Weak deactivators

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... 

A similar analysis suggests the absence of significant negative charge development as well. The fact that p-haloethyl derivatives have only a weak deactivating effect, rather than an activating effect on SN2 reactivity (Hine and Brader, 1953 Hine, 1962 Streitwieser, 1962) implies that there is no... 

Most known iodonium salts of this category have one phenyl and one aryl group. Various hypervalent iodine precursors can be used as effective electrophiles, notably the combination of (diacetoxyiodo)benzene with triflic acid. The reagent is formed in situ and is suitable for a range of activated to weakly deactivated arenes. The reactions proceed at room temperature and in some cases, e.g. with anisole, only the p-isomer was produced. Strongly deactivated arenes such as nitrobenzene did not react. 

The first mode of deactivation is clearly shown with HZSM5, At low coke content, Vr/Va is close to 1 4 coke molecules are needed to deactivate one acid site, This weak deactivating effect can be explained by a competition for adsorption on the acid sites between the reactant and the coke molecules which are too weakly basic to be "irreversibly adsorbed at the reaction temperature. However limitations in the rate of diffusion of the reactant can also be responsible for deactivation. The size and the basicity of the coke molecules increase with the coke content, which causes an increase in the deactivating effect of the coke molecules. Beyond a certain size of the coke molecules the channel intersection is completely inaccessible to the reactant and to the adsorbates and Vr/Va can be lower than T This first mode of deactivation occurs also with USHY. However the deactivating effect of coke molecules is initially very high because coke molecules are formed on the strongest (hence the most active) acid sites. 

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