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.
Among the following which is feasible?
A. `X^(-) +CH_(3) -CH_(2)-H rarr CH_(3)-CH_(2)-X +H^(-)`
B. `X^(-)+CH_(3)-OH rarr CH_(3)-X +overset(bar(O)H)`
C. `X^(-) +H_(3)C -underset(H)underset(|)overset(o+)(OH)rarr CH_(3)-X +H_(2)O`
D. `X^(-)+CH_(3)-CH_(3)rarr CH_(3)-X +overset(bar(CH_(3)))`
