Question

16. Which has the smaller heat of hydrogenation, (E)-2-pentene or (Z)-2-pentene? What is the structural origin of this difference? Please give your answer here:

i know with E pentene it would be trans and Z pentene it would be cis. but im confused at to what that has to do with heat of hydrogenation

Answer 1

Answer:

Alkene hydrogenation is the syn-addition of hydrogen to an alkene, saturating the bond. The alkene reacts with hydrogen gas in the presence of a metal catalyst which allows the reaction to occur quickly. The energy released in this process, called the heat of hydrogenation, indicates the relative stability of the double bond in the molecule.

The stability of alkene can be determined by measuring the amount of energy associated with the hydrogenation of the molecule. Since the double bond is breaking in this reaction, the energy released in hydrogenation is proportional to the energy in the double bond of the molecule. Lower energy molecules are more stable than higher energy molecules. More substituted alkenes are more stable than less substituted ones due to hyperconjugation. They have a lower heat of hydrogenation.

In disubstituted alkenes, trans isomers are more stable than cis isomers due to steric hindrance.

Hence, (E)-2-pentene has the smaller heat of hydrogenation. There is less steric strain between the alkyl substituents in the (E)-isomer than in the (Z)-isomer.

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