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Why aren't aromatic compounds with 3, 4 or 5 carbon atoms taught at high school level chemistry? We are instead directly taught about aromatic compounds with 6 carbons/benzene rings. Why?

My core question is: is because the lower members are tough to understand? Do they have any strange properties?

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Hi tvamsisai, I'm afraid this question is rather subjective (and depends on local cursus, etc.). It seems to me that the simple reason is that benzene-based aromatics are the most common of these compounds… – F'x Feb 14 '13 at 11:38
F'x: maybe, but its surely not off topic. ah, availability! that partly answers. are lower members tough to study/learn/understand/teach? – tvamsisai Feb 14 '13 at 12:08
I was taught these at high school level. At first, benzene was taught, and then these compounds were used as examples for aromaticity. Including the crazy positively-charged triangle. – ManishEarth Feb 14 '13 at 12:56
This is a very interesting question, as I took two years of chemistry in high school, and was taught no organic chemistry whatsoever. – Ben Norris Feb 15 '13 at 15:30
up vote 17 down vote accepted

Hueckel's rule (i.e. that aromatic compounds have $4n+2$ $\pi$ electrons) places limits on aromaticity in cycles with less than 6 carbons.

As there are only 3 cases to look at, let's enumerate them:

5 carbons: cyclopentadiene ($\ce{C5H6}$) is not aromatic, but the cyclopentadienide anion ($\ce{[C5H5]^{-}}$) is. This species is well known in organometallic chemistry, due to its propensity to act as an $\eta^5$ ligand and form sandwich complexes with transition metals. The most famous of these is no doubt ferrocene, where two cyclopentadienide ligands sandwich an $\ce{Fe^{2+}}$ ion. This ion is didactically problematic and in my experience not covered in high school or early undergraduate curricula because most of its interest lies in its sandwich bonding mode with metal ions, which cannot be easily reconciled with the simplified VB/VSEPR/hybridisation theory used to teach introductory organic chemistry. The same goes for the under-loved tropylium cation ($\ce{[C7H7]^+}$), which has 7 carbons.

4 carbons: cyclobutadiene is antiaromatic, and as such only exists at very low temperatures. If I recall correctly, it possesses two unusually long bonds and two unusually short bonds, quite at odds with the bond equivalency found in benzene. This molecule would have to lose 2 e- to become aromatic. It does so happen that the cyclobutadienyl ligand is known in $\eta^{4}$ metal complexes such as $\ce{(C4H4)Fe(CO)3}$ (1) - I was not aware of these complexes until an expert in the field pointed them out to me.

3 carbons: I was not sure about this, but apparently the cyclopropenyl cation is a thing and is aromatic. Cotton (2) also mentions this being known as a $\eta^3$ ligand. I suspect that it is not taught because it's too weird.

In conclusion:

The smaller aromatic rings are principally known as polyhaptic ligands to metals due to their non-neutrality. This places them outside of the comfort zone of the theories of molecular structure and bonding used in introductory organic chemistry, and as such they are usually left until later.

(1) Cotton, F.A., Wilkinson, G.; Advanced Inorganic Chemistry; Wiley Interscience, 5e, (1988), p. 1170

(2) Ibid, p. 1168

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We were taught about the cyclopropenyl cation, and they even showed us some reactions where its aromaticity makes a difference (so yeah, it's a thing). But yeah, it's weird. – ManishEarth Feb 14 '13 at 12:58
@Manishearth - Most chemistry educators will be suddenly consumed by a pillar of eldritch flame if they attempt to reveal any unusual organometallic bonding patterns to any student before second year, due to the dark pact they enter into. – Richard Terrett Feb 15 '13 at 6:16
+1 for answering a question about high school chemistry with Cotton and Wilkinson! – Ben Norris Feb 15 '13 at 15:28
@BenNorris - Housecroft and Sharpe (one of my favourites) also covers cyclooctatetraenide through to cyclobutadienyl complexes and is somewhat more introductory than Cotton. Both books are heavy enough to cause serious injury if dropped or thrown. – Richard Terrett Feb 16 '13 at 9:26
An excellent answer, but perhaps for completeness 7-membered rings should be mentioned, even if not specifically asked in the question. Cycloheptatriene loses H- when treated with an appropriate oxidant, to form the tropylium cation C7H7+ which is aromatic because it has 6 pi electrons. – Level River St Nov 10 '14 at 12:12

I remember asking a very similar question when I was learning about aromatic compounds in high school.

The answers I was given is that the possible structures with less than 6 carbons are not stable under standard conditions.

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The possible structures under 6 carbons are all ions, and as intermediates or solvated ions they are apparently stable enough, at least the 5-ring. But yeah, not stable on their own, obviously. – ManishEarth Feb 16 '13 at 17:10

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