A qualitative scale for the electron withdrawing effect of substituted phenyl groups and heterocycles

Abstract

The electron withdrawing effect of a variety of differently substituted phenyl groups can be classified on the basis of the C O or C N distance of the corresponding phenolate or aryl amide ions (Ar–O − and Ar–NH −), respectively, which are reliably accessible by DFT calculations on B3LYP/6-311 + G(2d,p) level of theory. An increasing electron withdrawing effect of the aromatic group leads to a shortened C O and C N distance of the corresponding ions. Within the presented model it is also possible to characterize the electronic nature of different kinds of six membered heterocycles. The defined constants Δ( E) m,p – the difference of the E–C distances of the substituted derivatives X-C 6H 4-E and the non-substituted phenyl derivative, C 6H 5-E – exhibit the same tendency as the corresponding Hammett constants. The values of Δ( E) m,p strongly depend on the nature of E. With E = F, the resulting values Δ( F) m,p are found to be accidentally close to the corresponding Hammett constants. Therefore, the values of Δ( E) m,p , which can be easily determined by DFT calculations, are useful tools to classify the electronic nature of different kinds of substituents. The different electronic nature of the groups E, for example O −, NH − or F, gives rise to varying electronic interactions with the connected aromatic ring system. These interactions influence on their part the special interaction of a given substituent X. This is the reason why the constants Δ( E) m,p exhibits different values, which vary depending on the different electronic nature of the groups E. As a consequence, the values Δ( E) m,p open the possibility to classify the electronic nature of substituents X in relation to any neutral or even charged group E.