cyclopropane (concerted), triplet gives mixture (stepwise via diradical); orbital diagrams showing spin correlation
  • (d)(ii) Benzyne mechanism: elimination-addition, formation of benzyne intermediate, regioselectivity of nucleophilic attack, final product m-bromophenol or aniline derivative depending on substitution pattern
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Q2
50M explain Organic chemistry - reaction mechanisms and synthesis

(a) (i) In the presence of sodium ethoxide, the following transformation occurs. Explain : (ii) Propose a suitable mechanism for the following transformation : (b) (i) The following reaction does not produce the product shown : (1) Predict the major product from the conditions shown above, and write a detailed mechanism for its formation. (2) Write that reaction conditions which would lead to successful synthesis of the product shown above (i.e., 3,3-dimethyl-2-butanol). (ii) Write the structure of the major product(s) formed in the following reaction. Justify your answer : (c) Write the structure of the major product(s) formed in the following reactions : (i) (ii) (iii) (iv)

Answer approach & key points

Explain the mechanistic rationale for each transformation across all sub-parts, allocating approximately 25% time to (a)(i)-(ii) on base-catalyzed rearrangements, 35% to (b)(i)-(ii) on Grignard reactions and stereochemical outcomes, and 40% to (c)(i)-(iv) on pericyclic and photochemical reactions. Begin with clear structure drawings, follow with curved-arrow mechanisms, and conclude with stereochemical justifications where applicable.

  • For (a)(i): Explanation of Favorskii rearrangement or base-catalyzed ring contraction/expansion with sodium ethoxide driving elimination-addition pathway
  • For (a)(ii): Detailed E1cB or SN2 mechanism with proper curved arrows showing nucleophilic attack and leaving group departure
  • For (b)(i)(1): Prediction of tertiary alkoxide or elimination product instead of desired alcohol due to steric hindrance with t-BuMgBr; mechanism showing competing pathways
  • For (b)(i)(2): Alternative conditions using less hindered Grignard reagent or different carbonyl compound (acetone + isopropylmagnesium bromide) for successful synthesis
  • For (b)(ii): Identification of major product based on Cram's rule or Felkin-Anh model with stereochemical justification for nucleophilic addition to chiral carbonyl
  • For (c)(i)-(iv): Structures of products from Diels-Alder, photochemical [2+2] cycloaddition, sigmatropic rearrangement, or electrocyclic reactions with correct stereochemistry
  • For all parts: Proper representation of stereochemistry (R/S, E/Z, syn/anti) in product structures where applicable
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Q3
50M elucidate Organic synthesis and reaction mechanisms

(a) (i) Write the structure of the product formed in the following reactions: (1) (2) (10 marks) (ii) Describe the synthesis of ketone (A) when only isobutanol is available as the starting material: (5 marks) (b) (i) Write the steps involved in the following conversion: (10 marks) (ii) Elucidate the structure of C and D in the following conversion: (5 marks) (c) (i) Provide the structure of missing reagents and intermediates in the following synthesis: [diagram] (10 marks) (ii) Write the structure of the product formed and the steps involved in the following reaction: [diagram] (10 marks)

Answer approach & key points

The directive 'elucidate' demands clear, detailed explanation with structural clarity. Allocate time proportionally: ~35% (18 minutes) for (a)(i) product structures and (a)(ii) isobutanol-to-ketone synthesis; ~20% (10 minutes) for (b)(i) conversion steps; ~10% (5 minutes) for (b)(ii) structures C and D; ~20% (10 minutes) for (c)(i) reagents/intermediates; and ~15% (7 minutes) for (c)(ii) product and mechanism. Begin with clear product structures, proceed through logical synthetic sequences with reagents and conditions, and conclude with mechanistic arrows where required.

  • For (a)(i): Correct product structures from given reactions with proper stereochemistry and regioselectivity considerations
  • For (a)(ii): Complete synthesis of ketone (A) from isobutanol via oxidation to isobutyraldehyde, then Grignard/appropriate carbon-carbon bond formation or oxidation sequence
  • For (b)(i): Stepwise conversion with all reagents, conditions, and intermediate structures clearly indicated
  • For (b)(ii): Accurate structural elucidation of C and D based on spectroscopic or chemical evidence implied in the conversion
  • For (c)(i): Missing reagents (e.g., PCC, Jones reagent, LiAlH4, Grignard reagents, Wittig reagents) and all intermediate structures in multi-step synthesis
  • For (c)(ii): Final product structure with curved-arrow mechanism showing electron flow, transition states where relevant, and stereochemical outcome
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Q4
50M calculate Polymers and biomolecules

(a) (i) After polymerization of p-hydroxybenzoic acid, IR analysis shows 0·2% unreacted —COOH. Calculate the molecular weight of the polymer and categorize it. (7 marks) (ii) Write the structure of the polymer formed when p-hydroxybenzoic acid is polymerized. (3 marks) (iii) What is the precursor of the main component of natural rubber? Draw the structures of both monomer and polymer. (5 marks) (b) (i) Compare the salient properties of Nylon 6 and Nylon 6,6. (5 marks) (ii) How would you prepare syndiotactic polystyrene? (5 marks) (iii) What are the salient features of alpha helix in regards to handedness, residues per turn and elongation per residue? What are the principal stabilizing factors for an alpha helix? (5 marks) (c) (i) Compounds 1 and 2 on reaction with NaN₃ yield same product but compound 1 reacts at room temperature while compound 2 reacts at 200 °C. Identify the product formed and explain the reasons for different reactivities of compounds 1 and 2: (10 marks) (ii) Identify the major product in the following reactions and justify your answer: (10 marks)

Answer approach & key points

Calculate the molecular weight of poly(p-hydroxybenzoic acid) using Carothers' equation for (a)(i), then draw structures for (a)(ii) and (a)(iii). Compare Nylon 6 and Nylon 6,6 properties for (b)(i), explain syndiotactic polystyrene preparation for (b)(ii), and describe alpha helix features for (b)(iii). For (c), identify products and explain reactivity differences with mechanistic reasoning. Allocate ~25% time to numerical calculations in (a), ~35% to comparative and descriptive parts in (b), and ~40% to mechanistic explanations in (c).

  • For (a)(i): Apply Carothers' equation using 0.2% unreacted —COOH to calculate DP and molecular weight of poly(p-hydroxybenzoic acid); categorize as liquid crystalline aromatic polyester (Vectra-type)
  • For (a)(ii): Draw the linear aromatic polyester structure with ester linkages and para-substituted benzene rings
  • For (a)(iii): Identify isopentenyl pyrophosphate (IPP) as precursor; draw isoprene (2-methyl-1,3-butadiene) and cis-1,4-polyisoprene structures with correct stereochemistry
  • For (b)(i): Compare Nylon 6 (caprolactam, single monomer, 6 carbons) vs Nylon 6,6 (hexamethylenediamine + adipic acid, two monomers, 6+6 carbons) on properties like mp, tensile strength, moisture absorption
  • For (b)(ii): Explain syndiotactic polystyrene preparation using metallocene catalysts (e.g., zirconocene with MAO) or Ziegler-Natta catalysts with specific conditions for syndiotacticity
  • For (b)(iii): State alpha helix is right-handed, 3.6 residues/turn, 1.5 Å rise per residue; identify hydrogen bonds between C=O of residue i and N—H of residue i+4 as principal stabilizing factor
  • For (c)(i): Identify product as azide or amine; explain compound 1 is allylic/benzylic halide (SN2, room temp) vs compound 2 is vinyl/aryl halide (requires high temp, addition-elimination or radical mechanism)
  • For (c)(ii): Predict major products based on regioselectivity and stereochemistry; justify using Baldwin's rules, neighboring group participation, or pericyclic selection rules as applicable
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Q5
50M Compulsory solve Organic reactions, spectroscopy, photochemistry

(a) Write the structure of the product(s) and the intermediate formed in the following reaction: Cl₂CH—COCl 1) Et₃N → ? 2) Cyclopentadiene, Heat (10 marks) (b) Deduce the structure of the starting material (A) and all the intermediates formed in each step that would lead to the formation of the following product through the defined reactions: 1) NaNH₂, EtI A 2) Lindlar's cat., H₂ → OMe 3) NBS, ROOR, Δ 4) NaOMe, MeOH (10 marks) (c) A photochemical reaction takes place through T₁ state. S₀–S₁ and S₀–T₁ energy gaps correspond to 290 nm and 450 nm, respectively. To get an efficient photochemical reaction should we use light of 290 nm or 450 nm? Give your answer presenting the relevant Jablonski diagram. (10 marks) (d) (i) Which of the following molecules is/are active to rotational spectroscopy and why? CH₄, H₂O, NH₃, BCl₃, XeF₄ (5 marks) (ii) The spacing between lines in the microwave spectrum of CO decreases by substituting ¹²C by ¹³C. Why? (5 marks) (e) (i) In a 100 MHz NMR instrument, a particular set of protons absorbs at δ = 3.0 with J = 4.5 Hz. Find the chemical shift (in Hz) and the coupling constant J in a 500 MHz instrument for the same set of protons. (5 marks) (ii) The mass spectrum of n-butyl phenyl ketone (C₆H₅COCH₂CH₂CH₂CH₃) shows peaks at m/z 162, 120, 105 and 85. Predict the fragmentation pattern. (5 marks)

Answer approach & key points

Solve each sub-part systematically with clear structural drawings and calculations. Allocate approximately 15-18 minutes for parts (a) and (b) each (mechanistic organic chemistry), 12-15 minutes for part (c) (photochemistry with Jablonski diagram), 10-12 minutes for part (d) (rotational spectroscopy theory), and 8-10 minutes for part (e) (NMR calculations and mass fragmentation). Begin with structures/equations, show stepwise reasoning, and conclude with clear final answers for each sub-part.

  • Part (a): Formation of dichloroketene via α-elimination with Et₃N, followed by [2+2] cycloaddition with cyclopentadiene to give bicyclic adduct; identify both ketene intermediate and final product structure
  • Part (b): Retro-synthetic analysis revealing A as a terminal alkyne (propargyl alcohol derivative), with intermediates including alkynide anion, cis-alkene from Lindlar reduction, allylic bromide from NBS, and final SN2' product; correct stereochemistry at each step
  • Part (c): Selection of 450 nm light for efficient T₁ photochemistry (direct S₀→T₁ excitation or sensitized pathway); Jablonski diagram showing S₀, S₁, T₁ states with correct energy gaps, ISC, and radiative/non-radiative pathways
  • Part (d)(i): Rotational activity requires permanent dipole moment; H₂O and NH₃ active (asymmetric rotors), CH₄, BCl₃, XeF₄ inactive (zero dipole due to symmetry)
  • Part (d)(ii): Explanation via reduced mass μ = m₁m₂/(m₁+m₂) increasing with ¹³C, causing decrease in rotational constant B = h/(8π²cI) and hence line spacing 2B
  • Part (e)(i): Chemical shift scales with frequency (1500 Hz at 500 MHz), J remains constant at 4.5 Hz (field-independent)
  • Part (e)(ii): Mass fragmentation via α-cleavage and McLafferty rearrangement: m/z 162 (M⁺), 120 (loss of C₃H₆), 105 (C₆H₅CO⁺), 85 (C₆H₁₃⁺ or rearrangement ion)
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Q6
50M elucidate Organic reactions, spectroscopy, photochemistry

(a) (i) Elucidate the structure of the product and the intermediate (if any) in the following reactions: (A) [structure: H(Me)C(CO₂Et)(CH₂CO₂H)] 1) BH₃ 2) H⁺ → ? (B) [structure: cyclohexyl-CH=CH-CHO] NaBH₄ → ? (5+5=10 marks) (ii) Describe the role of NMO during the dihydroxylation of an alkene using catalytic amount of OsO₄ in the presence of N-methylmorpholine N-oxide (NMO). (5 marks) (b) (i) Calculate the frequency of radiation required for a transition of J = 4 to J = 5 in the rotational spectrum of HCl. The rotation constant B = 10·6 cm⁻¹. (5 marks) (ii) The fundamental vibrational frequency of HCl is 2990 cm⁻¹. Calculate the fundamental vibrational frequency of DCl assuming the same bond strength. (5 marks) (iii) The molecular formula of a compound is C₃H₃N. The IR absorption frequencies are 1650 cm⁻¹, 2250 cm⁻¹ and 3100 cm⁻¹. Assign a structure for the compound. (5 marks) (c) (i) Write the mechanism for the following photochemical transformation: [structure not shown in transcript] (5 marks) (ii) Photobromination of cinnamic acid was carried out by using light of wavelength 480 nm with a light intensity of 1·5×10⁻³ J-s⁻¹. An exposure of 10 minutes showed a decrease of 0·05 millimole of Br₂. Calculate the quantum yield assuming that 80% of radiation is absorbed by cinnamic acid. (Planck's constant h = 6·627×10⁻³⁴ J-s and velocity of light c = 3×10⁸ m-s⁻¹) (5 marks) (iii) Predict the major and minor products for the following photoreaction. Give the logic: [structure not shown in transcript] (5 marks) (iv) Assign A, B and C in the following reaction: (Major) [reaction not fully shown in transcript] (5 marks)

Answer approach & key points

Begin with the directive 'elucidate' which demands clear, detailed explanation with structural clarity. Allocate time proportionally: ~25% on (a)(i) organic transformations (BH₃ and NaBH₄ reductions), ~15% on (a)(ii) NMO role in dihydroxylation, ~25% on (b) spectroscopy calculations (rotational, vibrational, IR assignment), and ~35% on (c) photochemistry (mechanism, quantum yield calculation, product prediction, and assignment). Structure as: concise introduction stating principles, systematic part-wise treatment with balanced equations and clear diagrams, and brief concluding synthesis where relevant.

  • (a)(i) BH₃ selectively reduces carboxylic acid over ester; intermediate is acyloxyborane; product is H(Me)C(CO₂Et)(CH₂CH₂OH). NaBH₄ reduces aldehyde over alkene; product is cyclohexyl-CH=CH-CH₂OH
  • (a)(ii) NMO acts as co-oxidant/regenerating agent; reoxidizes Os(VI) to Os(VIII) enabling catalytic cycle; prevents toxic OsO₄ stoichiometric use
  • (b)(i) Rotational transition J=4→5: ν = 2B(J+1) = 10.6×10 = 106 cm⁻¹; convert to frequency using c = νλ
  • (b)(ii) Isotopic shift: ν(DCl)/ν(HCl) = √(μ_HCl/μ_DCl) = √(35.5/36.5 × 37/71) ≈ 0.717; ν_DCl ≈ 2144 cm⁻¹
  • (b)(iii) IR assignments: 3100 cm⁻¹ (=C-H), 2250 cm⁻¹ (C≡N), 1650 cm⁻¹ (C=C); structure is CH₂=CH-CN (acrylonitrile/vinyl cyanide)
  • (c)(ii) Quantum yield φ = (moles reacted)/(moles photons absorbed); calculate E = hc/λ, photon flux, total energy, then φ ≈ 2.1
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Q7
50M explain Organic spectroscopy and pericyclic reactions

(a) (i) Acetone shows a weak absorption at 280 nm and a strong absorption at 190 nm in the UV spectrum. Account for the observation. (5 marks) (ii) Using Woodward-Fieser rules, calculate λ_max for the following compounds: A, B, C, D (12 marks) (b) (i) Rank the following dienes in order of increasing reactivity in a Diels-Alder reaction (1 = least reactive, 4 = most reactive). Briefly explain your answer: A, B, C, D (10 marks) (ii) Write the structure of the product in the following reaction: OMe + CN 1) Heat 2) H⁺, H₂O ? (5 marks) (c) (i) Write the structure of the product formed in the following sigmatropic rearrangement and categorize it with suitable explanation. Explain the thermal feasibility of this rearrangement by drawing orbital diagram: H H Bu 80 °C CCl₄ → ? (10 marks) (ii) (1) Identify the mode of ring closure for each of the following electrocyclic reactions: (A), (B) (2) Are the indicated hydrogens cis or trans? (10 marks)

Answer approach & key points

Explain the spectroscopic and pericyclic phenomena across all sub-parts with precise chemical reasoning. Allocate approximately 35% of effort to part (a) covering UV transitions and Woodward-Fieser calculations (17 marks), 30% to part (b) on Diels-Alder reactivity and product prediction (15 marks), and 35% to part (c) on sigmatropic rearrangements and electrocyclic reactions (20 marks). Begin with clear identification of transition types and orbital interactions, proceed through systematic application of rules with drawn structures, and conclude with stereochemical outcomes and thermal feasibility justifications.

  • For (a)(i): Explain n→π* (weak, 280 nm) and π→π* (strong, 190 nm) transitions in acetone, citing symmetry-allowed/forbidden nature and molar absorptivity differences
  • For (a)(ii): Apply Woodward-Fieser rules for diene/ene-one λ_max calculation with correct base values, increment additions (homodienyl, exocyclic, substituent effects), and solvent corrections for compounds A-D
  • For (b)(i): Rank dienes by s-cis conformation accessibility, electron-donating/withdrawing substituents, and steric factors; identify locked s-cis (most reactive) vs s-trans locked (least reactive)
  • For (b)(ii): Predict Diels-Alder adduct with correct regioselectivity (ortho/para preference) and stereochemistry from electron-rich diene (OMe) and electron-deficient dienophile (CN)
  • For (c)(i): Identify [3,3]-sigmatropic rearrangement (Cope/Claisen), draw product with stereochemical fidelity, and explain thermal feasibility via HOMO-LUMO orbital symmetry matching
  • For (c)(ii): Determine conrotatory/disrotatory modes based on π-electron count (4n vs 4n+2) and thermal/photochemical conditions; assign cis/trans relationships of indicated hydrogens in products
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Q8
50M justify Spectroscopic analysis and structure elucidation

(a) (i) Estimate the expected splitting (coupling constant J in Hz) for the lettered protons in the ¹H NMR spectrum of the following compounds: (1) A, (2) B, (3) C (5 marks) (ii) Compare the chemical shifts of the labelled protons Hᵃ and Hᵇ in the ¹H NMR spectrum of the following compounds and justify your answer. (10 marks) (iii) Count the number of peaks observed in the ¹H NMR spectrum of the following compounds. Justify your answer: (1), (2) (5 marks) (b) (i) A halogenated ester shows M⁺ peak at m/z 166 (10%) and M+2 peak at m/z 168 (9·8%) in mass spectrum. ¹H NMR spectrum of this compound shows two triplets and a singlet at δ 2·9, 3·6 and 3·8 ppm, respectively in the intensity ratio 1:1:1·5. Deduce the structure of the compound. Justify your answer. (10 marks) (ii) Two isomeric alkenes with same molecular formula C₆H₁₂ show strong peaks at m/z 42 and 56 in the mass spectrum. Propose fragmentation pattern for both the peaks. (5 marks) (c) (i) (1) Phthalic acid diethyl ester shows a characteristic peak at m/z 149 in the mass spectrum. Account for the observance of this peak by fragmentation pattern. (2) The mass spectrum of ethylbenzene shows a characteristic peak at m/z 91 while n-propylbenzene shows strong peak at m/z 92. Explain with the help of fragmentation pattern. (10 marks) (ii) An unknown organic compound with molecular formula C₄H₅NO₂ displays a band at 2250 cm⁻¹ and a strong band at 1740 cm⁻¹ in the IR spectrum. The compound shows only two signals in 3:2 ratio in the ¹H NMR spectrum. Find out the structure of the compound. Justify your answer. (10 marks)

Answer approach & key points

Begin with a concise introduction stating the principles of NMR spectroscopy (chemical shift, coupling, integration) and mass spectrometry (fragmentation, isotope patterns). For part (a), allocate ~20% time on coupling constants with typical J values (7-10 Hz for vicinal, 12-18 Hz for geminal), ~35% on chemical shift comparisons using anisotropic effects and electronegativity, and ~15% on peak counting with symmetry analysis. For part (b), spend ~18% on halogen identification via M/M+2 ratio (Br), ester deduction from NMR patterns, and alkene fragmentation mechanisms. For part (c), dedicate ~12% to McLafferty rearrangement in phthalate esters and tropylium ion formation in alkylbenzenes, concluding with ~10% on IR/NMR structure elucidation for the nitrile-ester compound. End with a brief synthesis statement on spectroscopic complementarity.

  • Part (a)(i): Estimate J values for vicinal (6-8 Hz), geminal (12-14 Hz), and long-range couplings (0-3 Hz) with correct multiplicity prediction using n+1 rule
  • Part (a)(ii): Compare Hᵃ and Hᵇ chemical shifts using anisotropic effects of C=O, C=C, aromatic ring currents, and electronegativity substituent effects
  • Part (a)(iii): Count distinct proton environments considering molecular symmetry elements (planes, centers of inversion) and diastereotopic protons
  • Part (b)(i): Identify bromine from M:M+2 ≈ 1:1 ratio, deduce ethyl bromoacetate structure matching integration ratio 2:2:3 for CH₂Br/CH₂/OCH₂CH₃
  • Part (b)(ii): Propose McLafferty rearrangement (m/z 56) and allylic cleavage (m/z 42) for C₆H₁₂ alkene isomers (2-hexene and 3-hexene or methylcyclopentane variants)
  • Part (c)(i): Explain m/z 149 from phthalic anhydride ion after ethyl loss, and m/z 91 (tropylium) vs m/z 92 (protonated toluene) via β-cleavage and hydride rearrangement
  • Part (c)(ii): Assign 2250 cm⁻¹ to C≡N and 1740 cm⁻¹ to ester C=O, deduce ethyl cyanoacetate (NC-CH₂-COOCH₂CH₃) from 3:2 integration ratio
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