Ortho, para-directing groups

Consider the nitration of toluene. In the first step, the nitronium ion may attack a ring carbon that is ortho, meta, or para to the methyl group. 

In one of the three resonance contributors to the benzenonium ion intermediate for ortho or para substitution (shown in blue dashed boxes), the positive charge is on the methyl-bearing carbon. That contributor is a tertiary carbocation and more stable than the other contributors, which are secondary carbocations. However, with 

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Consider now the other ortho,para-directing groups listed in Table 4A.In each of them, the atom attached to the aromatic ring has an unshared electron pair. 

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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) —... 

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... 

Do you expect the aldehyde functional group (CHO) to act as a meta-directing or as an ortho, para-directing group Explain. 

When an ortho-para directing group is on a ring, it is usually difficult to predict how much of the product will be the ortho isomer and how much the para isomer. Indeed, these proportions can depend greatly on the reaction conditions. For example, chlorination of toluene gives an ortho/para ratio anywhere from 62/38 to 34/66. Nevertheless, certain points can be made. On a purely statistical basis there would be 67% ortho and 33% para, since there are two ortho positions and only one para. However, the phenonium ion (9), which arises from protonation of benzene, has the... 

When the ortho-para directing group is one with an unshared pair (this of course applies to most of them), there is another effect that increases the amount of para product at the expense of the ortho. A comparison of the intermediates involved (p. 683) shows that C is a canonical form with an ortho-quinoid structure, while D has a para-quinoid structure. Since we know that para-quinones are more stable than the ortho isomers, it seems reasonable to assume that D is more stable than C, and therefore contributes more to the hybrid and increases its stability compared to the ortho intermediate. 

When chlorination or bromination is carried out at high temperatures (e.g., 300-400°C), ortho-para directing groups direct meta and vice versa. " A different mechanism operates here, which is not completely understood. It is also possible for bromination to take place by the SeI mechanism, (e.g., in the r-BuOK catalyzed bromination of 1,3,5-tribromobenzene). ... 

Bromine is an ortho, para-directing group. If it is introduced first, nitration of the resulting bromobenzene yields a mixture of o-bromonitrobenzene and p-bromonitrobenzene. 

Answer The Br atom is an ortho para directing group. However, the Br atoms on Xli-b are hll meta to one another. 

The results show that the methyl group is a typical ortho-para directing group. The relative reactivity of all three positions is greater than that of benzene. This... 

The electrophile in the Friedel-Crafts acylation appears to be a large, bulky complex, such as R—C=0 A1C14. Para substitution usually prevails when the aromatic substrate has an ortho, para-directing group, possibly because the electrophile is too bulky for effective attack at the ortho position. For example, when ethylbenzene reacts with acetyl chloride, the major product is p-ethylacetophenone. 

If the substituent is an electron-repelling anion, then the meta product is formed exclusively because this puts the product anion as far as possible from the anion already there. This again is a useful result as it creates a meta relationship between two ortho, para-directing groups. 

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