# Why is selenophene aromatic?

According to Wikipedia,

The selenophene molecule is flat and aromatic. Being aromatic, it undergoes electrophilic substitution reactions at the 2- or 2,5-positions. These reactions are slower than that of furan, but faster than thiophene.

Selenium is a bulky 4th period element and I think the size of selenium is too large to have conjugation with carbon atom and make the compound aromatic. Why is selenophene still aromatic?

Further, faster electrophilic aromatic substitution (EAS) than thiophene sounds suspicious. Why and how?

• +1 I have edited your question a little bit, I wasn't sure what EAS meant. If the edit is not appropriate, feel free to roll it back. Also, adding a picture of selenophene to the question would be helpful. Jan 11 at 9:54
• The molecular single bond radius of Sulfur is 103 pm and Selenium 116 pm (source webelements.com/selenium/atom_sizes.html). Se is only 10% larger, I don't think your argument holds water. Jan 11 at 13:14
• Also, "aromatic" isn't a binary yes/no label (sometimes we like to think of it as being so, but really it isn't). It's an interesting question, I think there's a good discussion to be had. Jan 11 at 19:33
• The discussion to be had is going to be at least difficult and probably opinion-based. Unfortunately there is no objective way to quantify aromaticity and presentations like "It is a metallole with reduced aromatic character compared to thiophene." on Wikipedia are missing the reference framework. As long as the definition of aromaticity includes "behaves similar to benzene" I'm afraid there will be no straight answers to that. So to know more, one should read the publication which Wikipedia uses as a source for that quote (or not because it doesn't really say). Jan 11 at 23:14
• I have no access, but this looks promising: Pelkey, E. Selenophenes. Comprehensive Heterocyclic Chemistry III 2008, 975–1006. DOI: 10.1016/B978-008044992-0.00313-8. Jan 11 at 23:20

Selenium — like sulfur — is a chalkogen/member of group 16. It is heavier than sulfur, and thus more metalloid. In this regard similar to the trend in the adjacent group 17 (the halogens) where iodine has more metal-like character than the other members, or group 15 (the tetrels) with carbon; yet silicon, germanium, tin, and lead. Thus, formally, selenophene is isoelectronic to thiophene, a with six π-electrons over five ring atoms an electron enriched heteroaromatic compund (compared to benzene).

But, comparing pyrrole, furan, thiophene and selenophene, Yadav arguments

«The electron count [of π electrons for the Hueckel rule], however, is not enough. Along with the number of requisite electrons, all the bond lengths in the ring must be the same or very similar, as in benzene. In five-membered heterocylces such as pyrrole, furan, and thiophene, the size of the heteroatom lone pair orbital must be similar to that of the $$p$$ orbital on ring π bonds. Both the requirements are not fulfilled in thiophene because σC–S bond is significantly longer than σC–C bond and, in comparison to the p orbital on carbon, the size of lone pair orbital on sulfur is too large to allow effective overlap.»

(abstract)

Given the larger radius of selenium ($$\pu{115 pm}$$) vs. sulfur ($$\pu{100 pm}$$), this might be even more the valid for selenophene, than for thiophene.

The discussion however isn't a new one. By older recordings, the aromaticity of tellurophen and selenophen were considered between the one of furan and thiophene:

(Fringuelli et al., 1974)

References:

Fringuelli, F.; Marino, G.; Taticchi, A.; Grandolini, G. A comparative study of the aromatic character of furan, thiophen, selenophen, and tellurophen. J. Chem. Soc., Perkin Trans. 2 1974, 332-337; doi 10.1039/P29740000332.

Yadav V.K. (2021) Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels–Alder Stereoselectivity. In: Steric and Stereoelectronic Effects in Organic Chemistry, 191-216. Springer, Cham. doi 10.1007/978-3-030-75622-2_9.

Note, Yadav's publication is behind a paywall for which our library has no subscription. I only was able to access the abstract above.

• @andselisk I hope the edits you made are back again. It looks possible that both you and me edited the attempt simultaneously. For future reference, what is your argument to type π (apparently) directly, rather than by $\pi$? Is entering the math mode a reason why you suggest to type π directly, to keep consistency with the text-mode around? Jan 17 at 21:25
• No worries, my edit is present. There isn't much to add to my editing comment: here, letter pi should be upright (not a variable), and, since MathJax doesn't offer an alternative to upgreek package, it's better to use a Unicode version that isn't slanted. Jan 17 at 21:44
• I think you follow IUPAC's Green Book here. If I recall correctly, the 3rd edition (with Professor Quack on board) has/had a dedicated early section about when to use Roman, and when Italic fonts exactly on this point. Like in his short version, don't mix µ (upright) with $\mu{}$ (italic). Jan 17 at 21:47