# The naming of benzene

Why is benzene not called as a cyclo triene? Wikipedia says benzene is a IUPAC name, but isn't it a common name?

• Benzene is not a triene, the bonds are delocalized. – ron Jan 25 '16 at 17:43
• @ron I think that entirely exceeds the scope of the question. (Also note that the cyclopentadienyl anion is generally called that, although the bonds and charge are also delocalised.) – Jan Jan 25 '16 at 18:23
• @Jan Well, just my opinion, nothing more. Note that derivatives of 1,3,5-cyclohexatriene have been prepared; their properties are noticeably different from those of benzene. – ron Jan 25 '16 at 20:51
• @Jan See Peter Vollhardt's work on the synthesis and characterization of 1,3,5-triscyclopropabenzene and 1,3,5-triscyclobutabenzene. – ron Jan 25 '16 at 23:05
• how about phenyl hydride? – LiamH Jan 26 '16 at 9:44

Cyclohexa-1,3,5-triene does not exist.

It is just a hypothetically possible structure for the formula $\ce{C6H6}$.

The whole discussion is intended to explain that if it were to exist it would have such and such properties but it does not exist.

Instead we have benzene.

• Perhaps you could give a brief explanation of why it doesn't exist. At the moment this seems a bit unsatisfactory as an answer. – bon Jan 25 '16 at 18:22
• Not to say that this answer is probably a mile away from what the question is actually asking. – Jan Jan 25 '16 at 18:24
• [citation needed] Ron’s evidence seem to show that derivates of cyclohexatriene exist. – Jan Jan 25 '16 at 23:33

It would be wrong to say cyclotriene, rather cyclohexatriene would be an appropriate name. And it would also be a highly systematic IUPAC name for said compound, following the rules of e.g. the cyclopentadienyl anion, the cycloheptatrienyl cation, cyclooctatetrene and more.

Let’s look at benzene’s history. It was discovered as early as 1825 by Michael Faraday who called it bicarburet of hydrogen. In 1833, Eilhard Mitscherlich isolated it and named it benzin. In 1836 it was isolated by Auguste Laurent who called it phène. The name benzene also dates back to the first half of the 19th century and, like benzin, derives from the gum benzoin. Note that this already predates the founding of IUPAC by about 80 years. The first IUPAC nomenclature was not established until 1921.

At the time when it was discovered, chemists had no clue as to what structure it had. It wasn’t until 1865 when Friedrich Kekulé published his ground-breaking paper[1] on the structure of benzene which in the end turned out to be correct (-ish). At that time, nomenclature of chemicals was still the Wild West, since there was hardly any way for structure elucidation and thus most chemicals acquired trivial names — some of which survive up to date (acetic acid being a prime example).

When IUPAC finally derived comprehensive nomenclatures for organic (and inorganic) compounds, and when structure elucidation was finally good enough to prove structures, the names benzene, benzin and possibly also phène had been in use for many decades, much like many other trivial names. Had IUPAC decided to obsolete them all altogether, they could have just as well trashed their own recommendations, because nobody would have followed them — even today, many chemists are too traditional to take up newer nomenclature schemes even if they have been around for two decades.

Probably to help gain acceptance, IUPAC has always allowed a certain set of retained names to persist under official nomenclature. While the list has gotten smaller over the years, it is extremely unlikely that benzene, phenol, phenyl etc. will ever get removed from it, since calling those compounds that way is so prevalent — and short.

Similar things happened to carboxylic acid. Hardly anybody says methanoic acid or butanoic diacid in place of formic acid and succinic acid. And sometimes, trivial names even become part of the official nomenclature, as was the case with fullerene because it provided a much shorter way to label a certain class of compounds.

So the short answer to your question: IUPAC has retained a common name. The reasons are tradition, prevalence and simplicity.

Note:

‡: Yes, as a chemist I know that the structure of benzene is incorrectly represented by drawing three pure double bonds and three pure single bonds, that in reality the molecule has a very high $D_\mathrm{6h}$ symmetry rendering all bonds equal and the electrons in the π system are delocalised all across the molecule. However, that is not what the question is asking about, and not something IUPAC nomenclature cares about.

Reference:

[1]: F. A. Kekulé Bull. Soc. Chim. Paris 1865, 3, 98.

• I disagree, cyclohexatriene is an incorrect name for benzene. As I mentioned in my comments under the OP's question, derivatives of 1,3,5 cyclohexatriene have been prepared and characterized by Vollhardt's team. These 1,3,5-cyclohexatrienes behave quite differently from benzene - the names benzene and cyclohexatriene describe different compounds. – ron Jan 25 '16 at 23:26
• @ron In this and in many other cases concerning nomenclature, IUPAC looks at the compound to be named in an entirely theoretic manner and decides solely by theoretic considerations what to call it. IUPAC’s aim in nomenclature is to be able to take a compound of which you only know a skeletal formula and turn it into a name. Therefore, IUPAC nomenclature would never distinguish between a resonance-stabilised an a non-resonating cyclohexatriene. – Jan Jan 25 '16 at 23:30
• Sounds like a serious shortcoming to me, as 3 double bonds do not exist in benzene. Since the properties of 1,3,5-cyclohexatriene and benzene differ, it just seems wrong to me to call them both by the same name - but understand, that's just my opinion. – ron Jan 25 '16 at 23:36
• I second @ron's concern, but following that logic we would have to give the cyclopentadienyl-anion a new name too, because there are certainly no localised double bonds in there, as well as no localised negative charge. In that regard I support the decision to ignore delocalised systems completely and name such species according to it's resonance structures. – Martin - マーチン Feb 5 '16 at 1:59
• great, I was always wondering about phenyl etymology – mykhal May 11 '17 at 22:05