Page 743 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

726 Cyclobutadiene can also be prepared by photolysis of several different precursors
at very low temperature in solid inert gases. 47 These methods provide cyclobutadiene
CHAPTER 8 in a form that is amenable for spectroscopic study. Analysis of the infrared spectrum of
Aromaticity the product and deuterated analogs generated from labeled precursors have confirmed
the theoretical conclusion that cyclobutadiene is a rectangular molecule. 48

1.567 Å
1.346 Å

A number of alkyl-substituted cyclobutadienes have been prepared by related
methods. 49 Increasing alkyl substitution enhances the stability of the compounds. The

tetra-t-butyl derivative is stable up to at least 150 C, but is very reactive toward
50
oxygen. This reactivity reflects the high energy of the HOMO. The chemical behavior
of the cyclobutadienes is in excellent accord with that expected from the theoretical
picture of the structure of these compounds.
While simple HMO theory assumes a square geometry for cyclobutadiene, most
MO methods predict a rectangular structure as the minimum-energy geometry. 51
The rectangular structure is calculated to be strongly destabilized (antiaromatic) with
respect to a polyene model. 52 With HF/6-31G* calculations, for example, cyclobu-
tadiene is found to have a negative resonance energy of –54.7 kcal/mol, relative to
1,3-butadiene. Furthermore, 30.7 kcal of strain is found, giving a total destabilization
of 85.4 kcal/mol. 53 G2 and MP4/G-31(d,p calculations arrive at an antiaromatic
destabilization energy of about 42 kcal/mol. 54 A homodesmotic reaction incorpo-
rating polyradicals gives a value of 40.3 as the antiaromatic destabilization at the
MP4(SDQ)/6-31G(d,p level. 55 Recently, the technique of photoacoustic calorimetry
provided the first experimental thermodynamic data on cyclobutadiene. The H f
value of 114 ± 11 kcal/mol that was found leads to a total destabilization of
87 kcal/mol, of which 32 kcal/mol is attributed to ring strain and 55 kcal/mol to
antiaromaticity. 56

47
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