Читать книгу Organic Mechanisms - Xiaoping Sun - Страница 28

Substituents on aromatic (phenyl) rings can increase or decrease the electron density in the ring by donating or withdrawing electrons. This alters rates and/or equilibrium constants for the reactions occurring in the ring and in the side group attached to the ring. Such electronic effects of substituents can be quantitatively defined on the basis of their influence on the acidity (dissociation constant) of para‐ or meta‐substituted benzoic acid (XC₆H₄COOH) relative to benzoic acid (C₆H₅COOH) (Eq. 1.56):

(1.56)

In Equation 1.56, K_A and K_H are the acid dissociation constants of XC₆H₄COOH and C₆H₅COOH, respectively. lg is the common logarithm (10‐based logarithm). The σ value defined in the equation is called Hammett substituent constant for a given substituent –X at para‐ or meta‐position. The para‐ or meta‐substituted benzoic acids (XC₆H₄COOH) with different –X groups are in general commercially available or easy to synthesize. The pK_A (–lgK_A) value for each XC₆H₄COOH is numerically equal to pH of a solution containing equalmolar concentrations of the acid and the sodium salt of the conjugate base and can be readily determined experimentally. Therefore, the σ constants for various substituents can be obtained readily [1].

The electron‐donating and electron‐withdrawing effects of various substituents originate from combination of inductive and conjugation effects. In general, an electron‐withdrawing group (EWG) on the phenyl ring of XC₆H₄COO⁻, such as –NO₂ (nitro), –C(O)R (acyl), –SO₃H (sulfonic acid), and –CN (cyanide), lowers the electron density in the ring facilitating delocalization of the negative charge of the carboxylate to the ring. This stabilizes the XC₆H₄COO⁻ anion, resulting in an increase in K_A relative to K_H. Therefore, the σ constants for EWGs are usually positive (σ > 0 for EWGs). On the other hand, an electron‐donating group (EDG) on the phenyl ring of XC₆H₄COO⁻, such as –R (alkyl), –OH (hydroxyl), –OR (alkoxy), and –NH₂ (amino), enhances the electron density in the ring to disfavor the delocalization of the negative charge of the carboxylate. This destabilizes the XC₆H₄COO⁻ anion, giving rise to decrease in K_A relative to K_H. Therefore, the σ constants for EDGs are usually negative (σ < 0 for EDGs). The substituents on the ortho‐position of the phenyl rings often produce steric hindrance on the side group next to them. Therefore, the electronic effects of the ortho‐substituents are usually not considered.

For a given substituent (EWG or EDG), the extent of its electronic effects on the side group (–COO⁻) of XC₆H₄COO⁻ is different when the substituent is placed on the para‐position and on the meta‐position. Therefore, the σ constants for the substituent on para‐position (σ_para) and on meta‐position (σ_meta) are different. For example, the electron withdrawing effects of –NO₂ on both its para‐ and meta‐carbons are mainly due to its conjugation effect. The conjugation effect of a para–NO₂ to the side group –COO⁻ is stronger than a meta–NO₂. Therefore, the σ_para (0.81) is greater than the σ_meta (0.71) for –NO₂ (σ > 0 for EWGs) [1]. This is also true for some other EWGs (–CN, –CF₃, and –CO₂Me) whose major electronic effects on both their para‐ and meta‐carbons are conjugation effects, and we have σ_para = 0.70 and σ_meta = 0.62 for –CN; σ_para = 0.53 and σ_meta = 0.46 for –CF₃; and σ_para = 0.44 and σ_meta = 0.35 for –CO₂Me [1]. For EDGs whose major electronic effects are conjugation effects, the absolute value of σ_para is greater than the absolute vale of σ_meta (σ < 0 for EDGs). For example, we have σ_para = –0.14 and σ_meta = –0.06 for –CH₃; and σ_para = –0.32 and σ_meta = –0.10 for –N(CH₃)₂ [1].