Ortho/ para positions

However, when strongly electron-attracting groups are located on the ring at the ortho-para positions, the intermediate anion is stabilized by delocalization of electrons from the ring carbons to more favorable locations on the substituent groups. As an example, consider the displacement of bromine by OCH3 in the reaction of 4-bromonitrobenzene and methoxide ion ... 

Alkyl groups are activating groups and direct substitution to the ortho, para positions. Electron withdrawing substitutions (Following fig.) have the opposite effect. They deactivate the ring, make the ring less nucleophilic and less likely to react with an electrophile. 

The effect of the 1,2,5-thiadiazole system on carbon-bound substituents (see Section 4.02.3.3.1) can be summarized as follows (1) stabilization of carbanions (2) destabilization of carbenium ions (3) enhanced SN2 reactivity and repressed SN1 reactivity (68AHC(9)107). Aryl substituents are rendered more reactive to nucleophiles (72IJS(A)25) and deactivated in reactions with electrophiles, which are directed to the ortho/para positions by the thiadiazole ring (78MI42601, 72IJS(A)25). 

The second group contains the halogens, which also direct the incoming electrophile to the ortho/para positions, but exert such a strong -I effect that they withdraw sufficient electron density from the benzene ring to deactivate it. 

Orientation rules are just beginning to be formulated a) Substitution meta to a nitro group is favorable, b) Alkoxy and probably other electron donating groups direct to ortho-para positions, c) The a-position of polycyclic aromatic compounds is favored, d) Resonance stabilization is important during product formation. The present level of understanding is illustrated with several examples. 

Arenium ions possessing a ring fragment of the CHR type or ortho(para)-positioned OH-, NHR- and CHR-type substituents mainly react with rising nucleophilicity of the medium by loss of the proton, as marked earlier. In some cases the Iok of the proton from the substituent competes with the addition of a nucleophile to the ion. For example, arenium ions formed by ipso-addition of N02 capture an acetate-anion (nitroacetoxylation 2-665)j. 

On this evidence the Mc3SiCH2 group attached to an aromatic ring should be quite strongly activating and ortho/para directing. Crestoni and Fornarini have studied the gas-phase reactivity of benzyltrimethylsilane and have confirmed that it is highly activated towards electrophilic attack directed to the ortho/para positions. 

Note Knowledge of the electronic effects of substiments in the aromatic ring, expressed by o-values, and the Hammett equation helps in planning the syntheses of polysubstimted aromatic compounds [18-20]. Orientation of electrophilic aromatic substitution is determined by the electronic properties of the present substiments. Electron-acceptor substiments (with positive o-values) deactivate ortho/para positions, resulting in the orientation of electrophilic substimtion to the meta-position. This effect is quantitatively expressed by Hammet o-values and for most usual substiments is presented in Table 5.1 [21]. 

There are four pairwise equivalent, respective ortho, para positions to the two CHjO substituents, only one set of which is marked.)... 

Most main-chain sulfonated diamines are synthesized by direct sulfonation of a non-sulfonated precursor (Pig. 6) or its dinitro analogue [69]. 4,4 -Diaminodiphenyl ether-2,2 -disulfonic acid (ODADS), 4,4 -bis(4-aminophen-oxy)biphenyl-3,3 -disulfonic acid (BAPBDS) [71,72], 2,2 -bis[4-(4-amino-phenoxy)hexafluoroisopropane disulfonic acid (BAHFDS) [73,74], 9,9-bis (4-aminophenyl)fluorene-2,7-disulfonic acid (BAPFDS) [35], and 2,2 -bis(4-aminophenoxy)biphenyl-5,5 -disulfonic acid (o-BAPBDS) [75] were synthesized accordingly. Because of the electrophihc nature of the sulfonation reaction, the sulfonic acid groups are usually introduced in the ortho/para position to the electron-donating substituents. Polymer-grade monomers are obtained with excellent yields (83, 85, and 90%, respectively, for BAPFDS, ODADS, and p-BAPBDS), except in some cases (o-BAPBDS, 48%). 

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