Halogens show evidence of both electron withdrawal and donation

Nitro is the most eiectron-withdrawing of these groups and some of the other compounds are nearly as reactive (in the meta position, of course) as benzene itself. It is easy, for example, to nitrate methyl benzoate and the m-nitro ester can then be hydrolysed to meta-nitrobenzoic acid very easily. 

One group of substituents remains and they are slightly odd. They are ortho, para-directing but they are also deactivating. They are the halogens. 

Halogens show evidence of both electron withdrawal and donation 

So far we have steered clear of the reactions of halogenated derivatives of benzene. Before we explain their reactions, have a look at the table, which shows the rates of nitration of fluoro, chloro, bromo, and iodobenzene relative to benzene itself, and also gives an indication of the products formed in each case. ---- 

With aii this in mind, how would you expect fluorobenzene to react Most electron density is removed first from the ortho positions by induction, then from the meta positions, and then from the para position. Any conjugation of the lone pairs on fluorine with the it system would increase the eiectron density in the ortho and para positions. Both effects favour the para position and this is where most substitution occurs. But is the ring more or less reactive than benzene This is hard to say and the honest answer is that sometimes fluorobenzene is more reactive in the para position than benzene (for example, in proton exchange and in acetyiation—see later) and sometimes it is less reactive than benzene (for example, in nitration, as shown by the table above). In all cases, fluorobenzene is significantly more reactive than the other halobenzenes. We appreciate that this is a rather surprising conclusion, but the evidence supports it. For example, fluorobenzene reacts with bromine and an iron catalyst (it does need a cataiyst it is not as reactive as phenol) at only -20 °C to give the para-biomo derivative. 

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HALOGENS SHOW EVIDENCE OF BOTH ELECTRON WITHDRAWAL AND DONATION... 

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