Electronic effects in certain chemical reactions

Abstract

It has been shown that certain relationships exist between ionization constants. In the light of these relationships the influence of inductive effects upon velocity constants of chemical reactions has been considered. Regularities have been found for reactions involving the sidc chain of phenyl compounds with mem or p a n subsritucnts and indications are given of the SRUCNre of complexes formed as intermediates. The study of inductive effects in a series of biologically active compounds may give some indication of the mode of action : for example, the toxic action of certain phenols towards some micro-organisms seems to depend upon the formation of hydrogen bonds.Branch and Calvin' considered that the effect upon the free energy (AGO = -RT log,K) for the dissociation of a proton from hydroxylic acids in dilute aqueous solution at 25" c., of a formal charge on the atom to which the hydroxyl group was attached was f 16.8 kg.ca1. and that this effect was reduced by the fraction 1/2.8 every timc a saturated atom was placed between the charged atom and the oxygen atom ot the hydroxyl group. McGowan? has pointed out that the value 14 kg.-cal. and the fraction I/;? give better results than the figures proposed by Branch and Calvin. Values of AGO are listed below for some pairs of related hydroxylic acids, one having a charge which the other has not. It will be seen that the differences between the AGO values for thc pairs come close to 14, 7, 3.5, 1-75, and 0.875 kg.-cal. Other examples have been given elsewhere,?. and this treatment is not restricted to hydroxylic acids. The last three pairs are interesting for all three acids H,C(CHJ,,COOH, where N equals I, 2, and 3, have roughly the same AGO value of 6.7 and the AGO values of the other set of acids H,N(CHJ,,COOH are responsible for the differences.In this second set for N equals I, AGO = + 3.2 ; and for N equals 2 and 3 AGO= 3.2 + 1.75 and-3.2 + 1-75 + 1.75/2 respectively. When N equals 4,AG0 would be expected to be 3.2 + 1-75 + 1-75/2 3.1.7514 but this value has not been reported in the literature. The value of AGO for n equals % . can be found by the usual formula for the sum of a geometrical progression and equals 3.2 + I.75 i.e., 6.7 so that with the NH, group at infinite distance from the -OH, the value of AGO does become as one would expect, equal to the acids with a simple straight hydrocarbon chain.Certain atoms and radicals consisting of a group of atoms have a considerable effect upon a neighbowing dissociatingproton. These effects can be conveniently described by the allocation to the atom or radical of an I' effective charge " Q which may be defined as the charge required to produce an effect equivalent to the atom or radicaL2 From the effective on the structure separating Y from the dissociating proton. When however a group is involved in an electromeric shift in one series of acids the value of Q derived from another series of acids in which there is no electromeric shift cannot necessarily be used in the Hammed found that the effect of a substituent ...