Problems and exercisesBenzene's Puzzle

1. Heats of formation

Calculate the heats of formation of cyclohexene, 1,4-cyclohexadiene, 1,3-cyclohexadiene and benzene from their heats of combustion: −896.9, −853.3, −854.6 and −780.8 kcal/mol, respectively (with liquid water as product). What conclusions can you draw?

Click here to recall how it is calculated.

2. Kekulé isomers

If benzene were a cyclohexatriene rapidly interconverting between its two Kekulé structures, how many isomers of 1,2-dimethylbenzene should be isolable? How many are known?

3. Hydrogenation

The hydrogenation of cyclohexene releases 28.6 kcal/mol and that of benzene 49.3. Calculate the resonance energy of benzene.