What is the correct order of exothermicity for hydrogenation of the following butenes upon treatment with H₂/Pd (more exothermic less exothermic)?

Question

What is the correct order of exothermicity for hydrogenation of the following butenes upon treatment with H₂/Pd (more exothermic > less exothermic)?

Quizplus · Accepted Answer

The exothermicity of hydrogenation depends on the stability of the butene isomers formed upon hydrogenation. The more stable the isomer, the less exothermic the reaction will be.

1-butene will form n-butane upon hydrogenation, which is a fully saturated, linear hydrocarbon. It is the most stable out of the three possible isomers and therefore, the least exothermic.

2-butene will form a branched isomer, 2-methylpropane, upon hydrogenation. This isomer is less stable than n-butane, but more stable than the remaining isomer, 2-butene. Therefore, the hydrogenation of 2-butene will be more exothermic than 1-butene, but less exothermic than 3-butene.

3-butene will form a cis-2-butene isomer upon hydrogenation, which is the least stable out of the three isomers. Therefore, the hydrogenation of 3-butene will be the most exothermic out of the three butenes.

Putting it all together, the correct order is: 1 > 2 > 3.

What Is the Correct Order of Exothermicity for Hydrogenation of the Following