Ortho/para

A is a parameter that can be varied to give the correct amount of ionic character. Another way to view the valence bond picture is that the incorporation of ionic character corrects the overemphasis that the valence bond treatment places on electron correlation. The molecular orbital wavefimction underestimates electron correlation and requires methods such as configuration interaction to correct for it. Although the presence of ionic structures in species such as H2 appears coimterintuitive to many chemists, such species are widely used to explain certain other phenomena such as the ortho/para or meta directing properties of substituted benzene compounds imder electrophilic attack. Moverover, it has been shown that the ionic structures correspond to the deformation of the atomic orbitals when daey are involved in chemical bonds. 

The ortho - para directing groups include those which are saturated or only weakly unsaturated at the point of the attachment of the ring. These are (—N(CHj)j, —NHj[ and —OH are by far the most powerful) —... 

The usual directive influences are not operative in this and similar reactions for ortho - para substitution occurs (this may be modified by steric hindrance) irrespective of the nature of R in the aromatic liquid CsHjR, e.g. phenyldlazo hydroxide and nitrobenzene yield 4-nitrodiphenyl this supports the assumption that neutral free radicals are formed. 

The catalysed nitration of phenol gives chiefly 0- and />-nitrophenol, (< 0-1% of w-nitrophenol is formed), with small quantities of dinitrated compound and condensed products. The ortho para ratio is very dependent on the conditions of reaction and the concentration of nitrous acid. Thus, in aqueous solution containing sulphuric acid (i 75 mol 1 ) and nitric acid (0-5 mol 1 ), the proportion of oriha-substitution decreases from 73 % to 9 % as the concentration of nitrous acid is varied from o-i mol l i. However, when acetic acid is the solvent the proportion of ortAo-substitution changes from 44 % to 74 % on the introduction of dinitrogen tetroxide (4-5 mol 1 ). 

The theory that the catalysed nitration proceeds through nitrosation was supported by the isolation of some />-nitrosophenol from the interrupted nitration of phenol, and from the observation that the ortho.-para ratio (9 91) of strongly catalysed nitration under aqueous conditions was very similar to the corresponding ratio of formation of nitrosophenols in the absence of nitric acid. ... 

This additivity of incremental indices has enabled the identification of a large number of arylthiazoles variously substituted in the ortho, para, and meta positions of the aromatic ring (174, 175). 

Because substitution in toluene occurs primarily at positions ortho and para to methyl we say that a methyl substituent is an ortho, para director... 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

All alkyl groups not just methyl are activating substituents and ortho para direc tors This IS because any alkyl group be it methyl ethyl isopropyl tert butyl or any other stabilizes a carbocation site to which it is directly attached When R = alkyl... 

Our analysis of substituent effects has so far centered on two groups methyl and triflu oromethyl We have seen that a methyl substituent is activating and ortho para directing A trifluoromethyl group is strongly deactivating and meta directing What about other substituents ... 

Halogen substituents are slightly deactivating but are ortho para directing... 

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

Very strongly activating —NH2 (ammo) Ortho para directing... 

Activating Standard of comparison Deactivating —R —Ar —CH=CR —H —X (X = F Cl —CH2X (alkyl) (aryl) 2 (alkenyl) (hydrogen) (halogen) Br 1) (halomethyl) Ortho para directing Ortho para directing... 

The greater stability of the intermediates arising from attack at the ortho and para posi tions compared with those formed by attack at the position meta to the oxygen sub stituent explains the ortho para directing property of hydroxyl alkoxy and acyloxy groups... 

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and ortho para directing... 

Because we have come to associate activating substituents with ortho para directing effects and deactivating substituents with meta the properties of the halogen substituents appear on initial inspection to be unusual... 

The mix of inductive and resonance effects varies from one halogen to another but the net result is that fluorine chlorine bromine and iodine are weakly deactivating ortho para directing substituents... 

Often the directing effects of substituents reinforce each other Brommation of p mtrotoluene for example takes place at the position that is ortho to the ortho para directing methyl group and meta to the meta directing nitro group... 

When the ortho para directing bromine is introduced first the major product is p bro moacetophenone (along with some of its ortho isomer from which it is separated by distillation)... 

Sometimes the orientation of two substituents m an aromatic compound precludes Its straightforward synthesis m Chloroethylbenzene for example has two ortho para directing groups m a meta relationship and so can t be prepared either from chloroben zene or ethylbenzene In cases such as this we couple electrophilic aromatic substitution with functional group manipulation to produce the desired compound... 

Give reagents suitable for carrying out each of the following reactions and wnte the major organic products If an ortho para mixture is expected show both If the meta isomer is the expected major product wnte only that isomer... 

Wnte equations showing how to prepare each of the following from benzene or toluene and any necessary organic or inorganic reagents If an ortho para mixture is formed in any step of your synthesis assume that you can separate the two isomers... 

As actually performed a 62% yield of a mixture of ortho and para nitration prod ucts has been obtained with an ortho-para ratio of about 1 3... 

The reactivity of arylamines was noted m Section 12 12 where it was pointed out that —NH2 —NHR and —NR2 are ortho para directing and exceedingly powerful... 

Protecting the amino group of an arylamine in this way moderates its reactivity and per mits nitration of the ring to be achieved The acetamido group is activating toward elec trophilic aromatic substitution and is ortho para directing... 

Electrophilic aromatic substitution (Section 12 14) Halo gen substituents are slightly deactivating and ortho para directing Br... 

The effect of low temperatures affecting the ortho para ratio is more important for light... 

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