UPSC Indian Forest Service (IFoS) Main Previous Year Question Paper 2024 Chemistry -2,

UPSC Indian Forest Service (IFoS) Main Previous Year Question Paper 2024 Chemistry -2

1. (a) Which one of the following is aromatic? Justify your answer. Give names of the given annulenes :

[IMAGE: Three annulene structures labeled I, II and III]

UPSC Indian Forest Service (IFoS) MAIN Previous Year Question Paper 2024 – Chemistry-II

(b) What is secondary kinetic isotope effect? Explain it using the reaction of HCN with the following molecules :

[IMAGE: Two structures labeled I and II: p-methoxybenzaldehyde and its deuterated analogue]

(c) Comment on aromatic/non-aromatic/anti-aromatic behaviour of the following compounds and justify your answer :

[IMAGE: Five structures labeled I, II, III, IV and V]

(d) Predict the products [A], B and C. Also give the reaction mechanism of the following reaction sequence :

[IMAGE: Reaction of p-bromotoluene with NaNH₂/NH₃(l) giving [A], then B + C]

(e) Giving mechanism, write down the product(s) in the following chemical transformation :

[IMAGE: Cyclopentanone derivative with side chain treated with KOH/Δ]

(f) Identify A and B in the following reaction/rearrangement with mechanism :

[IMAGE: Substituted cyclohexanone treated with Br₂/Et₂O, then NaOMe/MeOH]

(g) How does migration of R* group involving photochemical [1,5] sigmatropic rearrangement in the following compound proceed with inversion of configuration? Explain :

[IMAGE: Diene system with R* group and stereochemical structure of R*]

(h) Give the products in the following reaction sequences :

(i) CH₃—CHBr—CH₂—COCH₃ —(NH₂NH₂/t-BuOK, DMSO, Heat)→ ?

[IMAGE: Reaction scheme for (i)]

(ii) R—CO—CO—R —(NH₂—NH₂, Alc. KOH, Glycol)→ ?

[IMAGE: Reaction scheme for (ii)]

2. (a) (i) Indicate the product(s) in the following reaction. What information is obtained about the reaction mechanism from the product(s) observed?

[IMAGE: Two aryl ethers with allyl/butenyl side chains, heated]

(ii) Predict the generation of reactive chemical species giving suitable explanation in terms of thermodynamic/kinetic control in the following chemical equations :

[IMAGE: Ketone A treated with LDA at −78 °C and with RO⁻]

(b) (i) Identify the products [A] and B in the following reaction :

[IMAGE: o-tolyl diazomethane photolysis giving [A], then B]

(ii) Arrange the following chemical entities in order of their decreasing stability :

[IMAGE: Three carbocation structures labeled I, II and III]

(iii) Complete the following reaction and justify the formation of the major product with emphasis on intermediate formed in situ :

[IMAGE: Alkyl chloride with C₂H₅OH/Δ]

(c) (i) Define Hofmann product. Explain the formation of the major product with the help of Newman projection formulae by taking the example given below :

[IMAGE: Quaternary ammonium salt with (CH₃)₃COK/Δ]

(ii) Predict the major product and identify the reaction given below by giving its name :

[IMAGE: Xanthate ester with Δ]

3. (a) Complete the following reaction with detailed reaction mechanism :

[IMAGE: Pyrrole + N,N-dimethylacetamide with POCl₃ then Na₂CO₃/H₂O]

(b) Write the major product(s) of each of the following rearrangements with justification :

(i) [IMAGE: Diol with Aq. HClO₄]

(ii) [IMAGE: Cyclohexanone oxime with SO₂Cl₂]

(iii) [IMAGE: Acyl aromatic with CF₃CO₃H]

(iv) [IMAGE: α-chloro ketone with NaOH then H₃O⁺]

(v) [IMAGE: Decalin derivative with H₂SO₄]

(c) (i) In the cycloaddition reaction, endo product is preferred.

(ii) Write the product indicating stereochemistry when 2E,4Z-hexadiene is undergoing thermal as well as photochemical reaction.

4. (a) Complete the following reactions giving proper explanation for the formation of products with their spread :

[IMAGE: Cyclohexanone with 1) CH₃MgBr, 2) H₂O/H⁺ and with 1) t-BuMgBr, 2) H₂O/H⁺]

(b) Write the product of the reaction between cyclohexanone and diethyl succinate in the presence of a strong base followed by treatment with aqueous acid. Also give the mechanism of this transformation. 10

(c) (i) Write the product(s) in the following reaction sequence along with the mechanism :

[IMAGE: Benzoin + p-anisaldehyde with KCN/EtOH, H₂O, Heat]

(ii) Identify the products A and B in the following reaction sequence. Also write the mechanism of Step II :

[IMAGE: Phenethylamine with RCOCl (Step I) giving A, then POCl₃ (Step II) giving B]

## SECTION-B

5. (a) Calculate M_w for a polydispersed polymer having three fractions with molecular weights and mass % as follows :

Molecular weight : 200000 50000 100000
Mass % : 50 20 30

where M_w is average molecular weight of the polymer.

(b) What is the energy of the lowest vibrational level of the simple harmonic oscillator? Justify your answer.

(c) Complete the following reactions :

(i) [IMAGE: 2-methylbenzoic acid with Na/NH₃(l), t-BuOH, −78 °C]

(ii) [IMAGE: 1,3-dimethoxybenzene with Na/NH₃(l), t-BuOH, −78 °C]

(d) Draw the structure of a peptide bond giving the values of bond angles and bond lengths.

(e) Indicating the major/minor product(s), complete the following reaction :

CH₃CH₂CH₂CH₂CH₂CHO —(hν)→ ?

(f) Distinguish between acetanilide and N-methyl benzamide on the basis of IR spectroscopy. 5

(g) How can you identify the presence of (i) chlorine, (ii) bromine and (iii) both chlorine and bromine atoms in the molecule based on the percentage intensity of isotope peaks relative to the intensity of molecular ion peak? 5

(h) Define spin labels. Write the name and structure of the most commonly used spin label for macromolecules' structure investigation. 5

6. (a) (i) Write the structures of the products formed in the following reactions :

(1) [IMAGE: 2,3-dimethyl-2-butene with SeO₂/t-BuOOH]

(2) [IMAGE: 1-ethylcyclohexene with SeO₂/t-BuOOH]

(ii) Giving mechanism, complete the following reaction :

[IMAGE: 6-methyl-1-oxaspiro[2.5]octane with 1) LiAlH₄, 2) H₂O/OH⁻]

(b) (i) What is living polymer? Explain the reaction mechanism by taking example of a reaction of monomer styrene with BuLi. 10

(ii) Give the representative structure of silicone. Comment on its stability. 5

(c) (i) Complete the following photochemical reaction giving the most probable mechanism :

[IMAGE: Bicyclic ketone with hν]

7. (a) Write the structures of the products in the following reaction sequences :

(i) [IMAGE: Polyene with BH₃/Δ giving P, then 1) CO/Diglyme, 2) H₂O₂/OH⁻ giving Q]

(ii) [IMAGE: 1-methylcyclopentene with BH₃/Δ giving R, then H₂O₂/NaOH giving S]

(b) (i) Give and compare the approximate values of λ_max and ε of 3-pentanone and 3-penten-2-one due to π–π* and n–π* transitions. 10

(ii) Predict λ_max of the following molecules based on Woodward-Fieser rules :

[IMAGE: Two steroidal diene structures]

(c) Identify the structure of a compound having molecular formula C₆H₁₃NO₂ and displaying the following spectral data :

IR : ν_max 2981 cm⁻¹ and 1736 cm⁻¹

¹H NMR : δ 1.31 (t, 3H), 2.42 (s, 6H), 3.20 (s, 2H), 4.18 (q, 2H)

Explain the mass spectral fragmentation of the identified compound for m/z 131, 116, 103, 86 and 84 peaks. 15

8. (a) (i) How can you distinguish the following compounds on the basis of IR spectroscopy?

[IMAGE: Two chlorocyclohexanone isomers X and Y]

(ii) State which of the following molecules are infrared active :

CH₃OH, CH₃NH₂, CO₂, CH₄, CO (5)

(b) Differentiate between benzyl alcohol and anisole on the basis of mass spectral fragmentation. 10

(c) (i) Distinguish between (1) ethyl acetate and methyl propionate and (2) 2-nitroacetophenone and 4-nitroacetophenone with reference to number of signals, approximate chemical shift values and spin-spin couplings (multiplicity) in their ¹H NMR spectra. 10

(ii) Deduce the structure of the compound obtained by condensation of benzaldehyde and acetone in the presence of a base based on the following ¹H NMR spectral data :

δ 2.31 (s, 3H), 6.73 (d, 1H, J = 16 Hz), 7.32–7.40 (m, 5H), 7.52 (d, 1H, J = 16 Hz)

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