Chemistry UPSC PYQ 2025

(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.

(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]