Question

why cyclohexene and benzene cannot be same

Answer 1

The pi bonds in benzene are each more stable than the pi bond in cyclohexane due to the aromatic stabilisation energy. The reason heats of formation are more exothermic for cyclohexane is because you’re forming more CH bonds. On an absolute scale, heat of hydrogenation of benzene is still exothermic, so partially hydrogenating it, say, to cyclohexane, leads to a more stable product. The picture below shows all energies relative to benzene and in kJ/mol. As you can see, the loss of aromatic stabilisation energy on hydrogenation to 1,3-cyclohexadiene actually makes 1,3-cyclohexadiene less thermodynamically stable than benzene, but subsequent hydrogenation is favourable, because you’re breaking a C-C pi bond and a H-H bond, but you’re forming two C-H bonds, which are more stable overall.