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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3g3108

Which of the following statements about singlet and triplet methylene are correct?

  1. ATriplet adds to alkenes in a single concerted step with retention of alkene geometry.
  2. BSinglet adds to cis-but-2-ene to give only cis-1,2-dimethylcyclopropane.
  3. CTriplet adds to cis-but-2-ene to give both cis and trans cyclopropanes.
  4. DThe singlet state of methylene is its ground state.

Answer: B, C

A singlet carbene adds in a concerted step, so stereochemistry is retained. A triplet carbene must form two bonds in sequence: the first gives a triplet 1,3-diradical, which must undergo spin inversion before ring closure. During this time rotation about the single bond is possible, so cis and trans products form. The triplet is the ground state of , so the last statement is also false.
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