Sensitivity of the Five-Membered Heterocycles to Substituent Effects

Sensitivity of the Five-Membered Heterocycles to Substituent Effects 

In considering the results obtained to date, some general points need to be considered. 

It is thus probably impossible at present to interpret unambiguously the magnitude of p, and this difficulty is compounded by the paucity of data. Some p factors for electrophilic substitutions of benzene and thiophene are gathered in Table 6.14 only the acetylation, detritiation, and protio- 

Hammett p Factors eor Electrophilic Substitution ok Substituted Benzenes and Thiophenes 

The general trend seems to be for thiophene to give smaller p factors, consistent either with it being more reactive, or that it transmits substituent effects less readily. However, the relative values of the (well-documented) p factors for desilylation are puzzling and confirms that a single overall explanation will not suffice. It is probably relevant that in this reaction the demand for resonance stabilization of the transition state is substantially lower. 

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