Nucleophilic substitution reactions generally expressed as
`Nu^(-) +R-L rarr R-Nu +L^(-)`
Where `Nu^(-) rarr` Nucleophile , `R-L rarr` substrate, `L rarr` leaving group
The best leaving groups are those that become the most stable ions after they depart. since most leaving group leave as a negatibe ion, the best leaving groups are those ions that stabilize a negative charge most effectively. A good leaving group should be
(a) electron-withdrawing to polarize the carbon
(b) stable once it has left (not a strong base)
(c) polaristable to maintain partial bonding with the carbon in the transition state (both `S_(N)1` and `S_(N)2)`. This bonding helps to stabilise the transition state and reduces the activation energy.
(I) `CI^(-)` (II) `CH_(3)O^(-)` (III) `CH_(3)S^(-)` (IV) `I^(-)`
The correct order of increasing leaving group capability of above anions