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JEE Advanced levelSingle correct optionDifficulty 3 of 3o3102

Assertion (A): Ethanethiolate, , is a better nucleophile than ethoxide, , towards in ethanol, even though ethoxide is the stronger base. Reason (R): Sulfur is larger and more polarisable than oxygen, and the thiolate ion is less tightly solvated by hydrogen bonding in ethanol. Choose the correct option.

  1. AA is true but R is false.
  2. BBoth A and R are true and R is the correct explanation of A.
  3. CA is false but R is true.
  4. DBoth A and R are true but R is not the correct explanation of A.

Answer: B

A is true: thiolates are much more reactive than alkoxides in protic solvents. R is true and explains A. The soft, polarisable sulfur can begin to bond at a longer distance in the transition state, and weak hydrogen bonding to sulfur means little desolvation penalty. Basicity measures affinity for a proton, which is a small, hard acceptor, so it does not predict nucleophilicity towards carbon.
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