Chemistry UPSC PYQ 2025

(a) (i) Elucidate the structure of the product and the intermediate (if any) in the following reactions. (ii) Describe the role of N-methylmorpholine N-oxide (NMO) during the dihydroxylation of an alkene using catalytic OsO4. (b) (i) Calculate the frequency of radiation required for a transition of J = 4 to J = 5 in the rotational spectrum of HCl. (B = 10·6 cm⁻¹) (ii) The fundamental vibrational frequency of HCl is 2990 cm⁻¹. Calculate the corresponding frequency for DCl assuming the same bond strength. (iii) A compound has molecular formula C3H3N. Its IR absorptions are 1650 cm⁻¹, 2250 cm⁻¹ and 3100 cm⁻¹. Assign a structure. (c) (i) Write the mechanism for the following photochemical transformation. (ii) Photobromination of cinnamic acid was carried out with 480 nm light of intensity 1·5 × 10⁻³ J s⁻¹ for 10 min causing a decrease of 0·05 mmol of Br2. Calculate the quantum yield assuming 80 % radiation absorption (h = 6·627 × 10⁻³⁴ J s, c = 3 × 10⁸ m s⁻¹). (iii) Predict the major and minor products for the given photoreaction and give the logic. (iv) Assign A, B and C in the following reaction.2c:["$","div","MAINS_2025_Chemistry-II Elucidate the structure of the product and any intermediate formed in the given reactions. [10M] Describe the role of N-methylmorpholine N-oxide (NMO) during OsO4-catalysed dihydroxylation of an alkene. [5M] Calculate the frequency for the J = 4 → J = 5 transition in the rotational spectrum of HCl (B = 10·6 cm⁻¹). [5M] Given ν₀(HCl) = 2990 cm⁻¹, calculate the fundamental vibrational frequency of DCl. [5M] Assign a structure to a compound of formula C3H3N with IR absorptions at 1650 cm⁻¹, 2250 cm⁻¹ and 3100 cm⁻¹. [5M] Write the mechanism for the specified photochemical transformation. [5M] Calculate the quantum yield for photobromination of cinnamic acid under the given conditions. [5M] Predict the major and minor products for the stated photoreaction, giving reasons. [5M] Assign species A, B and C in the given reaction. [5M] <!--qid:MAINS_2025_Chemistry-II_Q6-->

(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. (ii) Write the structure of the polymer formed when p-hydroxybenzoic acid is polymerized. (iii) What is the precursor of the main component of natural rubber? Draw the structures of both monomer and polymer. (b) (i) Compare the salient properties of Nylon 6 and Nylon 6,6. (ii) How would you prepare syndiotactic polystyrene? (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? (c) (i) Compounds 1 and 2 on reaction with NaN3 yield the 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. (ii) Identify the major product in the following reactions and justify your answer.2a:[ After polymerization of p-hydroxybenzoic acid, IR analysis shows 0·2 % unreacted —COOH. Calculate the molecular weight of the polymer and categorize it. [7M] Write the structure of the polymer formed when p-hydroxybenzoic acid is polymerized. [3M] What is the precursor of the main component of natural rubber? Draw the structures of both monomer and polymer. [5M] Compare the salient properties of Nylon 6 and Nylon 6,6. [5M] How would you prepare syndiotactic polystyrene? [5M] 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? [5M] Compounds 1 and 2 on reaction with NaN3 yield the 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 the different reactivities of compounds 1 and 2. [10M] Identify the major product in the following reactions and justify your answer. [10M] <!--qid:MAINS_2025_Chemistry-II_Q4-->

8. (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) CH2Br-CH2Cl, (2) Cl-CH2-CH2-Cl, (3) Ha-C≡C-CH2-Cl. (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 : (D) Ha-CH2-OCH3, (E) Hb-CH2-OCOCH3. (iii) Count the number of peaks observed in the 1H NMR spectrum of the following compounds. Justify your answer : (1) CH3-CH2-CH2-Cl, (2) (CH3)3C-Cl. (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. (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. (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. (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.2e:["$","div Estimate the expected splitting (coupling constant J in Hz) for the lettered protons in the 1H NMR spectrum of the given compounds. [5M] Compare the chemical shifts of the labelled protons Ha and Hb in the 1H NMR spectrum of the given compounds and justify your answer. [10M] Count the number of peaks observed in the 1H NMR spectrum of the given compounds. Justify your answer. [5M] 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 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 and justify your answer. [10M] Two isomeric alkenes with molecular formula C6H12 show strong peaks at m/z 42 and 56 in the mass spectrum. Propose fragmentation pattern for both the peaks. [5M] (1) Phthalic acid diethyl ester shows a characteristic peak at m/z 149 in the mass spectrum. Account for this peak with the help of fragmentation pattern. (2) Ethylbenzene shows a characteristic peak at m/z 91 while n-propylbenzene shows a strong peak at m/z 92. Explain with the help of fragmentation pattern. [10M] An unknown organic compound (C4H5NO2) shows a band at 2250 cm−1 and a strong band at 1740 cm−1 in its IR spectrum. It displays only two signals in the 1H NMR spectrum in 3 : 2 ratio. Determine the structure of the compound and justify your answer. [10M] <!--qid:MAINS_2025_Chemistry-II_Q8-->

Complete the following chemical reactions and indicate the category of these reactions. Justify your answer. 2[Co(CN)5]3− + MeI ⟶ [Ru(CO)3(PPh3)2] + MeI ⟶ <!--qid:MAINS_2025_Chemistry-I_Q0-->

(a) Calculate the ratio of probability of finding the 1s electron of hydrogen atom at r = a0 and at r = 10 a0, where ‘r’ is the distance from the nucleus and a0 = radius of the first Bohr orbit. (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. (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. (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 J s–1 m–1 K–1. (e) Why do liquids become super-heated before boiling? Explain using Kelvin equation. (f) Arrange the following molecules in the ascending order of their dipole moment values. Justify your answer. NH3, NF3 and H2O (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? (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. (i) The surface area of an object to be gold plated is 49·8 cm2, and the density of gold is 19·3 g cm–3. 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–1) (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?21:["$","main",null,{"className":"py-12","children Arrange the following molecules in the ascending order of their dipole moment values. Justify your answer. NH3, NF3 and H2O [5M] 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? [5M] 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. [5M] The surface area of an object to be gold plated is 49·8 cm2, and the density of gold is 19·3 g cm–3. 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–1) [5M] 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? [5M] <!--qid:MAINS_2025_Chemistry-I_Q1-->

(a) Write the structure of the product(s) and the intermediate formed in the following reaction. (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 given product through the defined reactions. (c) A photochemical reaction takes place through the 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. (d) (i) Which of the following molecules is/are active to rotational spectroscopy and why? CH4, H2O, NH3, BCl3, XeF4. (ii) The spacing between lines in the microwave spectrum of CO decreases by substituting 12C by 13C. Why? (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. (ii) The mass spectrum of n-butyl phenyl ketone (C6H5COCH2CH2CH2CH3) shows peaks at m/z 162, 120, 105 and 85. Predict the fragmentation pattern.2b:["$" Which of the following molecules is/are active to rotational spectroscopy and why? CH4, H2O, NH3, BCl3, XeF4. [5M] Explain why the spacing between lines in the microwave spectrum of CO decreases on substituting 12C by 13C. [5M] For protons absorbing at δ = 3·0 with J = 4·5 Hz on a 100 MHz NMR instrument, calculate the chemical shift in Hz and the coupling constant J on a 500 MHz instrument. [5M] Predict the fragmentation pattern corresponding to the peaks at m/z 162, 120, 105 and 85 in the mass spectrum of n-butyl phenyl ketone (C6H5COCH2CH2CH2CH3). [5M] <!--qid:MAINS_2025_Chemistry-II_Q5-->

(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. (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 m–2 <!--qid:MAINS_2025_Chemistry-I_Q3-->

(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. (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? (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. (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? <!--qid:MAINS_2025_Chemistry-I_Q2-->

Answer the following sub-parts: Write the structure of the product formed in the following reactions: (1) (bromocyclohexanone with MeO−); (2) Ethoxycarbonyl-sulfonamide system in the presence of NEt3. [Reaction schemes as shown] [10M] Describe the synthesis of ketone (A) when only isobutanol is available as the starting material. [5M] Write the steps involved in the following conversion: ClCO2tBu → (cyclopropane dicarboxylate) under tBuOK. [Scheme as shown] [10M] Elucidate the structure of compounds C and D in the following conversion involving ozonolysis followed by hydrogenation. [Scheme as shown] [5M] <!--qid:MAINS_2025_Chemistry-II_Q3-->

Answer the following sub-parts: <!--qid:MAINS_2025_Chemistry-II_Q1-->

(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. (ii) Using Woodward–Fieser rules, calculate λmax for the following compounds (structures A, B, C and D are provided). Explain why acetone exhibits weak absorption at 280 nm but strong absorption at 190 nm in its UV spectrum. [5M] Using Woodward–Fieser rules, calculate λmax for the given compounds A, B, C and D. [5M] <!--qid:MAINS_2025_Chemistry-II_Q7-->

Answer the following sub-parts: The following reaction does not produce the product shown. [Reaction scheme given] (1) Predict the major product from the conditions shown above and write a detailed mechanism for its formation. (2) Write the reaction conditions which would lead to successful synthesis of the product shown above (i.e., 3,3-dimethyl-2-butanol). [10M] Write the structure of the major product(s) formed in the following reaction. Justify your answer. Ph —BrCCl3, hν → ? [5M] <!--qid:MAINS_2025_Chemistry-II_Q2-->