Directing effects benzene

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. 

This difference in selectivity makes it necessary to consider both the substituent and the reagent when discussing the directing effects of substituents to a much larger extent than is necessary in the benzene series. 

There are some inherent limits to the usefulness of such reactions. Radical substitutions are only moderately sensitive to substituent directing effects, so that substituted reactants usually give a mixture of products. This means that the practical utility is limited to symmetrical reactants, such as benzene, where the position of attack... 

The directing effect of strained ring annelation on the nitration of benzene has been examined in two classes of compounds simple cycloannelated and bicycloan-nelated. Within the series of benzocycloalkenes, significant P-selectivity is ob-... 

The structural theory of organic chemistry was developed in the last half of the nineteenth century. It led to the concept that chemical, physical and biological properties of all kinds must be a function of structural change. The earliest structure-property relationships (SPR) were qualitative. Examples are the directional effect of substituents on the benzene ring with respect to electrophilic aromatic substitution and orientation in... 

Studies with chloroarenes and Cp/Re(CO)3 under UV irradiation show oxidative addition products derived from C - Cl bond activation. For example, pentachlorobenzene at 350 nm yields fra s-( p,Rc((X))2(Ar)Cl (Ar is 2,3,4,5-C6CI4H) as the primary type of product [121]. Studies of di- and tri-chloro benzenes [122] and of disubstituted dichloroarenes [123] also show C-Cl bond activation. In the latter case, unusual directing effects were observed, for example, a C - Cl bond across the ring from either CF3 and CH3 is selectively activated depending upon the system [123]. 

The halogenation of ketones must be carried out in acid solution to avoid polyhalogenation.1 So the synthesis of reagent 22, used to make derivatives of carboxylic acids in chapter 6, is simple providing that we notice the directing effects of the two groups on the benzene ring in 23 and disconnect with Friedel-Crafts in mind. 

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