Selectivity pattern

At this point, attention can be given to specific electrophilic substitution reactions. The kinds of data that have been especially useful for determining mechanistic details include linear ffee-energy relationships, kinetic studies, isotope effects, and selectivity patterns. In general, the basic questions that need to be asked about each mechanism are (1) What is the active electrophile (2) Which step in the general mechanism for electrophilic aromatic substitution is rate-determining (3) What are the orientation and selectivity patterns ... 

TCDF is not very toxic to the mouse (acute oral LDjq < 6000 pg/kg) but is highly toxic to the guinea pig (acute oral LD50 5-10 pg/kg). Symptoms of toxicity in the gninea pig were similar to those fonnd with 2,3,7,8-TCDD. Thus, the selectivity pattern was similar to that for 2,3,7,8-TCDD, but toxicity was considerably less (see Environmental Health Criteria 88). 

To Illustrate the utility of the technique, we have addressed the question of the anomalous chemlsorptlve behavior of tltanla-supported group VIII metals reduced at high temperatures. The suppression of strong H2 chemisorption on these catalysts has been ascribed to a strong-metal-support Interaction (SMSI) ( ). It has also been found that the reaction activity and selectivity patterns of the catalysts are different In normal and SMSI states... 

A wide variety of soUd acid catalysts has been examined using the methanol/isobutanol reaction mixture to establish activity and selectivity patterns for alcohol coupling and dehydration reactions (Table 1). 

The regioselectivity and stereoselectivity of electrophilic additions to 2-benzyl-3-azabicyclo[2.2.1]hept-5-en-3-one are quite dependent on the specific electrophile. Discuss the factors that could influence the differing selectivity patterns that are observed. 

This selectivity is attributed to the steric bulk of the aluminum reagent favoring the migration of the larger alkyl group. The same selectivity pattern is observed with unbranched substituents. 

C and 4 h for Raney catalysts. Due to lower reducibility and stronger interaction of Co- and Ni-oxides with alumina, 10 wt % metal was used. Despite the higher metal content of these catalysts they were less active than the alumina supported noble metals and their selectivity to RNH2 was lower than that of Ru. The selectivity pattern on noble metals was in good agreement with literature data [1,4],... 

Use of the same benzyl(methyl)phenylphosphine and DIOP rhodium(I) catalysts with ,/3-unsaturated carbonyls has led to some interesting selectivity patterns (297, 298). Dihydrosilanes undergo 1,2-addition at the carbonyl of substrates such as 2-methylcyclohexenone, mesityl oxide,... 

The limitations of homogeneous systems have been recognized, and this has stimulated the development of supported catalysts to the extent that a few of them now reach activity levels comparable to those of the homogeneous counterparts. Highly selective catalysts have been reported. although the selectivity patterns are often different to those of a homogeneous analog (see Section IV). Supported asymmetric catalysts... 

Other sensors are mostly grouped towards the triethylamine. In the case of porphyrins 6-8, 13 the coordinated metal is no longer able to drive the selectivity pattern and the presence of the peripheral alkyl chains completely shadows the coordination interactions. This result can explain the failure to observe the coordination interaction in the sensing mechanism of the metal complexes of the closely related alkyl chains functionalized phthalocyanines reported in the past by Gopel and coworkers [22]. 

Layer doping with catalytic metals can be done either during the powder preparation or later in the deposition and annealing process. Doping is an important procedure in tuning the gas-sensor characteristics (selectivity pattern of the sensitive layers) [67,68]. 

Results of analogous permeation studies for the hydrophobic toluene molecule are shown in Fig. 20. Now the opposite selectivity pattern is observed i.e., toluene is preferentially transported in the R = -CieHjj membranes. This can be illustrated by defining the alternative selectivity coefficient aci6/0H (Table 3). As was the case for aoH/cia. the acie/on values increase with decreasing tubule diameter. In addition to toluene, acie/on values were determined for p-xylene and naphthalene in the i.d. = 1.9 nm membranes. The following acu/on values were obtained 2.8 for toluene, 6.2 for j9-xylene, and 16 for naphthalene. 

These types of switchable electrode surfaces have been used to selectively pattern two different cell populations onto a surface [151] and additionally these surfaces can selectively release different cells at different applied potentials [152]. However, it is important to recognize that electrochemically switching a surface from inactive to conjugation and active to conjugation has been well explored with nitro-terminated aryl diazonium salts. In such studies, the application where very anodic potential resulted in a six-electron reduction to an amine [139], to which proteins could be attached [153-155]. The key difference is that the interaction of the biological medium with the surface is controlled by the presence of the antifouling layer. In many ways these electrode surfaces developed by Mrksich and coworkers [150-152, 156] are very similar to the antifouling surfaces with molecular wires discussed in Section 1.4.2 [131, 132, 138, 142]. In both cases the electrode is... 

Some conclusions can be drawn from comparison of the TOF values obtained using biphasic catalysis with literature data for the same reactions catalyzed by a polymer-supported Pd(0) catalyst (Table 2) [161]. First, although the TOFs for the dendrimer-encapsulated catalysts are consistently lower than for the polymer-supported catalysts, the values are in some cases comparable. Second, the selectivity pattern exhibited by the two types of catalysts is somewhat different. Specifically, the range of TOF numbers for biphasic catalysis is far greater than for conventional polymer catalysis, which suggests the possibility of the type of... 

Zirconia-modified silica impregnated with Co2(CO)s and activated under H2 at 300 °C renders a catalyst more active and selective to diesel fraction, in the CO hydrogenation reaction, than that conventionally prepared from a nitrate salt solution. The selectivity patterns followed a Schulz-Flory distribution and catalysts prepared from carbonyl precursor exhibited low water-gas shift activity [146]. 

Shenhar R, Jeoung E, Srivastava S, Norsten TB, Rotello VM. Crosslinked nanoparticle stripes and hexagonal networks obtained via selective patterning of block copolymer thin films. Adv Mater 2005 17 2206-2210. 

Figure 15.10 Selectivity pattern of PPy(NO ) electrodes. The anions tested include (1) nitrate, (2) bromide, (3) perchlorate, (4) sahcylate, and (5) phosphate AB is the difference between the steady-state potential and the starting potential. Reprinted from Hutchins and Bachas (1995). Copyright 1995 American Chemical Society.

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