Annulene and Acyclic Polyenes

Let us turn to [18]annulene, 149, for which there are two sets of measurements. The first set consists of direct enthalpy of combustion and thus of formation96. From the 

On the basis of some judicious measurements and relevant estimations, the enthalpy of formation of gas phase [18]annulene was derived97 to be ca 519 22 kJmol-1. From this we conclude that benzene is ca 15 kJmol-1 more aromatic per carbon than [18]annulene. This value seems too large. 

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- 

---