Additivity rules thiophene

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

The syn addition mode was also confirmed by ab intio calculation of the reaction between thiophene 1-oxide 99 and ethylene. They stated that the selectivity can be explained by the orbital mixing rule (Scheme 51). The ir-HOMO of the diene part of 99 is modified by an out-of-phase combination with the low lying n-orbital of... 

Is there any evidence that this rule can be contravened To answer this question, the complexes of vinyl fluoride, furan and thiophene with HC1 and ClF will be considered. Vinyl fluoride, CH2CHF, is an example of a mixed n-pair/jt-pair donor in which, unlike CO, HCN, CH3CN or CH2O, the pairs of electrons (a Tt-pair shared between Ci and C2 and an n-pair on F) do not have an atom in common. In addition, its complexes with HC1 and ClF are important in the context of linear/non-linear hydrogen and halogen bonds. On the other hand, furan and thiophene are examples of mixed n-pair/n-pair aromatic donors in which the n-pair can be withdrawn into the ring. 

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