Hammett correlation anhydrides

Lastly, nitration of 2-substituted thiophenes by nitric acid-acetic anhydride was said to give a Hammett correlation with p = -6.7 (82CHE127). However, this is invalid since r values were used instead of cr+, it was assumed that all substitution occurred at the 5-position (certainly incorrect), and the correlation line missed the origin by a very large amount. 

An other correlation has been found between log endolexo) in the cyclopentadiene addition to ring-substituted rrun.v-cinnamic acids and Hammett-type substituent constants the low absolute value of the slope (—0.30)"" indicates very similar substituent effects on the two reaction paths. For the reaction of 2-substituted anthracenes with maleic anhydride" the influence of substituents on log (synlanti) is even weaker"". 

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