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Why does thiophene possess a greater tendency to undergo sulphonation than benzene? I came across this reaction which employs the use of sulphuric acid to separate thiophene from commercially prepared benzene.

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  • $\begingroup$ The activation energy is less for the the thiophene reaction because thiophene is less aromatic than benzene. $\endgroup$ – ron Jan 25 '18 at 16:12
  • $\begingroup$ Not because aromatic conjugation in thiophene imparts negative charge to the carbon? Draw thiponene Kekule type structures and see ... . $\endgroup$ – Oscar Lanzi Jan 25 '18 at 20:10
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    $\begingroup$ As ron said thiophene is less aromatic than benzene, but perhaps more importantly thiophene is an electron-rich ring which leads to greater reactivity with electrophiles (which are electron-poor). Pyridine is as aromatic or slightly less aromatic than benzene, but pyridine is very unreactive towards electrophiles. $\endgroup$ – orthocresol Jan 25 '18 at 20:25
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    $\begingroup$ I came across a Quora answer saying that in thiophene, the 6 electrons are delocalised over 5 atoms but in benzene, the delocalisation is over 6 atoms. Therefore, electron density is higher in the former. $\endgroup$ – Tan Yong Boon Jan 25 '18 at 23:09
  • $\begingroup$ @Orthocresol as an aside -- pyridine can be made to "wake up" for electrophiles by N-oxidizing it. $\endgroup$ – Oscar Lanzi Jan 26 '18 at 1:47
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In the chapter Aromatic five-membered ring heterocycles with one heteroatom in Organic Chemistry by J. William Suggs (2002) the following explanation can be found (pp. 403-404) which summarizes the thoughts already pointed out in the comments:

The resonance stabilization energy of benzene is greater than that of these heteroaromatic compounds. The order of aromaticity is benzene > thiophene > pyrrole > furan.

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All three of these ring systems undergo electrophilic aromatic substitution and are much more reactive than benzene. In part, this reactivity difference arises because the rate-determining step in electrophilic aromatic substitution is the first step, which breaks up the aromatic $\pi$ system. Since thiophene, pyrrole, and furan have less stabilization to lose than benzene, the intermediate is lower in energy and the overall reaction proceeds more rapidly.

[…]

All three of these heteroaromatic rings undergo electrophilic aromatic substitution, preferentially at C-2. The reactivity order is pyrrole > furan > thiophene because of several factors, including the electronegativity of the heteroatom and the resonance stabilization of the aromatic ring.

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