Bromination of aniline

Mono-substitution products of primary amines cannot easUy be prepared by direct action of the appropriate reagent for example, bromination of aniline yields largely the 2 4 6-tribomo derivative and nitration results in much oxidation. If, however, the amino group is protected as in acetanilide, smooth substitution occurs. Thus with bromine, />-bromoacetanilide is the main product the small quantity of the ortlio isomeride simultaneously formed can be easily eliminated by crystallisation. Hydrolysis of p-bromoacetanilide gives/ -bromoaniline ... 

Finally we mention in this section the non-catalytic selective bromination of aniline by the application of a zeolite pre-loaded with Bt2 as a slow release reagent (ref. 27). Aniline, dissolved in CCI4 was treated with Br2 adsorbed onto various zeolites and zeolite CaA was found to be most selective for monosubstitution (92%). The addition of organic bases improved the performance, probably due to scavenging of HBr. Also the toluidines could be monobrominated with this system with >95% selectivity. 

The direct bromination of aniline and phenol derivatives in solution results in polybromination to give a complex mixture. But bromination by gas-solid and solid-solid reactions proceeds more efficiently and selectively. Bromination by gas-solid reaction can be accomplished through a very simple procedure. For example, the powdered crystalline aniline 13 and Br2 were placed in... 

Benzene, toluene, ethylbenzene and chlorobenzene have been shown to be suitable substrates under these conditions, and the reaction rates are 50 to 70 times faster than for uncatalysed reactions. This reaction can be further enhanced by use of methanol as a co-catalyst, which allows bromination of anilines in quantitative yield with complete selectivity for the para- isomer [54],... 

Conversion of secondary and tertiary alcohols into the corresponding chlorides has been shown to proceed efficiently when the powdered solid alcohol is exposed to HCl in a desiccator [28]. Solid-state bromination of anilines and phenols with gaseous bromine and solid bromination reagents has been shown to proceed with higher yields than in solution [29]. 

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 bromination of benzene because bromine is an ortho, para director. Instead, advantage is taken of the powerful activating and ortho, para-directing effects of the amino group in aniline. Bromination of aniline yields 2,4,6-tribromoanihne in quantitative yield. Diazotization of the resulting 2,4,6-tribromoaniline and reduction of the diazonium salt gives the desired 1,3,5-tribromobenzene. 

Tabic 1 Rate Constants for the Bromination of Aniline, the Anilinium Ion and a Metal Complex"... 

The Lewis acidity of a particular metal ion will be primarily a function of the charge density on the metal centre. Complexes of Cu11, Zn11 and Co111 should therefore display properties intermediate between those of the free ligand and its conjugate acid. Early measurements on the bromination of aniline, the anilinium ion and coordinated aniline provide a good illustration of this point (Table 1). Since this reaction is an electrophilic substitution, protonation and metal... 

The free amino group strongly activates the aromatic ring towards electrophilic attack and aromatic substitution of amines often results in polysubstitution. For example, the bromination of aniline yields largely 2,4,6-tribromoaniline (Expt 6.59). 

Figure 8-2. The bromination of aniline in the complex [Cr(PhNH2)3Cl3] to give a complex containing 2,4,6-tribromoaniline. The use of a kinetically inert metal centre allows us to ensure that the reaction is one of the co-ordinated ligand, even though the organic product is the same as that obtained with the free ligand.

Bromination of aniline in homogeneous solution immediately proceeds to tribromina-tion. Using bromine adsorbed on 5A zeolite leads to a selective monobromination mostly at the para position146. 

In bromination of anilines, the ortholpara ratio is significantly increased in presence of cetyltrimethylammonium bromide (CTAB) but is not altered with a symmetrical ammonium salt (TBAB). Surprisingly, increasing steric hindrance in the ortho position by N-alkylation has resulted with higher ortho-selectivity. This phenomenon is linked to the orientation of the aniline at the surface of the micellar aggregates1017. 

Draw all the resonance forms for the sigma complexes corresponding to bromination of aniline at the ortho, meta, and para positions. 

Nitration followed by reduction forms aniline (C6H5NH2) from benzene (Steps [1] and [2]). Bromination of aniline yields the tribromo derivative in Step [3]. 

The direct chlorination or bromination of aniline takes place more easily than that of benzene, the result being the symmetrical tri-chlor or tri-brom aniline, viz., i-amino 2-4-6-tri-chlor benzene, C6H2CI3-(NH2) and I-amino 2-4-6-tri-brom benzene, CeH2Br3(NH2). The mono-halogen anilines are prepared by reducing mono-chlor nitro benzenes, or by halogenating acetanilide and then hydrolyzing. 

Derivation Steam distillation ofp-bromoacetanilide and sodium hydroxide or bromination of aniline. 

Bromination of anilines. Good selectivity (p-bromination) is observed with KBr and sodium perborate in HOAc at room temperature. 

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