Question

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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 hydrogenation) 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 (CH,-CH2) I least stable 7 monosubstituted cis disubstituted geminal disubstituted trans disubstituted trisubstituted -32.8 kcal/mol -30.1 kcaVmol -28.6 kcal/mol -28.0 kcal/mol 27.6 kcal/mol -26.9 kcal/mol 26.6 kcal/mol 2 tetrasubstitutedmost stable 1 This data is for isolated alkenes. Conjugated alkenes are lower energy (more stable) and release less energy upon hydrogenation. Select three pairs of compounds with approximately equal heats of hydrogenation. Write the letters for each pair on the lines below. (1.5 pts)

Answer 1

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