Which has a greater -I effect, -OCH3 or -OH ? Please explain the reason.

HELLO THERE!

 

Negative inductive effect (-I effect) or electron withdrawing effect is shown by the groups which have more electronegativity.

 

OCH₃ group is more electron withdrawing (i.e, shows more -I effect) than the OH group.

 

WHY?

 

The reason is that, there are two lone pairs of oxygen. Oxygen has a smaller size, so in case of OCH₃, the methyl group is close to the lone pair electrons, which leads to Steric repulsion. Due to this, a slight positive charge is developed on Oxygen atom, and we know that positively charged ions are more electronegative, as they are electron deficient, and they definitely are in want of electrons. So, this positive charge increases the electronegativity of O atom, due to which its electron withdrawing effect increases.

 

However, in case of OH, the H atom is comparatively much much smaller than O, so here no Steric repulsion takes place. OH shows -I effect just due to the electronegativity value of Oxygen.

 

THANKS!

OCH3 showed more eletron withdrwing effect compaired to OH even though CH3 is electron rich than H, There are two reasons for this anomlous behaviour.

O-H bond lenth is shorter and weak asewell. The weak bond of O-H means the electron cloud Shaired between H and O is more towards O, which brings more -ve charge on oxygen ( actully here we have -O-ev and H+)  this made OH A  less electron widrawing. While as the bond between O and CH3 is having suffecent bond length, strength aswell and only inductive effective is oprative, only delta -ve charge is carried by O which is less compaired when OH is there. (Claden and Grives Book)