Nucleophilic attack tetrahedral carbon center

The linear or open arrangement of a three-ci iter-four-electron system H3 bears a close resemblance to the transition state geometry associated with a nucleophilic attack on a tetrahedral carbon center shown in 7.24. Corre ondtngly, the triangular or closed arrangement of a three-center-two-electron system H3 is related to a front-side electrophilic attack as shown in 7.25. The relevant orbitals at the tetrahedral carbon consists of the C—X bonding and antibonding orbitals OcX Sxt... 

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

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