The Effect of Extra-index substituents

The part of the multiplet theory considering the effect of substituents not in contact with the catalyst resembles the theory of the color 416). 

The index group corresponds to the chromophoric one. The position of the maximum of the volcano-shaped curve along the axis q corresponds to the position of the center of the absorption band in the spectrum while the effect of the index substituents on E corresponds to the shift of the absoprtion band under the influence of the auxo-chromic groups. Similar views were also expressed by Langenbeck 417). 

On account of the change in electron density the substituent, e.g., at A, affects the bond energy Qab and Qak, hence E too. For the quantitative calculation let us again take the variation in relation to A of Eq. (II.9). However, now we shall denote by Ai and A2 not different atoms, as above, but one and the same atom Ai with different extra- 

Formula (11.58) contains a result of fundamental importance in catalytic reactions, unlike noncatalytic ones, the effect of substituents on the reaction rate and on E should be taken into account twice, viz., the effect on Qab and on Qak, the two effects being opposed. This explains, in particular, the results, which seemed contradictory, that in the homologous series, in some instances, the rate diminishes (this happens when SQab 8Qak), in other instances it increases (when Qab Qak), and sometimes it does not change (when Qab = Qak). The last case is realized in the dehydrogenation of primary alcohols where e for normal primary alcohols from C2 to C9 has proved to be constant (on an oxide catalyst). In alcohol dehydration this compensation of Qab and Qak does not occur and a-, jS-, and y-substitution each introduces its increment into e. 

The hydrogenation rate of olefins on Pt falls as the extent of the hydrogen substitution at the double bond by alkyl groups increases [Lebedev s rule (418)-, as the number of phenyl radicals substituting the alkyl radicals at the double bond in olefins increases the rate of hydrogenation on Pt decreases 419)]. 

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