The GATE Grind

GATE 2022 CY – Question 27

Organic Chemistry · Stereochemistry: Chirality and symmetry of organic molecules with or without chiral centres and determination of their absolute configurations and optical purity. Relative stereochemistry in compounds having more than one stereogenic centre. Homotopic, enantiotopic and diastereotopic atoms, groups and faces. Conformational analysis of acyclic and cyclic compounds. Geometrical isomerism and optical isomerism. Configurational and conformational effects, atropisomerism, and neighbouring group participation on reactivity and selectivity/specificity. Stereoselective and stereospecific synthesis. Enantiomeric excess. · 1 mark · Multiple select

Which statements about the $^1H$ NMR spectra of P and Q are correct?

Diagram for GATE 2022 CY question 27
  1. P shows a sharp singlet at δ = 3.70 ppm (for Ha and Hb)
  2. Q shows a sharp singlet at δ = 3.70 ppm (for Ha and Hb)
  3. P shows a AB-quartet centered at δ = 3.63 ppm (for Ha and Hb)
  4. Q shows a AB-quartet centered at δ = 3.63 ppm (for Ha and Hb)

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (A) P shows a sharp singlet at δ = 3.70 ppm (for Ha and Hb); (D) Q shows a AB-quartet centered at δ = 3.63 ppm (for Ha and Hb)

Explanation

For P, the symmetric cis substitution makes the benzylic hydrogens chemically equivalent in the achiral measurement, so they give a singlet, A. The trans stereochemistry of Q makes them diastereotopic; their different shifts and geminal coupling produce an AB quartet, D.

Thus **A, D**; fast nitrogen inversion does not remove carbon-configurational diastereotopicity.