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GATE 2026 CY – Question 12

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 choice

The correct statement about the following quaternary ammonium ion is

Spiro quaternary ammonium ion with four methyl-substituted ring positions.
  1. It has only C2-symmetry, hence chiral
  2. It has S4-symmetry, hence achiral
  3. It has no symmetry, hence chiral
  4. It has a centre of symmetry, hence achiral

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Correct answer: (B) It has S4-symmetry, hence achiral

Explanation

A quarter turn followed by reflection exchanges the two substituted rings and maps all methyl positions. The S4 improper axis makes the ion achiral.