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GATE 2025 CY – Question 16

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

In the $^1$H-NMR spectrum of the following molecule, the signal of proton $H_a$ appears as

A benzylic proton Ha coupled to two deuterium atoms.
  1. singlet
  2. triplet
  3. quintet
  4. quartet

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Correct answer: (C) quintet

Explanation

In the group $-\text{CD}_2-$ the two deuterium atoms are **spin-1 nuclei** ($I=1$) and they are equivalent.

**Multiplicity rule.** A proton coupled to $n$ equivalent nuclei of spin $I$ appears as a multiplet with
$$2nI+1\text{ lines}.$$

For $n=2$ and $I=1$:
$$2(2)(1)+1=5\text{ lines}.$$

So $H_a$ appears as a **quintet** (option C). (For ordinary protons, with $I=\tfrac12$, the same rule gives the familiar $n+1$ pattern, which would be a triplet for two equivalent H.)