Friedel pair overlap

The pair of Bragg reflections Ti, and F h arise from the same stack of planes but they are the scattering from the opposite sides these are a Friedel pair of reflections. They always exactly overlap in a powder pattern since h = h, but they may not have the same value of F if the structure is noncentrosym-metric and there is significant resonant scattering from some of the atoms. Their average intensity is given by  

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You may be surprised that thiophene is the least reactive of the three but this is because the p orbital of the lone pair of electrons on sulfur that conjugates with the ring is a 3p orbital rather than the 2p orbital of N or O, so overlap with the 2p orbitals on carbon is less good. Both furan and thiophene undergo more or less normal Friedel-Crafts reactions though the less reactive anhydrides are used instead of acid chlorides, and weaker Lewis acids than AICI3 are preferred. 

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