Untersuchungen in der Benztropylium-Reihe. IV. Die Bildungsenthalpien des 2,7-Dimethyl-4,5-benztropons, des 2,7-Dodecamethylen-4,5-benztropons und des 2,7-Pentamethylen-4,5-benztropons

The enthalpies of formation of 2,7-dimethyl-4,5-benzotropone (I), 2,7 - pentamethylene- 4,5 - benzotropone (III) and 2,7 - dodeca-methylene- 4,5-benzotropone (II) have been determined by combustion calorimetric methods. The loss of 36 kcal/moles in delocalisation energy for III relative to I and II is due to the steric inhibition of coplanarity of the 4,5-benzo-tropone-system in III.