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JEE Main levelSingle correct optionDifficulty 2 of 3st175

Which of the following spiro compounds is chiral?

  1. ASpiro[3.3]heptane-2-carboxylic acid
  2. BSpiro[3.3]heptane-2,6-dicarboxylic acid
  3. CSpiro[3.3]heptane-2,2-dicarboxylic acid
  4. DSpiro[3.3]heptane-2,2,6,6-tetracarboxylic acid

Answer: B

In spiro[3.3]heptane the two cyclobutane rings lie in perpendicular planes, like the two ends of an allene. A single carboxyl at C2, or two identical groups at C2 (and C6), leave the ring-B plane as a mirror plane, so these are achiral. With one COOH at C2 and one at C6 each ring end has two different groups, there is no mirror plane, and the molecule shows axial chirality (a pair of enantiomers).
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