For example consider the bromination of phenol we get the tri-substituted product (even though bromine is deactivating so I suppose it should deactivate the ring with each addition) while sulphonation essentially gives the mono-substituted product (even at high temperatures).
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3$\begingroup$ The OH group is strongly activating, the bromines are only slightly deactivating. $\endgroup$– ronFeb 15, 2015 at 17:00
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$\begingroup$ @ron Okay.Then what about Nitration in phenol? It also gives tri-substituted product. NO2 is strongly deactivating. $\endgroup$– KaraFeb 15, 2015 at 17:17
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$\begingroup$ A lot of it is experience, you pick it up as you go along. That the nitration of phenol occurs so readily is a further reflection of just how activating the OH group is. $\endgroup$– ronFeb 15, 2015 at 17:27
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$\begingroup$ @ron Take a look at this link chemwiki.ucdavis.edu/Textbook_Maps/… It's a good activator not excellent. $\endgroup$– KaraFeb 15, 2015 at 17:53
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1$\begingroup$ @DelPate Yes, the hydroxyl group activates the ring enough that 3 successive nitrations can occur. As I mentioned in an earlier comment, even the weakly activating methyl group is strong enough to allow 3 successive nitrations under forcing conditions. $\endgroup$– ronFeb 16, 2015 at 16:08
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2 Answers
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It is like others have already noted that phenol is very strongly ortho-para directing, while bromine is only slightly deactivating (or sometimes slightly activating). It is so activating that you do not even need a Lewis acidic catalyst or elevated temperature to let the reaction go fast and in good yield. Moreover, if you want the mono substituted product you will actually need to cool the reaction down to < 5 °C and use the rather dangerously flammable solvent $\ce{CS_2}$, yielding 4-bromophenol in 85% yield (source: Vogel). I assume the para product is formed due to the fact that there is no significant difference in electron density between ortho and para carbon atom while ortho is slightly sterically hindered.
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1$\begingroup$ Okay, so basically the answer is that we will get a tri-substituted product(if activating) unless- 1)Steric hindrance prohibits it 2)We use reaction conditions(like temperature,solvent) to favour some product. Would that be correct to say? $\endgroup$– KaraFeb 16, 2015 at 18:22
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In aqueous medium Phenol get converted to phenoxide ion which has more +M and it highly activates the ring and hence all the positions get occupied
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