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13 - Heat evolved upon catalytic hydrogenation(DHo A...

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ENERGY (-) 27.4 kcal/mol (-) 28.3 kcal/mol (-) 30.3 kcal/mol Heat evolved upon catalytic hydrogenation ( D H o ) A MEASURE OF ALKENE STABILITY ALKENE ALKANE H 2 /Pd-C
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Catalytic hydrogenation of benzene and various cyclohexenes 3 x -28.6 kcal/mol 2 x -28.6 kcal/mol Magnitude of aromatic stabilization = 36 kcal/mol Isolated alkene Isolated diene Conjugated diene Benzene
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Benzene (C 6 H 6 ) is not “cyclohexatriene! Each sp 2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring 1879 Landenberg
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Br Br No reaction Br 2 Br 2 FeBr 3 /Br 2 Br Bonds broken Bonds made p bond Br-Br bond 2 C-Br If reaction occurred p bond Br-Br bond BUT would also loose AROMATIC STABILIZATION INSTEAD - SUBSTITUTION ADDITION EFFECT OF AROMATIC STABILIZATION ON REACTIONS OF BENZENE (and other aromatic compounds)
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Annulenes Initially, all cyclic conjugated hydrocarbons were proposed to be aromatic. However, cyclobutadiene is so reactive that it dimerizes before it can be isolated.
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