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However, the other effect called resonance add electron density back to the ring (known as the +M effect) and dominate over that of inductive effect. Hence the result is that they are EDGs and ortho/para directors. Phenol is an ortho/para director, but in a presence of base, the reaction is more rapid.
The ortho effect also occurs when a meta-directing group is positioned in a meta arrangement relative to an ortho–para-directing group, a new substituent introduced into the molecule tends to preferentially occupy the ortho position relative to the meta-directing group rather than the para position.
Examples of activated aromatic rings are toluene, aniline and phenol. The extra electron density delivered into the ring by the substituent is not distributed evenly over the entire ring but is concentrated on atoms 2, 4 and 6, so activating substituents are also ortho/para directors (see below).
Substrates containing two phenols (or an aniline and a phenol; see equation (8) below for a related example), undergo oxidative coupling in the presence of hypervalent iodine(III) reagents. Coupling of both the ortho and para positions is possible; however, the use of bulky silyl-protected phenols provides complete selectivity for para coupling ...
Because electron donating groups are both ortho and para directors, separation of these isomers is a common problem in synthetic chemistry. Several methods exist in order to separate these isomers: Column chromatography will often separate these isomers, as the ortho is more polar than the para in general.
Phenol is readily alkylated at the ortho positions using alkenes in the presence of a Lewis acid such as aluminium phenoxide: [citation needed] CH 2 =CR 2 + C 6 H 5 OH → R 2 CHCH 2 -2-C 6 H 4 OH More than 100,000 tons of tert-butyl phenols are produced annually (year: 2000) in this way, using isobutylene (CH 2 =CMe 2 ) as the alkylating agent.
The ortho to para selectivity is low: [7] No reaction takes place when the solvent is replaced by tetrachloromethane. In contrast, when the reactant is 2-phenylethylamine, it is possible to employ relatively apolar solvents with exclusive ortho-regioselectivity due to the intermediate formation of a chloramine, enabling the Intramolecular reaction.
For meta-directing groups (electron withdrawing group or EWG), σ meta and σ para are more positive than σ’. (The superscript, c, in table denotes data from Hammett, 1940. [11] [page needed]) For ortho-para directing groups (electron donating group or EDG), σ’ more positive than σ meta and σ para.