3. The conversion of an alkene into an alkane via catalytic hydrogenation (with

Question

3. The conversion of an alkene into an alkane via catalytic hydrogenation (with H2 and Pd-C) is always exothermic. The amount of energy released (known as the heat of hydrogenaton) depends on the stability of the double bond undergoing hydrogenation, the more stable the double bond, the lower (less exothermic) the heat of hydrogenation. Alkene Type Stability (1-7) Heat of Hydrogenation ethene (CH2 CH2) least stable-7 -32.8 kcalmol 30.1 kcalmol 28.6 kcaVmol 28.0 kcalmol 27.6 kcaU/mol -26.9 kca/mol 26.6 kcaUmol geminal disubstituted tetrasubstituted most stable 1 This datai for isolated akenes Conjugated akenes are lower energy (more stable) and release less energy upon hydrogenabon Select three pairs of compounds with approximately equal heats of hydrogenation. Write the letters for each pair on the lines below. (1.5 pts)

Explanation / Answer

1. pair a and h [ both contains 2 trisubstituted alkene, heat of hydrogenation = -53.8 kcal/mol

2. pair d and e [ both contains 1 cis disubstitution and 1 trisubstitution, heat of hydogenation is less than -55.5kcal/mol due to conjugation]

3 pair f and i [ both contains tetra substituted alkene]

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