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.

Explanation

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.