Answer:
1,4-dihydro = 113 kJ·mol⁻¹
1,2-dihydro = 101 kJ·mol⁻¹
Explanation:
In 1,4-dihydronaphthalene, the 2,3-double bond is isolated from the benzene ring.
In 1,2-dihydronaphthalene, the 3,4-double bond is conjugated with the benzene ring.
Thus, 1,2-dihydronaphthalene is partially stabilized by resonance interactions between the ring and the double bond (think, styrene).
1,2-Dihydronaphthalene is at a lower energy level because of this stabilization.
The heat of hydrogenation of 1,2-dihydronaphthalene is therefore less than that of the 1,4-isomer when each is hydrogenated to the common product, 1,2,3,4-tetrahydronaphthalene.
