Benzene, acylation chlorination

It is thus apparent that the selectivity of a reagent toward thiophene and benzene can differ appreciably, and this difference in selectivity is also strongly noticeable in the proportions of 2- and 3-isomers formed. Although in certain reactions no 3-isomer has been detected, appreciable amounts have been found in other reactions. Thus 0.3% of the 3-isomer has been found in the chlorination of thiophene.- Earlier results indicated that 5-10% 3-nitrothiophene is formed in the nitration of thiophene and a recent gas-chromatographic analysis by Ostman shows that the mononitrothiophene fraction contains as much as 16% of the 3-isomer. It appears that gas-chromatographic analysis should be very useful for the detection of small amounts of 3-isomers in other substitution reactions. However, from routine analyses of IR spectra, it appears to the present author that the amount of 3-isomers formed in acylation, formylation, and bromina-tion of thiophene are certainly less than a few per cent. 

Biphenyl and terphenyls may be regarded as substituted benzenes that undergo acylation, alkylation, halogenation, nitration, sulfonation, and other reactions common to benzene. The points of initial attack on chlorination, miration, and sulfonation of biphenyl occur at the 2- and 4-positions the latter group predominates. 

Since /8-D-gIucose pentaacetate had been shown to produce tetra-acetylglucosyl chloride in the presence of aluminum chloride,14 an attempt was made25 to employ the pentaacetate directly for the catalytic glucosylation of benzene. Such a scheme assumes that the acetylated aldose would undergo initial chlorination at the acylal function, and the intermediate chloride would then react with the aromatic hydrocarbon in the previously observed manner. Such a series of reactions was found to occur. 

The acid chloride of phenothiazine-1-carboxylic acid has been prejDared by treating the acid with the calculated amount of PCI5 in benzene, thus avoiding ring chlorination -(see Section V, A, 1, d). Another way to prevent ring chlorination in this reaction is first to acylate at position 10. ° Some amides have been prepared from phenothiazine carboxylic acid chlorides - ° and Curtius degradations performed. ... 

The characteristic reaction of benzene and its derivatives is electrophilic aromatic substitution. In these reactions, a hydrogen on the benzene ring is replaced by a chlorine (chlorination), a bromine (bromination), an alkyl or acyl group (Friedel-Crafts alkylation or acylation), a nitro group (nitration), or a sulfonic acid group (sulfonation). 

The reaction of benzene with acid chlorides in the presence of a Lewis acid catalyst (such as FeCl3 or A1C13) leads to the formation of acyl-benzenes. The Lewis acid increases the electrophilicity of the carbon atom attached to the chlorine. This leads to the formation of an acylium ion, which reacts with the electron-rich benzene ring. 

Tagawa, T., Amemiya, J., and Goto, S. 2004. Chlorine-free Friedel-Crafts acylation of benzene with benzoic anhydride on insoluble heteropoly acid catalyst. Appl. Catal. A Gen. 257 19-23. 

Thiophene is chlorinated by CI2 or SO2CI2. Bromination occurs by Br2 in acetic acid or with N-bromosuccinimide. Nitration is effected by concentrated nitric acid in acetic acid at 10°C. Further substitution predominantly yields 2,4-dinitrothiophene. Sulfonation with 96% H2SO4 occurs at 30°C within minutes. Benzene reacts extremely slowly under these conditions. This provides the basis for a method to remove thiophene from coal tar benzene (see p 77). Alkylation of thiophenes often gives only poor yields. However, more efficient procedures are the Vilsmeier-Haack formylation, which yields thiophene-2-carbaldehyde, and acylation with acyl chlorides in the presence of tin tetrachloride, to give 2-acylthiophenes. Like furan, thiophene is mercurated with mercury(II) chloride. 

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