Aryl groups electrophilic substitution

5-Methyl-3-phenylisoxazole is nitrated as a conjugate acid at the meta position but as the free base at the para position of the phenyl group (79AHC(25)147). Phenyl groups attached to oxazole rings are nitrated or sulfonated in the para position, with relative reactivities of the phenyl groups in the order 

3-Phenylisothiazole is nitrated predominantly in the meta position of the phenyl group, whereas 4-phenylisothiazole is nitrated ortho and para in the phenyl group (72AHC(l4)l). Nitration of 3-phenyl-1,2,4-oxadiazole gives a mixture of m- and p-nitrophenyl derivatives (63GI196). 

In the 1,2-dithiolylium ion system, 3- and 5-phenyl groups on nitration give mixtures of para and meta orientation, whereas nitration of a 4-phenyl group gives para substitution only (61JA2934). 

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Other C-Linked Substituents 4.02.3.4.1 Aryl groups electrophilic substitution... 

A different reaction takes place when electrophiles react with arenes Substitution is observed instead of addition If we represent an arene by the general formula ArH where Ar stands for an aryl group the electrophihc portion of the reagent replaces one of the hydrogens on the ring... 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

Electrophilic substitution occurs readily in Af-phenyl groups, e.g. 1-phenyI-pyrazoIes, -imidazoles and -pyrazolinones are all nitrated and halogenated at the para position. The aryl group is attacked preferentially when the reactions are carried out in strongly acidic media, where the azole ring is protonated. 

The intramolecular arylation of sp3 C-H bonds is observed in the reaction of l-/ r/-butyl-2-iodobenzene under palladium catalysis (Equation (71)) 94 94a 94b The oxidative addition of Arl to Pd(0) gives an ArPdl species, which undergoes the electrophilic substitution at the tert-butyl group to afford the palladacycle. To this palladacycle, another molecule of Arl oxidatively adds, giving the Pd(iv) complex. 

Explain the fact that tn aryl alkyl ethers (i) the alkoxy group activates the benzene ring towards electrophilic substitution and (ii) it directs the incoming substituents to ortho and para positions in benzene ring. 

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