UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q6(j). Halogens from the M, M+2 and M+4 peaks
Question
The unknown dihaloalkane gives a molecular ion peak at m/z=128 with a relative intensity of 75. The relative intensities of M+2 and M+4 peaks are 100 and 25 respectively. Identify the halides present in the compound and justify your answer.
Answer in brief
One bromine and one chlorine: M: M+2: M+4 =3:4:1 (75:100:25). The compound is CH₂BrCl (m/z 128 = CH₂⁷⁹Br³⁵Cl).
Key points
- Isotopes: Cl 35: 37 =3:1; Br 79: 81 =1:1.
- 6:1; two Br would give 1:2:1; Br + Cl: (3+1)(1+1)=3:4:1: M (³⁵Cl⁷⁹Br) 3, M+2 (³⁷Cl⁷⁹Br + ³⁵Cl⁸¹Br) 1+3=4, M+4 (³⁷Cl⁸¹Br) 1. Matches 75: 100: 25.
- Mass 128: 12+2+79+35=128, CH₂BrCl (bromochloromethane).
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- Isotopes: Cl 35: 37 =3:1; Br 79: 81 =1:1.
- Two Cl would give 9:6:1; two Br would give 1:2:1; Br + Cl: (3+1)(1+1)=3:4:1: M (³⁵Cl⁷⁹Br) 3, M+2 (³⁷Cl⁷⁹Br + ³⁵Cl⁸¹Br) 1+3=4, M+4 (³⁷Cl⁸¹Br) 1. Matches 75: 100: 25.
- Mass 128: 12+2+79+35=128, CH₂BrCl (bromochloromethane).
Coversisotope patternsHalogens from the MM+2 and M+4 peaks
All parts of Question 6
- 6(a) Hofmann and Schmidt reactions4 marks
- 6(b) Cannizzaro; LiAlH₄ on a lactam; selective reduction of an ester4 marks
- 6(c) Gramine from indole and its displacement by cyanide4 marks
- 6(d) Benzoin condensation and Darzens reaction4 marks
- 6(e) Two epoxidations of carvone4 marks
- 6(f) Photochemistry of stilbene4 marks
- 6(g) Racemisation of an optically active 1,5-diene4 marks
- 6(h) Identify the amide from its IR bands4 marks
- 6(i) Woodward–Fieser rules for three dienes4 marks
- 6(j) Halogens from the M, M+2 and M+4 peaks4 marks