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GATE 2021 CY – Question 17

Organic Chemistry · Experimental Techniques in Organic Chemistry: Optical rotation (polarimetry). Application of various chromatographic techniques such as thin-layer, column, HPLC and GC. Applications of UV-visible, IR, NMR spectroscopy and Mass spectrometry in the structural determination of organic molecules. · 1 mark · Multiple choice

An organic compound has molecular-ion peaks $[M]^+$, $[M+2]^+$ and $[M+4]^+$ in ratio 1:2:1 and a singlet at δ 7.49 in its $^1H$ NMR spectrum in $CDCl_3$. The compound is ______.

  1. 1,4-dichlorobenzene
  2. 1,4-dibromobenzene
  3. 1,2-dibromobenzene
  4. 1,2-dichlorobenzene

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Correct answer: (B) 1,4-dibromobenzene

Explanation

Two bromine atoms, each with approximately equal 79Br/81Br abundance, produce M, M+2 and M+4 in the binomial ratio 1:2:1. Chlorine would give approximately 9:6:1. The para-disubstituted dibromobenzene has symmetry-equivalent aromatic hydrogens and therefore one proton chemical shift.

Hence **1,4-dibromobenzene, B**.