Question

Compare substituion vs addition in aromatic compounds through the thermochemical changes for bromine substitution and addition with benzene as substrate. Threat the bonds in benzene as ethylenic bonds, a reasonable approximation, and use the following data:
`B.E.(C-H)=414.2kJmol^(-1), B.E.(Br-Br)=188.3kJmol^(-1)`
`B.E.(C-C)=347.3 kJ mol^(-1), B.E.(H-Br)=364.0kJmol^(-1)`
`B.E.(C=C)=606.7 kJ mol^(-1),B.E.(C=Br)=284.5 kJ mol^(-1)`
Strategy: Energy is required to break bonds while energy is released during the formation of bonds.

Answer 1

Sustitution is therefore, theromodynamically favorable, whereas addition is thermodynamically unfavorable. In substitution there is no loss of resonance energy but there is a loss of 136 kJ in addition. It is this loss that is the deciding factor.