Active power

If, on the other hand, the encounter pair were an oriented structure, positional selectivity could be retained for a different reason and in a different quantitative sense. Thus, a monosubstituted benzene derivative in which the substituent was sufficiently powerfully activating would react with the electrophile to give three different encounter pairs two of these would more readily proceed to the substitution products than to the starting materials, whilst the third might more readily break up than go to products. In the limit the first two would be giving substitution at the encounter rate and, in the absence of steric effects, products in the statistical ratio whilst the third would not. If we consider particular cases, there is nothing in the rather inadequate data available to discourage the view that, for example, in the cases of toluene or phenol, which in sulphuric acid are nitrated at or near the encounter rate, the... 

Some of the most powerful activating substituents are those m which an oxygen atom IS attached directly to the nng These substituents include the hydroxyl group as well as alkoxy and acyloxy groups All are ortho para directors... 

Section 12 15 When two or more substituents are present on a nng the regioselectiv ity of electrophilic aromatic substitution is generally controlled by the directing effect of the more powerful activating substituent... 

A hydroxyl group is a very powerful activating substituent and electrophilic aro matic substitution m phenols occurs far faster and under milder conditions than m ben zene The hrst entry m Table 24 4 for example shows the monobrommation of phenol m high yield at low temperature and m the absence of any catalyst In this case the reac tion was carried out m the nonpolar solvent 1 2 dichloroethane In polar solvents such as water it is difficult to limit the brommation of phenols to monosubstitution In the fol lowing example all three positions that are ortho or para to the hydroxyl undergo rapid substitution... 

Although a hydroxyl group strongly activates an aromatic ring toward electrophilic attack an oxyanion substituent is an even more powerful activator Electron delocaliza tion m phenoxide anion leads to increased electron density at the positions ortho and para to oxygen... 

Ying, W.C., R.R. Bonk and S.A. Sojka. Treatment of Landfill Leachate in Powered Activated Carbon Enhanced Sequencing Batch Bioreactors. In Proc. of the 18th Mid-Atlantic Ind. Waste Conference, Technomic Publishing Company, Inc., Lancaster, Pennsylvania, 1986. 

The second point is somewhat less obvious but is readily illustrated by the synthesis of 1,3,5-tribromobenzene. This particular- substitution pattern cannot be obtained by direct brornination of benzene because bromine is an ortho, para director. Instead, advantage is taken of the powerful activating and ortho, para-directing effects of the fflnino group in aniline. Brornination of aniline yields 2,4,6-tribromoaniline in quantitative yield. Diazotization of the resulting 2,4,6-tribromoaniline and reduction of the diazonium salt gives the desired 1,3,5-tribromobenzene. 

The bromination of 5,8-dimethoxyquinoxaline in methanol gives a mixture of 6-bromo and 6,7-dibromo compounds/ Treatment of 2-methylquinoxaline with bromine in acetic acid yields a mixture of 27% of 2 bromomethyl- and 37% of 2-dibromomethyl-quinoxaline." Thus in the absence of powerfully activating groups, side-chain rather than nuclear substitution takes place. 

If t he directing effects of the two groups oppose each other, the more powerful activating group has the dominant influence, but mixtures of products often result. For example, bromination of p-methylphenol yields primarily 2-bromo-4-methylphenol because —OH is a more powerful activator than -CH3. 

Where does nonproliferation stand in the myriad of institutional, regulatory, public acceptance, economic and, at times, even technical problems now affecting the future of the nuclear option Is the potential contribution of nuclear power activities to further proliferation seen by policy makers and, of equal importance, the public as a significant obstacle to its revival Or is it viewed as a potential benefit capable of reducing proliferation risks. 

Fig. 5.42 Power Activation Mode -operation sequence for Ga203-natural gas sensors to avoid false alarm in the presence of ethanol and other interfering air components [20], (With kind permission of D. Kohl, D. Skiera and M. Lam-mer, Institute of Applied Physics, University of Giessen, Germany)...

You have read earlier that aniline is a resonance hybrid of five structures. Where do you find the maximum electron density In these structures Ortho- and para-posltlons to the -NHg group become centres of high electron density. Thus -NH group Is ortho and para directing and a powerful activating group. 

The city of Columbus, Ohio, installed a PACT water treatment system for 1.2 million. The system was purchased to remove atrazine (a pesticide) from the water supply. In 1997, Columbus spent 150,000 on powered activated carbon. Additional monitoring costs were estimated to be over 10,000 per year (D19947H, p. 11). 

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