Benzene bromination, illustration

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

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 benzene illustrates the difference between addition to alkenes and substitution of arenes. First, to achieve the bromination of benzene it is necessary to use a catalyst, such as iron(III) bromide. The catalyst acts as a Lewis acid, binding to the bromine molecule (a Lewis base) and ensuring that the outer bromine atom has a pronounced partial positive charge ... 

The principal intermolecular donor-acceptor interactions of this weakly bound complex are found to be of 7tcc-OBrBr form (3 x 0.20 kcalmol-1), as illustrated in Fig. 5.41(b). A prominent feature of Br2 (and other heavy halogens) is the nearly pure-p character of the bromine bonding hybrid, resulting in a conspicuous backside lobe on the OBrBr antibond (see Fig. 5.41(b)) that is effective in end-on complexation to the pi-donor face of benzene. The unusually small energy separation between donor and acceptor NBOs,... 

The influence of the OH-group on the reactivity of the benzene ring is of great importance. All substitution processes which are traced back to preliminary addition reactions take place much more easily with the phenols than with the hydrocarbons as far as possible the groups which enter occupy the o- and p-positions. A number of reactions illustrating this statement is discussed below and in subsequent parts of this book. It may be mentioned here that by the action of bromine water, o-o-p-tnbromophenol is at once precipitated from an aqueous solution of phenol. (Method for the quantitative determination of phenol.)... 

The acid-catalyzed hypobromous acid bromination is the only direct substitution reaction extended to very deactivated substituted benzenes. The available results are illustrated in Fig. 15. de la Mare and Hilton (1962) evaluated the slope as — 6.2. They also point out that the serious discrepancy exhibited by the m-trimethylamino substituent is not readily identified with an error in the c7+-value. It was suggested (Brown and Okamoto, 1958) that the entropies of activation (Table 19) for the solvolysis of the m- andp-trimethylaminophenyldimethylcarbinyl... 

Figure 1.30 illustrates that the allyl radical intermediate of several Wohl-Ziegler brominations can be accessed from isomeric alkene substrates as starting materials. This is worth considering when one substrate is more easily accessible or cheaper than its isomer. The price of allyl benzene, for example, is just a fraction of what has to be paid for 1 -phenylpropene and would thus be preferred for the synthesis shown in Figure 1.30. 

The radical bromination of alkyl benzenes is a useful reaction because the resulting benzylic halide can serve as starting material for a variety of substitution and elimination reactions, thus making it possible to form many new substituted benzenes. Sample Problem 18.7 illustrates one possibility. 

The effect of the benzene ring on orientation can be well illustrated by a single example, addition of HBr to 1-phenylpropene. In the absence of peroxides, bromine becomes attached to the carbon adjacent to the ring in the presence of peroxides, bromine becomes attached to the carbon once removed from the ring. According to the mechanisms proposed for these two reactions, these products are formed as follows ... 

Introduction. The action of halogens on saturated open chain hydrocarbons, as for example, pentane or hexane, gives several monohaJogen derivatives. Since the separation of the isomeric monohalides is difficult in the laboratory, they are usually prepared from alcohols. Direct halogenation is used industrially. The cyclic hydrocarbons, such as cyclohexane and benzene, jdeld only one monohalide. The present experiment illustrates direct bromination of a hydrocarbon. Chlorination is more difficult it is described in the latter part of the text (page 229). The catalyst used for bromination is iron other substances which can be used for the same purpose are anhydrous aluminum chloride and pyridine. 

The increasing importance of electron-transfer reactions with increasing aromatic hydrocarbon size is illustrated in the reaction of bromine with various aromatic compounds. With benzene (with a Lewis acid) and with naphthalene, electrophilic substitution occurs, and with anthracene, oxidative addition occurs (6) however, with graphite, only oxidation to the exclusion of carbon-bromine bond formation occurs, even at a stoichiometry of C8Br (II, 12). 

The case of electrophilic aromatic substitution can illustrate a situation in which it is useful to discuss transition state structure in terms of a reaction intermediate. The ortho-para- and mera-directing effects of aromatic substituents were among the first structure-reactivity relationships to be developed in organic chemistry. Certain functional groups were found to activate aromatic rings toward substitution and to direct the entering electrophile to the ortho and para positions whereas others were deactivating and led to substitution in the meta position. The bromination of anisole (methoxybenzene), benzene, and nitrobenzene can serve as cases for discussion. 

We can illustrate the usefulness of these generalizations by considering the synthesis of two different disubstituted derivatives of benzene. Suppose we wish to prepare bromonitrobenzene from benzene. This conversion can be carried out in two steps nitration and bromination. If the steps are carried out in just that order, the major product is indeed m-bromonitrobenzene. The nitro group is a meta director and directs bromination to a meta position ... 

Aromatic compounds that contain activating groups can be brominated without the use of the Lewis add catalyst because the ir electrons in the benzene ring are more available and polarize the bromine molecule suffidently to produce the required electrophile Br+. This is illustrated by the first step in the reaction between anisole and bromine ... 

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