# Difference between tendency of benzene and thiophene to undergo sulfonation

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.

• The activation energy is less for the the thiophene reaction because thiophene is less aromatic than benzene. – ron Jan 25 '18 at 16:12
• Not because aromatic conjugation in thiophene imparts negative charge to the carbon? Draw thiponene Kekule type structures and see ... . – Oscar Lanzi Jan 25 '18 at 20:10
• 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. – orthocresol Jan 25 '18 at 20:25
• 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. – Tan Yong Boon Jan 25 '18 at 23:09
• @Orthocresol as an aside -- pyridine can be made to "wake up" for electrophiles by N-oxidizing it. – Oscar Lanzi Jan 26 '18 at 1:47

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.