1- Butene heat of hydrogenation

Butylenes can be catalyticaUy hydrogenated in the presence of Pt, Pd, or Ni in an exothermic reaction. In the absence of a catalyst, this reaction proceeds at a negligible rate, even at elevated temperatures. Heats of hydrogenation in kj /mol are as follows 1-butene, —126.8 isobutylene, —118.8 <7j -2-butene, —119.7 and trans-2-huX.en.e —115.5. 

The differences between the heat of dissociation and the bond energy for the C—H bond in propylene and toluene and for the C—C bond in butene-1 and diphenylethane are interesting. The cause lies here not so much in the bonds of the molecules in question, as can be seen from the total binding energy deduced from the heat of combustion (or heat of hydrogenation), but in the peculiar stability of the ally] and toluyl radicals formed (p. 237). 

Manufacture. Butenediol is manufactured by partial hydrogenation of butynediol. Although suitable conditions can lead to either cis or trails isomers (111), die commercial product contains almost exclusively cis-2-butene- 1,4-diol. Trans isomer, available at one tune by hydrolysis of l,4-dichloro-2-butene, is unsuitable for die major uses of butenediol involving Diels-Alder reactions. The liquid-phase heat of hydrogenation of butynediol to butenediol is 156 kj/mol (37.28 kcal/mol) (112). 

Heats of hydrogenation can often give us valuable information about the relative stabilities of unsaturated compounds. For example, of the isomeric 2-butenes, the cis-isomer has a heat of hydrogenation of 28.6 kcal, the trans-homer one of 27.6 kcal. Both reactions consume one mole of hydrogen and yield the same product, /i-butane. Therefore, if the trans-homer evolves 1 kcal less... 

The hydrogenation of 1,3-butadiene to 1-butene cannot be carried out quantitatively but we know the total heats of hydrogenation at 355 K of both components to give -butane as the product (Conant and Kistiakowsky, 1937). 

Evidence for the extra stability of conjugated dienes comes from mea surements of heats of hydrogenation (Table 14.1). We saw earlier inthedit cussion of alkene stabilities (Section 6.7) that alkenes of similar substitution pattern have remarkably similar AHgydrog values. Monosubstituted alkenes such as 1-butene have values for near -126 kJ/mol... 

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