Deactivating group explanation

According to Heller and Blattmann ( 2.) the rotation of the hydroxyphenyl group around the central C-N bond may contribute to a rapid radiationless deactivation of the excited states. To understand this the items a) and b) of point 2° with regard to a rotational vibration around the C-N bond can be offered as an explanation. 

The nitro group is deactivating and directs the incoming electrophile to the meta position. Again, examination of the arenium ions provides an explanation for these results. First, consider the arenium ion produced by attack of the electrophile at an ortho position ... 

A serendipitous deprotection of only one equatorial PMB group was observed with 1 eq. of DDQ (CH2CI2, 0°C, 70% No explanation was offered for this result, but it may be that the electron withdrawing axial acetate deactivates the adjacent OPMB toward oxidation. 

From the slope of the plots of Fig. 6, the initial deactivation rate was calculated as do = [- da/dt ]h). In Fig. 7, the do values obtained for all the samples are represented in open squares as a fimction of the carbon content measured after the catalytic runs (Table 2) clearly, it does not exist any correlation between do and the amount of coke. Initial deactivation is lower on AI2O3 (do = 0.14 h" ) than on MgO (do = 0.53 h ) in spite that alumina forms more coke during reaction and that the coke is more difficult to oxidize as compared to MgO (Table 2 and Fig. 4). These results show that neither the coke amount nor its polymerization degree account for the catalyst deactivation order observed in Fig. 6. A better explanation is obtained by considering the nature of the surface sites that are responsible for the formation of coke precursors on pure AI2O3 or MgO. Alumina contains Brbnsted (OH groups) and Lewis (metal... 

These initial ideas were inadequate to explain the orientation effect of all groups. The problem appeared with the methyl group in toluene, which is activating, but for which no resonance structures could be constructed. The proper explanation of the regioselectivity (the preference for certain substituent positions) only came with the development of the theory of reaction mechanisms by Robinson and Ingold. As we have seen at the beginning of this chapter, the electrophilic attack causes the formation of a reaction intermediate, the carbocation. Since we know that the rate of chemical reaction depends mostly on the stability of the reaction intermediate, let us discuss the structure of the activated and deactivated cations. 

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