[mimong06]

Can you explain the effect of solvent in the reaction of acetanilide and Br2? thank you sa makakasagot.

[AhBeng]

A strongly polar (eg. protic) solvent is able to induce (via inter electron repulsions) a stronger temporary dipole in the non-polar Br2 molecule, making it more electrophilic, thereby reducing the activation energy required for the weak benzene ring nucleophile to attack the more strongly polarized Br2 electrophile.

If a non-polar solvent was used, the instantaneous dipole on the non-polar Br2 molecule would be much weaker, so much so that the weak benzene ring nucleophile wouldn't have sufficient energry to overcome the much higher activation barrier to carry out the electrophilic aromatic substitution.

Of course, the reason why benzene is such a weak nucleophile, is because it's pi electrons are delocalized by resonance (via the sideways or side-on overlapping of its unhybridized p orbitals), and thus less available for nucleophilic attack.

Although the N atom outside the benzene ring has a lone pair which is delocalized by resonance, but this lone pair is delocalized both ways : towards the benzene ring, and towards the amide group (in point of fact, delocalized to a greater extent towards the amide group, due to the strongly electronegative, electron-withdrawing O atom).