(i.e., both ortho/para or both meta), the third substituent depends on the stronger one. When both group have similar directing effect and are para to each other, the third substituent depends on the less hindered one. Rationalization of directing effects While steric effects are a consideration, the major rationalization of electron-donating and electron-withdrawing groups is their perturbation of the electronic distribution of the aromatic ring, mostly via mesomeric effects which extend through the entire conjugated system, to create regions of excessive or deficient π electron density. The consideration of resonance forms is useful in this regard, since they provide a convenient means of determining the locations of these perturbations. More specifically, any formal negative or positive charges in minor resonance contributors (ones in accord with the natural polarization but not necessarily obeying the octet rule) reflect locations having a larger or smaller coefficient, respectively, in the high energy occupied π molecular orbital(s). A carbon atom with a larger coefficient will be preferentially attacked, due to more favorable orbital overlap with the electrophile. The perturbation of a conjugating electron-withdrawing or electron-donating group causes the π electron distribution to resemble that of the electron-deficient benzyl cation or electron-excessive benzyl anion, respectively, although the change in electron distribution occurs to a smaller degree than in these limiting cases. Thus, we can use these simple species, whose π electron distribution can be calculated using simple Hückel theory, as models to rationalize the regiochemical outcome of electrophilic aromatic substitution. As can be seen, the π electron population at the ortho and para positions is depleted for the case of an electron-withdrawing group, causing meta attack to be occur as the least disfavourable option. In contrast, when an electron donating group is present, the ortho and para positions have an increased π electron population compared to the meta position, favoring attack at the ortho and para positions over the meta position. This is precisely the result that the drawing of resonance structures would predict. For example, in nitrobenzene the resonance structures have positive charges around the ring system (see the picture below) : Attack occurs at the meta position, since the ortho and para positions have (partial) formal positive charges that are indicative of π electron deficiency at these positions, leaving the meta positions a slightly higher electron density. On the other hand, in aniline the resonance structures have negative charges around the ring system (see