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7) Why are the polar aprotic solvents (DMSO, DMF, acetonitrile, acetone) so useful in substitution reactions?
a) They solvate cations well but anions poorly, making the anions less nucleophilic
b) They solvate anions well but cations poorly, making the anions more nucleophilic
c) They solvate cations well but anions poorly, making the anions more nucleophilic
d) They suppress carbocation rearrangements
Explanation: answer (c) is correct.
For SN1 reactions a polar solvent is essential: Dipole-dipole attractive forces between the polar solvent and the transition state will stabilize the transition state and thus lower its energy. Polar aprotic solvents solvate cations well, but anions poorly! This results in "naked" anions - anions without a solvent shell - which are very reactive....
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