Substituents Located on One Ring

Preparation by hydrolysis of 6-(4-hydr-oxybenzoyl)-benzoxazolinone in aqueous sodium hydroxide [564], according to [565]. 

Also obtained by reaction of o-methoxybenzoyl chloride with p-cresol in the presence of aluminium chloride at 100° for 24 h [683]. 

Qjj - Preparation by Fries rearrangement of o-cresyl / =./ salicylate with aluminium chloride without solvent 

Preparation by reaction of o-anisoyl chloride with 1,3-di-methoxybenzene in the presence of aluminium chloride in chlorobenzene, first at 0°, then at 88° (21%) [684,1294]. 

Preparation by partial demethylation of 2-hydroxy-2, 4 -dimethoxybenzophenone or 2,2, 4 -tri-methoxybenzophenone with aluminium chloride or aluminium bromide in chlorobenzene at 90-95° (good yield) [655]. 

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The greater stability of an equatorial methyl group, compared with an axial one, is another example of a steric effect (Section 3.2). An axial substituent is said to be crowded because of 1,3-diaxial repulsions between itself and the other two axial substituents located on the same side of the ring. 

Photochromic reactions of cw-l,2-diarylethenes are the extension of photochemical electrocyclization of cw-stilbene, which yields dihydrophenanthrene. When the aromatic stabilization energy (aromaticity) of at least one aryl group is low (such as furan, thiophene, benzothiophene) and the nonhydrogen substituents are located on the ring-forming carbon atoms, the thermally irreversible photochromism is observed. When the aromaticity of both aryl groups are high (such as phenyl, indolyl, or pyrryl), the diarylethene is thermally reversible [31]. 

The relative positions of two substituents on a benzene ring can be indicated either by numbers or by the prefixes ortho, meta, and para. Adjacent substituents are called ortho, substituents separated by one carbon are called meta, and substituents located opposite one another are designatedpara. Often, only their abbreviations (o, m,p) are used in naming compounds. 

Another way of generating arenium ions based on dihydroaromatic compounds is to break off, with a pair of electrons, a substituent located at one of the ring sp -hybridized carbon atoms. G. Olah et al. have used this method to obtain tetra-fluoroborate of the methylbenzenium ion by the action of AgBF on the bromination product of l-methylcyclohexa-l,4-diene at —60 °C. 

Pyridine is thus referred to as a n-deficient heterocycle and, by analogy with a benzene ring that carries an electron-withdrawing substituent, e.g. N02 (p. 151), one would expect it to be deactivated towards electrophilic attack. Substitution takes place, with difficulty, at the 3-position because this leads to the most stable Wheland intermediate (63) the intermediates for 2- and 4-attack (64 and 65, respectively) each has a canonical state in which the charge is located on divalent N—a highly unstable, i.e. high energy, state ... 

Diols in which at least one OH group is tertiary or is located on a carbon with aryl substituents can be cleaved by acid treatment.361 The reaction is most useful synthetically when at least one of the OH groups is on a ring.362... 

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