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Because aromaticity deals directly with double bonds and conjugation it is simpler to just count the number pi electrons in a compound.
This is true of aromatic compounds, meaning they are quite stable. With aromatic compounds, 2 electrons fill the lowest energy molecular orbital, and 4 electrons fill each subsequent energy level the number of subsequent energy levels is denoted by n , leaving all bonding orbitals filled and no anti-bonding orbitals occupied.
You can see how this works with the molecular orbital diagram for the aromatic compound, benzene, below. These 4 fill in the orbitals of the succeeding energy level. Notice how all of its bonding orbitals are filled, but none of the anti-bonding orbitals have any electrons.
If is 0 or any positive integer 1, 2, 3, More than years ago, chemists recognized the possible existence of other conjugated cyclic polyalkenes, which at least superficially would be expected to have properties like benzene. The most interesting of these are cyclobutadiene, whose shape and alignment of p orbitals suggested it should have substantial electron-delocalization energies. However, cyclobutadiene was found to be an extremely unstable molecules.
In fact, cyclobutadiene is even more reactive than most alkenes. The synthesis of cyclobutadiene eluded chemists for almost years. As more work was done, it became increasingly clear that the molecule, when formed in reactions, was immediately converted to something else. Finally, cyclobutadiene was captured in an essentially rigid matrix of argon at 8K. On warming to even 35K , it dimerizes through a Diels Alder reaction to yield a tricyclicdiene. Due to the square ring, cyclobutadiene was expected to have some degree of destabilization associated with ring strain.
However, estimations of the strain energies, though substantial, did not account for cyclobutadiene's high degree of instability. Also, why was it not being stabilized through cyclic conjugation in the same way as benzene. The answer can be seen in the molecular orbitals. We considering the molecular orbitals diagram of the analogous 1,3-butadiene, the four 2p atomic orbitals combine to form four pi molecular orbitals of increasing energy. The 4 pi electrons of 1,3-butadiene completely fill the bonding molecular orbitals giving is the additional stability associated with conjugated double bonds.
However, when placed into a ring the molecular orbitals undergo a significant change. The four p orbitals of cyclobutadiene combine to form the following 4 molecular orbitals:. The non-bonding orbitals represent that there is no direct interaction between adjacent atoms. When adding cyclobutadiene's 4 pi electrons to the molecular orbital diagram, the bonding orbital is filled and both non-bonding orbitals are singly occupied. If cyclobutadiene's double bonds were delocalized, all the pi electrons would be in low energy bonding orbitals.
However, only two of the pi electrons are in bonding orbitals; the other two are non-bonding. In addition, the molecular orbital diagram shows that two of the electrons are unpaired, a situation called a triplet state, which usually makes organic molecules very reactive.
For cyclobutadiene cyclic conjugation has made the molecule less stable. Antiaromaticity gives cyclobutadiene some interesting structural features. Cyclobutadiene's single and double bonds have different bond lengths, pm and pm respectively which give it a rectangular shape. If any of these conditions are violated, no aromaticity is possible. First, it must be cyclic.
Second, every atom in the ring must be conjugated. There are a few alternative ways to say the same thing. For example, how do we count electrons in the benzene anion below left or pyridine? Do we count the lone pair electrons as Pi electrons, giving a total of 8? Or do we ignore them? What about furan middle which has two lone pairs on oxgyen?
What about pyrrole, with its lone pair on nitrogen, or imidazole, with two nitrogens? Since that carbon is not involved in any pi-bonding, the answer is yes. Summary: Rules For Aromaticity This post went through the four conditions a molecule must meet to be aromatic.
Thanks to Matt Knowe for assistance with this post. Check out these worked examples Notes Fun fact. Pyrrole actually reacts with acid on carbon, not nitrogen! The latter can be verified by computational methods, as the article demonstrates. Global aromaticity at the nanoscale Rickhaus, M.
Nature Chem. Aromaticity is a difficult concept to accurately define, but one way to empirically measure it is to use computational methods.
Here, Prof. This can be measured experimentally or probed computationally. The Cyclodecapentaene System Dr. Vogel, Dipl. Roth Angew. The NMR spectrum shows a diamagnetic ring current of the type expected in an aromatic system. Synthesis of 1,6-didehydro[10]annulene. Observation of its exceptionally facile rearrangement to form the biradical 1,5-dehydronaphthalene Andrew G.
Myers and Nathaniel S. This is no exception, and the brevity of this communication belies the extreme difficulty of this experimental work, both in synthesis and characterization. This is because it cannot achieve planarity, thus proving that planarity is an essential criterion for achieving aromatic stabilization! On the structure and thermochemistry of [18]annulene Jerome M. Schulman and Raymond L.
The properties of [18]annulene are consistent with its being aromatic. Baldridge Prof. Jay S. Siegel Angew. Polar Aprotic? Are Acids! What Holds The Nucleus Together? Greetings from Brazil! This article was enough to clear my doubts. Thank you so much. The lone pair comes from the C-H bond, which is at right angles to the p-orbital. Nice post, easily understandable, clear and brief. Hi james , Will you please tell me that cyclohex- 1,3diene is aromatic or not???
Look at each of the two rings individually. Both rings are aromatic. Should we calculate it for the individual ring every time in case of the isolated systems?? The specific example you gave of 1,4-dihydronaphthalene was helpful! Article almost delivered everything perfectly. Got it all ;-.
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