Anthracene reactivity toward electrophiles

It is known that whereas benzene and naphthalene are relatively unreactive, the remaining compounds have much higher reactivity toward electrophiles and 1,2- or 1,4-additions. In the case of benzene, addition across a double bond will cause a 34 kcal/mol loss of stabilization (36 less 2 kcal/mol for the butadiene fragment) and addition to naphthalene will cause a 22 kcal/mol loss of stabilization (i.e. 60 kcal/mol less 38 kcal/mol for the remaining styrene unit). On the other hand, addition across the 9,10 positions in anthracene will only lead to an 8 kcal/mol loss... 

The first quantitative study of the reaction was carried out with anthracene-9-carboxylic acid (which possesses the necessary steric requirement by virtue of the peri-hydrogen atoms, and is very reactive at the 9 position towards electrophilic substitution). Schenkel632 found that the decarboxylation rate was increased in the presence of acid, and the first-order rate coefficients (believed to be in sec-1) are given in Table 205. It was subsequently concluded that in the absence of acid,... 

It is apparent from Table 6 that methyl and methoxy substituents enhance the reactivity of butadiene and anthracene, while chlorine atoms decrease that of butadiene and cyclopentadiene, towards electrophilic dienophiles. Phenyl substituents have very different effects in positions I and 2 [columns (i) and (iv)] here the substituent is large enough to exert a marked steric influence on the cisoid transo/c/equilibrium of the diene, so that the conformation of 2-phenyl-butadiene is favoured, and therefore the overall reactivity of 2-phenyl-butadiene is enhanced, while trans-1 -phenyl-butadiene is little affected. Cyclic unsubstituted dienes are more reactive than butadiene however, the difference between cyclopentadiene and cyclohexadiene is enormous the high reactivity of the former must be in part attributed to factors other than the cisoid conformation. 

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