Stabilities of Dienes

Chapter 15 Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy 

When a molecule has two isolated double bonds, the heat of hydrogenation is close to the sum of the heats of hydrogenation for the individual double bonds. For example, the heat of hydrogenation of 1,4-pentadiene is —252 kJ/mol (-60.2 kcal/mol), about twice that of 1-pentene. 

What happens if two double bonds are even closer together than in the conjugated case Successive double bonds with no intervening single bonds are called cumulated double bonds. Consider 1,2-pentadiene, which contains cumulated double bonds. Such 1,2-diene systems are also called allenes, after the simplest member of the class, 1,2-propadiene or allene, H2C = C=CH2- The heat of hydrogenation of 1,2-pentadiene is —292 kJ/mol (—69.8 kcal/mol). 

Rank each group of compounds in order of i ncreasing heat of hydrogenatioa (a) 1,2-hexadiene 1,3,5-hexatriene 1,3-hexadiene 1,4-hexadiene 1,5-hexadiene 2,4-hexadiene 

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