Question

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)

Short answer

a)

b)

Step-by-step solution

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.

1. 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

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.

Order of increasing heat of hydrogenation