General Aspects of Substitution on a Benzene Nucleus

Mechanism. As we shall see in later discussions of the individual ionic substitution processes, there is much evidence which indicates that substitution on a benzene ring generally proceeds through the steps- 

Usual electron-seeking attack at the olefinic linkage gives the carbonium ion (I). In this species a proton is eliminated from an adjacent carbon atom (p. 44), and the substituted benzene derivative (II) results. Elimination of a proton occurs, rather than the addition of an anion (Chapter 7), because restoration of the aromatic ring produces a species which is more stabilized by resonance than the dihydrobenzene derivative (III)  

When the resonance stabilization which can be gained by elimination of a proton is small, however, subsequent addition of an anion occasionally does occur. Phenanthrene, for example, can be brominated in the dark to give either 9,10-dibromophenanthrene 1 or 9-bromophenanthrene (see p. 247). 

The Effect of Various Groups on the Electron Distribution of an Aromatic Ring at the Moment of Reaction. Since attack on a benzene ring in substitution reactions is led by an electron-seeking species, it would be expected to occur at a point where the electron density is the highest. Consequently, electron-releasing groupsjiU ej ben- 

It is well known that substituents can be broadly classed as ortho-para or meta directing. The explanation of this fact becomes clear if we observe that any structure written with a charge in the ortho position is actually a resonance hybrid of two other almost equivalent resonance forms. One of these has a charge in the para position, and the other has a charge in the other ortho position. Thus when the substituent is electron repelling and cannot enter into resonance with the benzene ring, we have the extreme structures  

---