Ortho/para directors methoxy groups

Anticipating the products of electrophilic aromatic substitution can be more difficult when two or more substituents compete for control. For example, both methyl and methoxy groups are ortho para directors, and compete for control in electrophilic substitution of 2-methylanisole. The reaction product depends on which substituent has the stronger directing influence. 

From these results it can be seen that the methoxy group is also an ortho/para director and is a much stronger activating group than is the methyl group. Examination of the resonance structure for the para arenium ion that has the positive charge located on the carbon bonded to the methoxy group explains these results. 

There are two ortho positions in the first case, three in the second, and four in the third. We notice, of course, that although all four sites in the para case can react, there are only two different sorts of proton due to the plane of symmetry. The para substitution pattern is the example we look at first. Oxazoline 75 has two para related ortho-directors and two possible sites for lithiation on the ring. We can clearly see from the result that the oxazoline ring was the more potent director because the product 76 has the new methyl group ortho to it rather than ortho to the methoxy group. 

In summary, we have seen that both a methyl group and a methoxy group activate the ring and are ortho-para directors. This is in fact a general rule that will be used extensively throughout the rest of this chapter All activators are ortho-para directors. 

An aromatic ring is even more highly activated by a methoxy group, which is also an ortho-para director. 

The methoxy group is an ortho-para director, and para substitution is expected to predominate (because of steric factors), giving rise to the desired product. 

Eqn. (a) shows the interaction between thiophene-2-carboxylic acid chloride and 2, 3-dichloro-anisole in the presence of CS2, anhydrous aluminium chloride and subsequent hydrolysis yields [2, 3-dichloro-4-(2-thiophen carbonyl) phenoxy]-methyl (I). The methoxy function present in the latter reactant, being an ortho- and para-director, helps to hook on the thiophene-2-carbonyl moiety at the para-position of the 2, 3-dichloro anisole to give (I). Obviously, the ori/io-position is pre-occupied by a chloro group. 

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