Transition state – involves spreading out of charge compared to Nu- – not
much stabilised by polar or non-polar solvent
However -
Nucleophile needs to attack substrate in rate step
– solvent must dissolve Nucleophile and sufficiently polar solvent helps,
But
Nucleophile must also escape the solvent cage to get at substrate
In a protic solvent this is difficult because its too good a solvent
Protic solvent slows down S 2!
Typical solvents favouring S 2 - acetone, acetonitrile, DMF
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