Dewar-Breslow

Applications of Isomeric and Experimentally Realized Dewar-Breslow Reasoning... 

The enthalpy of this reaction is the same, within a numerical constant, as the difference of enthalpies of formation of C2H2XYZ and CH3XYZCH3. Were there a more or less constant enthalpy of formation difference between methyl and phenyl derivatives (an assumption employed in [27] that looked better in kilocalories than in kilojoules), the enthalpy of this reaction would be the same (again to an additive constant) as the difference of the enthalpies of formation of C2H2XYZ and C6H5XYZC6H5, the defining relationship in the experimental Dewar-Breslow approach [4—6]. 

Although we cannot as yet converge on a desired enthalpy of formation of gaseous [18]annulene, it is quite apparent that this last number is suspect in terms of at least two acyclic paradigms for aromaticity. Recall the Dewar-Breslow definition for aromaticity and antiaromaticity of an [njannulene in terms of the corresponding acyclic polyene with n/2 double bonds. There is no experimental measurement of the enthalpy of formation of all-( )-l,3,5,7,9,ll,13,15,17-octadecanonaene, species 151. However, we should be surprised if this value seriously differed from that of nine ethylenes and 8(5) kJmol , the 5 kJ mol being taken as the enthalpy of reaction 19 for unstrained olefins and dienes . The enthalpy of formation of 151 is thus ca 513 kJmol . This is somewhat less than the value for [18]annulene and so we would conclude that the cyclic species is essentially nonaromatic . Alternatively, consider the series of acyclic polyenes, ethylene, 1,3-butadiene, 1,3,5-hexatriene, The gas phase enthalpies of formation are respec-... 

Hosmane, Liebman, and co-workers have elabo-rated ° an experimental model for the quantitative estimation of aromaticity based on the Dewar— Breslow definition and the thermochemical relationship of substituted benzenes and ethylenes (Scheme 5). The model is based on the assumption... 

Table 2. Prediction of Relative Aromaticities Based on the Dewar—Breslow Model ...

While the majority of trapping experiments favors a biradicaloid structure, evidence for a bicyclic intermediate (butalene, 31) was also found. " Breslow et al. " reported the formation of 31 (the possible existence of which had been pointed out by Dewar before ) upon dehydrohalogenation of 3-chloro[2.2.0]bicyclohexadiene... 

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