ENTHALPY OF HYDROGENATION OF THE HEXADIENES AND CIS AND TRANS-1,3,5-HEXATRIENE

We present results for the enthalpies of hydrogenation (DELTA-H(h)) and enthalpies of formation of the cyclic unbranched hexadienes and the two hexatrienes by a method that is consistent with our earlier studies on the unbranched hexenes and which gives essentially gas-phase values. The DELTA-H(h) values are as follows: cis-hexa-1,3-diene, -53.9 +/- 0.3; trans-hexa-1,3-diene, -52.9 +/- 0.3; cis-hexa-1,4-diene, -58.4 +/- 0.4; trans-hexa-1,4-diene, -57.6 +/- 0.4; hexa-1,5-diene -60.3 +/- 0.4; cis,cis-hexa-2,4-diene, -52.4 +/- 0.4; cis,trans-hexa-2,4-diene, -51.4 +/- 0.4; trans,-trans-hexa-2,4-diene, -50.5 +/- 0.4; cis-hexa-1,3,5-triene, -81.0 +/- 0.6; trans-hexa-1,3,5-triene, -80.0 +/- 0.6 kcal/mol. Results are compared with three compounds for which literature values exist. A new hydrogenation calorimeter is briefly described. The device yields results as precise as those presently in the literature, but uses samples of 20-100 mg.