It is used to quantify the difference in energy between the π-system of the original molecule and the intermediate having the incoming electrophile or nucleophile attached.
It can be used to study important transformations such as the nitration of conjugated systems from a theoretical perspective.
The change in energy during the reaction can be derived by allowing the orbitals nearby the site i of attack to interact with the incoming molecule.
where β is the Huckel interaction parameter and ar and as are the coefficients of the highest energy molecular orbital at nearby sites r and s respectively.
Thus, by computation of the molecular orbital coefficients it is possible to evaluate Dewar's number for all the sites and establish which one will be the most reactive.
[2] The method is particularly efficient for alternant hydrocarbons in which the coefficients of the non-bonding orbitals involved are very easy to calculate.