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What about Other Neutral Hydrocarbons ?

The following table contains two hydrocarbons that have been notably absent so far in our discussions even though both are cyclic and have alternating C=C and C-C, but they contain either one less or one more C=C than benzene. These compounds are 1,3-cyclobutadiene and 1,3,5,7-cyclooctatetraene.

Experimental evidence such as reactivity, spectroscopic data and thermodynamic measurements suggest that 1,3,5,7-cyclooctatetraene is not aromatic but is more like a conjugated polyene

The experimental evidence tells us:

In terms of the aromaticity criteria described earlier , cyclooctatetraene is not aromatic since it fails to satisfy the 4n + 2 π electron Huckel rule (i.e. it doesn't have an odd number of  π electron pairs). It is actually an example of a 4n π electron system (i.e. an even number of π electron pairs). So, if cyclooctatetrene were planar it would be anti-aromatic, a destabilising situation. Molecules tend to try to adopt more stable shapes: think of cyclohexane being in the chair conformation rather than planar or the anti conformation of alkyl chains. Cyclooctatetraene prefers to adopt a more stable non-planar conformation.

Since cyclooctatetraene violates one of the first three aromaticity criteria (it's not planar), it is best described as non-aromatic.

8 pi electron system so 4n system
Cyclooctatetraene

 Resonance energy 
20 kJ/mol
5 kcal/mol

CC bonds 1.35 and 1.5 A

H NMR δH = 5.7 ppm

The fact that 1,3-cyclobutadiene is very difficult to prepare due to its very high reactivity tells us that unlike benzene, 1,3-cyclobutadiene is not a particularly stable compound. The evidence suggests cyclobutadiene has CC bonds of different lengths (i.e. alternating C=C and C-C).  In terms of the aromaticity criteria described earlier , 1,3-cyclobutadiene is not aromatic since it fails to satisfy the 4n + 2 π electron Huckel rule (i.e. it doesn't have an odd number of π electron pairs). Infact, it is an example of a 4n π electron system (i.e. an even number of  π electron pairs). However, since cyclobutadiene violates the final aromaticity criteria, it is described as anti-aromatic.  This term is used to reflect its lack of stability and high reactivity.

4 pi electron system, a 4n system
Cyclobutadiene
 

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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary