Bromination of aromatic rings

A benzene ring, with its six n electrons in a cyclic conjugated system, is a site of electron density. Furthermore, the benzene tt electrons are sterically accessible to attacking reagents because of their location above and below the plane of the ring. Thus, benzene acts as an electron donor (a Lewis base, or nucleophile) in most of its chemistry, and most of its reactions take place with electron acceptors (Lewis acids, or electrophiles). For example, benzene reacts with Br2 in the presence of FeBr3 as catalyst to yield the substitution product bromobenzene. 

CHAPTER 16 Chemistry of Benzene Electrophilic Aromatic Substitution 

Electrophilic substitution reactions are characteristic of all aromatic rings, not just of benzene and substituted benzenes. Indeed, the ability ofa compound to undergo electrophilic substitution is a good test of aromaticity. 

An electrophilic aromatic substitution reaction begins in a similar way but there are a number of differences. One difference is that aromatic ring.-are less reactive toward electrophiles than alkenes are. For example, Br2 in CH2CI2 solution reacts instantly with most alkenes but does not react at room temperature with benzene. For bromination of benzene to take plat a catalyst such as FeBr3 is needed. The catalyst makes the Br.2 molecu.. more electrophilic by polarizing it to give an FeBr4 Br species that reacts as if it were Br+. M 

The polarized Br2 molecule is then attacked by the 7r electron systeml of the nucleophilic benzene ring in a slow, rate-limiting step to yield a nonaromatic carbocation intermediate. This carbocation is doubly allylid (recall the allyl cation, Section 11.9) and has three resonance forms  

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Thomson 40W Click Organic Process to view an animation of the bromination of aromatic rings. 

Whereas pure NBS effects side chain bromination of aromatic hydrocarbons, aged material effects bromination of aromatic ring. 

Bromination of Aromatic Rings 593 Other Aromatic Substitutions 597 Alkylation of Aromatic Rings The Friedel-Crafts Reaction Acylation of Aromatic Rings 604 Substituent Effects in Substituted Aromatic Rings 605 An Explanation of Substituent Effects 610 Trisubstituted Benzenes Additivity of Effects... 

The bromination of aromatic hydrocarbons can occur either in a side chain or on the ring, depending on conditions. In the presence of sunlight aLkylben2enes are brominated predominately in the side chain (24). 

Alkyl Side Chains of Aromatic Rings. The preferential position of attack on a side chain is usually the one a to the ring. Both for active radicals such as chlorine and phenyl and for more selective ones such as bromine such attack is faster than that at a primary carbon, but for the active radicals benzylic attack is slower than for tertiary positions, while for the selective ones it is faster. Two or three aryl groups on a carbon activate its hydrogens even more, as would be expected from the resonance involved. These statements can be illustrated by the following abstraction ratios ... 

Table 13 Values of p and m for ring-substituent and solvent effects in the bromination of aromatic olefins trans-Ar—C(R)=CHR in methanol at 25°C.

This is the case for the synthesis of a thiol derivative of a paracyclo-phane starting from the corresponding bromine-substituted aromatic ring (Scheme 41) [106]. 

Whereas pure NBS effects side-chain bromination of aromatic hydrocarbons, aged material effects bromination of the aromatic ring. The responsible impurity can be removed by pumping out at 0.5 mm. over phosphorus pentoxide for 4 hrs. Caution Explosions have been reported in reactions of NBS. ... 

The pentacyclic core 63 was converted into codeine by a different approach from that of Taber (Scheme 14). Treatment of 63 with dibromohydantoin generated bromohydrin 64 stereoselectively. In this reaction, the reagent approaches from the less hindered (3-face and the resulting (3-cyclic bromonium ion was opened with water in trans-diaxial manner. Under the reaction conditions, the C-l position of aromatic ring was concomitantly brominated. Exposure of 64 to KOH/dioxane... 

B.1. Alkyl Cations and Regioselectivity. This chapter deals with generating carbocations that can be used to form carbon-carbon bonds. Just as alkenes react with cations to form cationic intermediates, the n bonds of aromatic rings also react with cations (sec. 12.4.A) to form a cationic intermediate identical to that found in electrophilic aromatic substitution (namely, 158). Just as in bromination, chlorination, nitration, and... 

Benzylic Bromination of Aromatic Compounds. An efficient and fast microwave-assisted method for the preparation of benzylic bromides has appeared The 2-trimethylsilylethyl substituent on the benzenoid ring of 3 undergoes benzylic bromination followed by elimination of MesSiBr and addition of bromine to produce the dibromo compound (4) (eq 31). The ketone (5) is also observed from the hydrolysis of (4). ... 

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