Q 1P Rank each group of compounds in ... [FREE SOLUTION]

Chapter 15: Q 1P (page 754)

Rank each group of compounds in order of increasing heat of hydrogenation.
(a) Hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene.
(b)

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Heat of hydrogenation of alkene

The heat of hydrogenation is the change of enthalpy when one mole of a compound gets wholly hydrogenated.In alkenes, the heat of hydrogenation is directly proportional to the number of C=C double bonds and is inversely proportional to the stability.

Order of increasing heat of hydrogenation

The heat of hydrogenation is greater for the compounds having a greater number of C=C double bonds. More is the conjugation, smaller is the heat of hydrogenation. More substituted is the alkene, smaller is the heat of hydrogenation.

In the given compounds, hexa-1,3,5-triene has the highest hydrogenation heat because it has a greater number of C=C bonds, is less conjugated, and is less stable.

Order of increasing heat of hydrogenation

b. In the given compounds, the triene compound has the highest heat of hydrogenation because it has a greater number of C=C bonds, is less conjugated, and is less stable.