Acetanilide: bromination nitration

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 ... 

Protection of the amino group by acetylation, as in acetanilide, therefore usually permits monosubstitution reactions with appropriate electrophilic reagents to proceed smoothly. Thus with bromine, p-bromoacetanilide is the main product the small quantity of the ortho isomer simultaneously formed can be easily eliminated by recrystallisation (Expt 6.67) hydrolysis of p-bromoacetanilide gives p-bromoaniline. Nitration leads similarly to p-nitroacetanilide which can be hydrolysed to p-nitroaniline (Expt 6.68). 

The electron-donating ability of an amino group is moderated by acetylation. Only monobromination of acetanilide occurs on reaction with bromine in acetic acid (Scheme 8.5). Acetanilide behaves similarly on nitration. 

Just by knowing the effects summarized in these short lists, we can now predict fairly accurately the course of hundreds of aromatic substitution reactions. We now know, for example, that bromination of nitro enzene will yield chiefly the /M-isomer and that the reaction will go more slowly than the bromination of benzene itself indeed, it will probably require severe conditions to go at ail. We now know that nitration of CeHsNHCOCH, acetanilide) will yield chiefly the o-and / -isomers and will take place more rapidly than nitration of benzene. 

Suppose, however, that we need o-nitroaniline. The synthesis that we just outlined would obviously not be a satisfactory method, for only a trace of o-nitroacetanilide is obtained in the nitration reaction. (The acetamido group is purely a para director in many reactions. Bromination of acetanilide, for example, gives/>-bromoacetanilide almost exclusively.)... 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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