Regiochemistry and Relative Rates of Aromatic Substitution

Substitution on benzene raises no regiochemical issues because every hydrogen is equivalent, but as soon as there is one substituent on the ring, isomeric products can result— namely, ortho, meta, and para. The reactivities of the different sites on substituted aromatic rings are quantified by what are known as partial rate factors (where n = o, m, or p for ortho, meta, or para, respectively, and R = substituent). These numbers reflect the rate constants (k ) for reaction of the individual ortho, meta, or para sites with an electrophile compared to the rate constant (k) for addition to benzene itself (Eqs. 10.113 A, B, and C). The rate constants for ortho and meta are divided by two because there are two ortho and meta hydrogens, and the rate constant for benzene is divided by six due to the six hydrogens. 

Aromatic substituents are broken into two categories regarding electrophilic aromatic substitution reactions activating or deactivating, and ortho/para directing or meta directing. When/n 1, we say that substituent R is activating, and when 1, we classify that 

Resonance structures resulting from electrophilic addition to a substituted benzene ring. A. Ortho, B. Meta, and C. Para. 

Now imagine X = CN, an electron withdrawing group. Once again the a complex derived from ortho and para addition of the electrophile has a resonance structure in which the 

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