Ch_322a_7.03

Ch_322a_7.03 - Relative Stabilities of Alkenes When two or...

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Relative Stabilities of Alkenes When two or more alkenes produce the same alkane product, their heats of hydrogenation provide a measure of their relative stabilities . The heat of hydrogenation, ! H o Hyd standard enthalpy change in the hydrogenation of an alkene: C=C + H 2 C-C H H ! H o ~ -30 kcal/mol Pt , is the
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Example: The Butenes CH 3 CH 2 CH=CH 2 + H 2 Pt CH 3 CH 2 CH 2 CH 3 ! H o (kcal/mol) 1-butene C=C + H 2 CH 3 CH 3 H H cis -2-butene Pt CH 3 CH 2 CH 2 CH 3 -30.3 -28.6 C=C CH 3 CH 3 H H trans -2-butene + H 2 Pt CH 3 CH 2 CH 2 CH 3 -27.6 Since the same product state is produced in each hydrogenation, the differences in the the heats of hydrogenation must reflect differences in the energies of the starting states. More specifically, they directly measure differences in the stabilities of the butene isomers .
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Heats of Hydrogenation of the Butenes A Measure of Relative Stability of the Isomers The state diagram below graphically shows the enthapy changes associated with the hydrogenation reactions of the butene isomers. Since there is a
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Ch_322a_7.03 - Relative Stabilities of Alkenes When two or...

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