Question

Place the following benzyl alcohols in decreasing order of reaction rate with `HBr`
`C_(6)H_(5)underset((A))(CH_(2))OH` , `p-O_(2)underset((B))(NC_(6))H_(4)CH_(2)OH`
`p-CH_(3)OC_(6)H_(4)Ch_(2)OH` , `p-Clunderset((D))(C_(6))H_(4)CH_(2)OH`
A. `C gt A gt D gt B`
B. `A gt C gt D gt B`
C. `C gt A gt B gt D`
D. `A gt C gt B gt D`

Answer 1

Correct Answer - A
Order of reactivity is decided by the order of stability of intermediate carbocation. The positive charge that develops on the benzylic `C` (in this `S_(N)1` reaction) is most effectively delocalized by the electron releasing `CH_(3)O^(-)` group :

The `p-NO_(2)` destabilizes the benzyl cation by withdrawing electron density from the ring by induction `(-I_("effect"))` as well as by resonance `(-R_("effect"))`:

Delocalization of the `+` charge from the benzylic carbon places the `+` charge in the ring adjacent to the positively charged N of `NO_(2)`, greately destabilizing the benzylic carbocation . `p-Cl` destablizes the `+`charge (of benzylic cation) only by induction `(-I_("effect"))` , not by resonance `(+R_("effect"))`, making the `p-Cl` compound less reactive than `phCH_(2)OH` but more reactive than the `p-NO_(2)` compound.