Identifying Activators and Deactivators

In the previous section, we learned how to predict the directing effects in a situation where you have more than one group on the ring. But in all of the cases in the previous section, I had to tell you whether each group was an activator or a deactivator and whether it was strong or weak. In this section, we will learn how to predict this, so that you won t have to memorize the characteristics of every possible group. In fact, very little memorization is actually involved here. We will see a few concepts that should make sense. And with those concepts, you should be able to identify the nature of any group, even if you have never seen it before. 

We will go through this methodically, starting with strong activators. 

We concluded in the previous section that this resonance effect is very strong and that the OH group is therefore donating a lot of electron density to the ring  

This is true, not only for the OH group, but also for other groups that have a lone pair next to the ring. The same kind of resonance structures can be drawn for an amino group connected to a ring  

Here are several examples of strong activators. Make sure that you can easily see the common feature (the lone pair next to the ring)  

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Benzylic halides and sulfonates show a wide range of reactivity towards nucleophiles. Activation and deactivation by o-/p-donors (e.g. OR) and acceptors (e.g. N02), respectively, are consistent with PAR. In each case the benzylic carbon atom is identified as acceptor or donor. The trends are also reflected in the relative acidities of the corresponding toluene derivatives. 

The typical industrial catalyst has both microscopic and macroscopic regions with different compositions and stmctures the surfaces of industrial catalysts are much more complex than those of the single crystals of metal investigated in ultrahigh vacuum experiments. Because surfaces of industrial catalysts are very difficult to characterize precisely and catalytic properties are sensitive to small stmctural details, it is usually not possible to identify the specific combinations of atoms on a surface, called catalytic sites or active sites, that are responsible for catalysis. Experiments with catalyst poisons, substances that bond strongly with catalyst surfaces and deactivate them, have shown that the catalytic sites are usually a small fraction of the catalyst surface. Most models of catalytic sites rest on rather shaky foundations. 

Figura 2.9 Dse of th Grob test Mixture to compare tbe activity of various glass surfaces coated with ov-ioi. Surface types A > Untreated pyrex glass, B pyrex glass deactivated by thermal degradation of Ceurbowax 20M, C < SCOT column, prepared with Silanox 101, D pyrex glass column coated with a layer of barium carbonate and deactivated as in (B), and E - untreated fused silica. Components are identified in Table 2.7 with ac - 2-ethylhexanoic acid. (Reproduced with permission from ref. 152. Copyright Elsevier Scientific Publishing Co.)...

Ceria-containing samples were prepared that showed a lower tendency towards coke formation, which was most pronounced for an Rh/Pt/Ce02 catalyst. It showed no measurable deactivation for all reaction temperatures applied in the test protocol. Hence this catalyst showed the best performance of all the samples discussed above. For this Rh/Pt/Ce02 catalyst, which was identified as the best catalyst regarding activity, selectivity and deactivation by coke formation, the effect of increasing the S/C ratio was determined at temperatures between 650 and 750°. These measure-... 

By way of example, Figure 3 shews the effect of steaming severity on zeolitic surface area (ZSA) for catalyst A and C. Also identified are typical values for equilibrium catalysts. What is seen is that the conditions needed to deactivate A to typical equilibrium ZSA are different than for C. If C is deactivated using the preferred conditions for A, then activity and surface areas are not in line with commercial experience. If the reverse is true, then A is deactivated too severely. 

Catalytic properties Phosphorus is known to have deactivation effects for some automotive catalysts and the formation of CeP04 has been identified in phosphorus contaminated catalysts (Uy et al., 2003). Nanocrystalline LaP04 would act as Lewis acid in a catalytic process, which could be determined by a temperature-programmed ammonia adsorption/desorption process (Onoda et al., 2002 Rajesh et al., 2004, 2007). In addition, the rare earth phosphate NCs could act as supports for example, Pd, Pt, or Rh supported on RPO4 show excellent catalytic reduction of NO into N2 and O2 (Tamai et al., 2000), and gold supported on RPO4 shows catalytic activity and stability for CO oxidation. 

Identify each of the following groups as an activator or deactivator and as an o,p-director or m-director ... 

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