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Addition To A Double Bond Synthlet

As a first approximation, the feasibility of addition – the adding of X–X or X–Y across a C=C, C=N, C=O 'double' or π-bond – can be determined by summing the average bond enthalpies and deducing the net enthalpy change, the ΔrxnH of the reaction.

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+  H2    

Ethene, Ethylene


Bond Enthalpy data, ΔA-BH, in kJmol-1

               C=C   =   614
               H-H   =   432

C-C   =   346
C-H   =   411
C-H   =   411

Calculated Reaction Enthalpy = sum of the left hand side minus the sum of the right hand side:

ΔrxnH   =   (614 + 432) - (346 + 411 + 411) =    -122    kJmol-1

The enthalpy of reaction is strongly negative, greater than -20 kJmol-1. The reaction is exothermic and the equilibrium lies to the right.

This synthlet does not consider the change in entropy, ΔS, and so it only predicts low temperature reactivity. This reasonable because elimination always dominates at high temperature.

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The Mechanism Matrix Pericyclic Chemistry

© Mark R. Leach 1999 –

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