Br2 (aq) vs Br2 (CCl4) ; and Methylbenzene vs Phenol

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Posted by Kyo on July 06, 2008 at 03:19:23:

In Reply to: NH3 vs PH3 ; H2O vs H2S posted by Kyo on June 26, 2008 at 15:24:39:

Q1) Both CH3 and OH substituents are activating (the benzene ring), so why is it that only one of these two compounds (methylbenzene and phenol) will react with Br2 (whether aq or in CCl4) at r.t.p?

Ans :
Although both substituents are indeed activating, but OH donates electrons strongly by resonance while CH3 donates electrons merely by induction; hence OH is a strong activator while CH3 is a weak activator.

Q2) Why is there a difference in product when phenol reacts with Br2(aq) versus Br2(in CCl4)?

Ans :
In both cases, electrophilic aromatic substitution occurs. However, because the polar solvent (ie. water) induces a stronger (albeit temporary) dipole in Br-Br as compared to the non-polar solvent (ie. CCl4), a stronger electrophile (Br+) is produced that undergoes electrophilic aromatic substitution with the benzene ring more readily. Consequently, you get tri-bromination with Br2 (aq) compared with mono-bromination with Br2 (in CCl4).

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