Activating, Ortho, Para-Directing Substituents

Energy profiles with an activating group. The methyl group of toluene stabilizes the sigma complexes and the transition states leading to them. This stabilization is most effective when the methyl group is ortho or para to the site of substitutioa 

Shown next is the reaction of ethylbenzene with bromine, catalyzed by ferric bromide. As with toluene, the rates of formation of the ortho- and para-substituted isomers are greatly enhanced with respect to the meta isomer. 

Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results. 

Resonance forms show that the methoxyl group effectively stabilizes the sigma complex if it is ortho or para to the site of substitution, but not if it is meta. Resonance stabilization is provided by a pi bond between the —OCH3 substituent and the ring. 

A methoxyl group is so strongly activating that anisole quickly brominates in water without a catalyst. In the presence of excess bromine, this reaction proceeds to the tribromide. 

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Qualitatively, the effect of the electropositive heteroatom of arsabenzene appears to be comparable to that of an activating ortho-para directing substituent on benzene. Presumably, the electrophile attacks only at the 4- (and 2-)positions because only these positions allow efficient electronic stabilization of the intermediate cr-com-plex 64. 

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and ortho para directing... 

The first compound has one electron-withdrawing substituent (CO2H), which is metn-directing md deactivating. The second has two identical ortho, para-directing substituents (alkyl groups), hich activate all positions. Steric hindrance decides where the nitro group will go. 

The remaining two compounds have competing ortho,para-directing substituents but in each case the one with the lone pair of electrons (N or O) is a more powerful director than the simple alkyl group. In the first case nitrogen directs ortho but in the second oxygen activates both ortho and para and steric hindrance makes the para position marginally more reactive. 

Draw appropriate resonance forms to explain the activating ortho, para-directing character of the phenyl substituent in biphenyl (below). 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

Our analysis of substituent effects has so far centered on two groups methyl and triflu oromethyl We have seen that a methyl substituent is activating and ortho para directing A trifluoromethyl group is strongly deactivating and meta directing What about other substituents ... 

Because we have come to associate activating substituents with ortho para directing effects and deactivating substituents with meta the properties of the halogen substituents appear on initial inspection to be unusual... 

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

Any substituent whose atom attached to the benzene contains a lone pair of electrons is ortho-para directing (but not necessarily a ring activator). Substituents without a lone pair on the atom attached to the ring are likely meta directors (with the exception of alkyl groups and aromatic rings, which turn out to be ortho-para directors). 

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