Aromatic group bromination

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

Methyla-arylacetates. These esters have been obtained by oxidative rearrangement of alkyl aryl ketones with thallium(III) nitrate in acidic methanol or trimethyl orthoformate (4,496 5, 656 7, 362). A new method, which avoids the toxic TTN, is based on the Woodward version of the Prevost reaction. Thus, treatment of the ketone with iodine (or bromine) and silver nitrate (2 equiv.) in refluxing methanol containing trimethyl orthoformate results in methyl a-arylacetates in 90% yield from simple substrates. Yields are lowered by electron-withdrawing substituents on the aromatic group and by a-branching in the alkyl group.2... 

Acetal Bromination and Formation. The bromination of an acetal by NBS under radical conditions does not require the presence of an aromatic group (eq 41). ... 

Chemical Tests. You may wish to perform several classification tests on the product (see Chapter 9). Carry out the ignition test to confirm the presence of an aromatic group. The Beilsfein tesf can be used to detect the presence of bromine. The silver nitrafe fesf for alkyl halides should also give a positive result. 

Bromine Water. Aromatic amines, since they possess an electron-rich aromatic ring, can undergo electrophilic aromatic substitution with bromine, to yield the corresponding arylamino halide(s). Therefore, if elemental tests indicate that an aromatic group is present in an amine, treatment with the bromine water reagent may indicate that the amine is attached to an aromatic ring. 

Detection of certain types of compounds (for example, of an aromatic skeleton). If the compound trasmits radiation (b < 1) between 220 and 800 nm, it may well be an aliphatic or alicyclic hydrocarbon, amine, nitrile, alcohol, ether, alkyl chloride, or alkyl fluoride, but the presence of conjugated double bonds, the carbonyl group, nitro group, bromine, or iodine is excluded. The detection of the aromatic nucleus in an unknown sample is carried out by measuring the absorption in the ultraviolet region of ethanolic, cyclohexane, or /i-hexane solutions of various concentrations (10 -10 mole/liter). 

For the identification of aromatic ethers use is made either of the reactivity of the aromatic nucleus (bromination, chlorosulfonation), or else they can be cleaved with hydriodic acid (see p. 201). In the latter case, however, only the alkyl group bound to oxygen is identified. The oxidation of side chains (compare p. 129) has only a limited use the alkoxy group on the aromatic nucleus increases the stability of addition compounds (see p. 126) with picric acid, which can also be used for identification. 

Because the position of electrophilic attack on an aromatic nng is controlled by the direct ing effects of substituents already present the preparation of disubstituted aromatic com pounds requires that careful thought be given to the order of introduction of the two groups Compare the independent preparations of m bromoacetophenone and p bromoace tophenone from benzene Both syntheses require a Friedel-Crafts acylation step and a bromination step but the major product is determined by the order m which the two steps are carried out When the meta directing acetyl group is introduced first the final product IS m bromoacetophenone... 

Poly(phenylene oxide)s undergo many substitution reactions (25). Reactions involving the aromatic rings and the methyl groups of DMPPO include bromination (26), displacement of the resultant bromine with phosphoms or amines (27), lithiation (28), and maleic anhydride grafting (29). Additional reactions at the open 3-position on the ring include nitration, alkylation (30), and amidation with isocyanates (31). 

Halogenation. The presence of the amino group activates the ortho and para positions of the aromatic ring and, as a result, aniline reacts readily with bromine or chlorine. Under mild conditions, bromination yields 2,4,6-trihromoaniline [147-82-0]. 

Electrophilic substitution reactions of diarylamines are easily accompHshed since the amino group activates the aromatic ring. Thus, DPA reacts with bromine or chlorine to form the 2,2H,4 tetrahalo derivative nitration usually produces the trinitro compound. 

Bromine can replace sulfonic acid groups on aromatic rings that also contain activating groups. PhenoHc sulfonic acids, for example, are polybrominated (24). 

Again, as with pyridopyrimidines, the main reaction is oxidation of di- or poly-hydro derivatives to fully aromatic structures, often merely by air or oxygen. In some cases the reagent of choice is mercury(II) oxide, whilst other reagents used include sulfur, bromine, chloranil, chromium trioxide-acetic acid, hydrogen peroxide, and potassium ferricyanide, which also caused oxidative removal of a benzyl group in the transformation (306) (307)... 

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