Difference between Halogenation of Phenol and Benzene

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username2889812
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I was wondering when a phenol undergoes halogenation, why does it usually form tri "chloro /etc" phenol...or in other words why does it undergo substitution thrice as compared to halogenation of benzene?

:ta: for all help and solutions!
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BobbJo
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The hydroxyl group is ring-activating and makes the ring more reactive. It reacts more readily and undergoes trisubstitution readily, forming a white ppt.

Benzene requires a halogen carrier to react
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MexicanKeith
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(Original post by Spannerin'moi)
I was wondering when a phenol undergoes halogenation, why does it usually form tri "chloro /etc" phenol...or in other words why does it undergo substitution thrice as compared to halogenation of benzene?

:ta: for all help and solutions!
The answer already given is good. It might be worth adding that the Halogen deactivates the ring, so each substitution becomes progressively more difficult. For benzene, achieving one substitution requires a lewis acid catalyst (halogen carrier) and so the second substitution on the now deactivated ring isn't achievable. For phenol the activating power of the OH group can overcome the deactivating power of the halogens until you have three substitutions, then further reaction again becomes unfavourable for a number of reasons (which you don't really need to fully understand unless you're doing a chemistry degree). The reasons are:

1)you have 3 deactivating groups so the ring is not as reactive
2)sterics make it less favourable to have lots of large (ish) halogen atoms next to each other
3) most importantly, substitution at the meta position of phenol derivatives is less favourable than it is at the ortho and para positions due to poorer transition state stabilisation.
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