Deactivators directing effects

Starting material Example Activating/deactivating Directing effect Page... 

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 effect of substituents on the reactivity of heterocyclic nuclei is broadly similar to that on benzene. Thus mem-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... 

Inductive and resonance effects account for the directing effects of substituents as well as for their activating or deactivating effects. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 16.13. 

A wide variety of aromatic compounds can be brominated. Highly reactive ones, such as anilines and phenols, may undergo bromination at all activated positions. More selective reagents such as pyridinium bromide perbromide or tetraalkylammonium tribromides can be used in such cases.18 Moderately reactive compounds such as anilides, haloaromatics, and hydrocarbons can be readily brominated and the usual directing effects control the regiochemistry. Use of Lewis acid catalysts permits bromination of rings with deactivating substituents, such as nitro and cyano. 

It has been observed that, the indirect effect of delta coke on catalyst deactivation, and the direct effect of delta coke on the blocking of acid sites (early) in the riser seems to be a prime factor, which dominates conversion and selectivity effects in a resid type of operation. 

Neither Friedel-Crafts alkylation nor acylation works on deactivated rings, and the directing effects are wrong also. 

We can, in a qualitative way, combine the directing effects of two or more substituents. In some cases the substituents both direct to the same positions, as in the syntheses of bromoxynil and ioxynil, contact herbicides especially used in spring cereals to control weeds resistant to other weedkillers. They are both synthesized from p-hydroxybenzaldehyde by halogenation. The aldehyde directs meta and the OH group directs ortho so they both direct to the same position. The aldehyde is deactivating but the OH is activating. 

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