LETTERS TO THE EDITORAs is usual in this type of calculation 6 we suppose these differences of the localization energies to be equal to the differences of the activation energies. As the activation entropies are constant to a good approximation,6 we may calculate the percentage of meta isomer in the reaction product from the values of e. and a.k, which in turn are obtained from the data on bond distances and dipole moments from the literature. Some of the results of our calculations are shown in Table 1.The agreement between our results and the experimental values is surprisingly good for the first group of substituents. For the second group all theoretical values are higher than the experimental results, which is ascribed to the hyperconjugation of the -CH3 and -CH2X group with the pentadienate cation, which tends to lower the energy of the transition state for ortho and para substitution. This effect of hyperconjugation was not taken into account in the present calculations. 1 M. J. S. Dewar, The Electronic Theory of Organic Chemistry (The Clarendon Press, Oxford, 1949). 2 We use the same convention of sign as Dewar (I.e.) '

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