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.