Chapter 11 : Arenes and Aromaticity

What about Other Neutral Hydrocarbons ?

The following table contains two hydrcarbons 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 cyclobutadiene and cyclooctatetraene.

Experimental evidence such as reactivity, spectroscopic data and thermodynamic measurements suggest that cyclooctatetraene is not aromatic but is more like a conjugated polyene.  Evidence suggests cyclooctatetraene has CC bonds of different lengths (i.e. alternating C=C and C-C) and is non-planar (rotate the CHIME image below to see this). In terms of the aromaticity criteria described earlier , cyclooctatetraene is not aromatic since it fails to satisfy the 4n + 2 p electron Huckel rule (i.e. it doesn't have an odd number of  p electron pairs). Infact, it is an example of 4n p electron systems (i.e. an even number of  p electron pairs). However, since cyclooctatetraene violates one of the first three aromaticity criteria, it is best described as non-aromatic.
 

Cyclooctatetraene

Resonance energy  20 kJ/mol 5 kcal/mol

The fact that cyclobutadiene is very difficult to prepare due to its very high reactivity tells us that unlike benzene, cyclobutadiene is not a particularly stable compound. 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 , cyclobutadiene is not aromatic since it fails to satisfy the 4n + 2 p electron Huckel rule (i.e. it doesn't have an odd number of p electron pairs). Infact, it is an example of 4n p electron systems (i.e. an even number of  p 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.
 

Cyclobutadiene