Benzene deactivating, meta-directing

The effect of substituents on the reactivity of heterocyclic nuclei is broadly similar to that on benzene. Thus meta-directing groups such as methoxycarbonyl and nitro are deactivating. The effects of strongly activating groups such as amino and hydroxy are difficult to assess since simple amino compounds are unstable and hydroxy compounds exist in an alternative tautomeric form. Comparison of the rates of formylation and trifiuoroacetylation of the parent heterocycle and its 2-methyl derivative indicate the following order of sensitivity to substituent effects furan > tellurophene > selenophene = thiophene... 

The entrance of a third or fourth substituent can be predicted by Beilstein s rule. If a substituent Z- enters into a compound C H XY, both X and Y exert an influence, but the group with the predominant influence directs Z- to the position it will occupy. Since all meta-directing groups are deactivating, it follows that ortho—para activating groups predominate when one of them is present on the benzene ring. 

A more detailed exploration of the reactivity of biphenyl resolves the problem. The ra-phenyl substituent reduces the rate of substitution in the benzene nucleus (Table 7). Qualitatively, this effect is in agreement with the predictions based on the rate of solvolysis of ra-phenylphenyl-dimethylcarbinyl chloride (Brown and Okamoto, 1958) and with the expected electron-withdrawing properties of the phenyl group. The data conform to the Selectivity Relationship with reasonable precision (Fig. 31). In view of the activation of the ortho and para positions, direct evaluation of the partial rate factors for the deactivated meta position is not always possible. Hence, indirect kinetic procedures were employed in several cases, halogenation and acylation, to estimate the values. Graphical analysis of the data shows that mfb is independent of the reagent selectivity. Deviations from the relationship are no greater than for the ordinary side-chain reactions. 

So far, groups have been either activating and ortho/para directors or deactivating and meta directors. The halogens are exceptions to this generalization. They are slightly deactivating compared to benzene but still direct to the ortho and para positions. For example, chlorobenzene is nitrated 17 times slower than benzene and produces predominantly ortho- and /Mra-chloronitrobenzene. 

The aldehyde group deactivates the ring and is meta directing. There are few useful examples, since not only is electrophilic attack more difficult than for benzene, but also the aldehyde group is prone to oxidation during the attack. Substituted benzaldehydes are therefore usually synthesized by functional group transformations or by direct formylation. 

We have seen that certain groups activate the benzene ring and direct substitution to ortho and para positions, and that other groups deactivate the ring and (except halogens) direct substitution to meta positions. Let us see if wc can account for these eflects on the basis of principles we have already learned. 

Substituents on the benzene ring affect both the reactivity of the rii toward further substitution and the orientation of that substitution. Grouj can be classified as ortho- and para-directing activators, ortho- an para-directing deactivators, or meta-directing deactivators. Sul stituents influence aromatic rings by a combination of resonance and indu five effects. Resonance effects are transmitted through -ir bonds indu tive effects are transmitted throu [Pg.654]

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