EQUIVALENT (DBE) OR INDEX OF HYDROGEN DEFICIENCY (IHD)
1.6 RESONANCE AND RESONANCE EFFECT OR MESOMERIC EFFECT
1.6.6 Effect of Resonance on the Properties of Molecules
1.6.6.2 Effect of ring substituents on the acid strength of phenols
The acidity of phenols depends on whether a ring substituent is electron-attracting or electron-releasing and also its position with respect to the —OH group.
An electron-attracting substituents like —X, —NO2, —CHO, —CN, etc., withdraws electrons from the ring and thereby stabilizes the phenoxide ion more than the phenol.
Thus, the energy difference between the phenol and its conjugate base decreases and as a result, the energy required for ionization decreases. Consequently, the ionization equilibrium becomes favoured. Therefore, an electron-attracting substituent increases the acidity of phenol. An electron-releasing substituent, like —CH3, —OCH3, —NH2, etc., donates electron to the ring and thereby destabilizes the phenoxide ion relative to the phenol by intensifying the negative charge on it. For this reason, the difference in stability between the phenol and its conjugate base, i.e., the phenoxide ion increases and consequently, the strength of the phenol as an acid decreases. Therefore, an electron- releasing substituent decreases the acidity of phenol.
A substituent can exert its –R or +R effect only when it is at ortho or para position. Therefore, the acid-strengthening or acid-weakening effect of a substituent is very much pronounced when it is present at otho or para position but not at the meta position. The acidic strength of nitro substituted phenols, for example, increases in the following order: phenol <
m-nitrophenol < o-nitrophenol < p-nitrophenol. A nitro (—NO2) group present in ortho- or para–position is capable of withdrawing electrons from the negatively charged oxygen atom of the phenoxide ion by its –I and –R effects. It, therefore, stabilizes the conjugate base by dispersing the negative charge. In fact, there is a relatively stable structure (P) in which the negative charge is placed on the highly electronegative oxygen atom of the
—NO2 group and its contribution makes the hybrid stable. A similar resonance structure
(Q) makes the o-nitrophenoxide ion considerably stable. Hence, o- and p-nitrophenoxide ions are relatively more stable than phenoxide ion. In fact, the relative stabilization of the phenoxide ion with respect to the undissociated phenol is more effective with these nitrophenols compared to phenol and for this reason, o- and p-nitrophenols are more acidic than phenol.
The m-nitrophenoxide ion is stabilized only by the –I effect of the –NO2 group and this is because the nitro group is unable to delocalize the negative charge on oxygen due to lack of proper conjugation. For this reason, m-nitrophenol is a stronger acid than phenol, but weaker acid as compared to its other isomers.
In the o-isomer, there occurs intermolecular hydrogen bonding (chelation) which, in fact, disfavours proton release to some extent. For this reason, the o-isomer is less acidic than the p-isomer, even though the —I effect of the relatively close —NO2 group is much stronger than the former.
1. Acidic character of active methylene compounds An active methyl compound contains a
—CH2— or —CHR— group fl anked by two strong electron-attracting (—I and —R) groups.
Acetylacetone (CH3COCH2COCH3), diethyl malonate (EtO2C – CH2—CO2Et), ethyl acetoacetate (CH3COCH2CO2Et), ethyl cyanoacetate (EtO2C — CH2 — CN), dinitromethane
(O2N — CH2 — NO2), etc. are the most important active methylene compounds. The acidic methylene hydrogens of these compounds can be easily abstracted by base because the corresponding conjugate bases are well stabilized by resonance. However, their acidity differs due to the presence of different electron-withdrawing groups. For example, acetylacetone is a stronger acid than ethyl acetoacetate. A resonance argument can be used to explain the difference in acidity between these two active methylene compounds.
Acetylacetone and ethyl acetoacetate ionize in the presence of a base (:B) as follows:
Charge delocalization occurs in both the conjugate bases and also the same number of resonance structures can be written for both of them. The resonance structures of acetylacetonate anion are all signifi cant contributors. On the other hand, two of the three resonance structures of acetoacetate anion (the fi rst and second) contribute signifi cantly to the hybrid. The contribution of the last structure to the hybrid is, in fact, very small because delocalization of electrons within the ester group itself lowers the ability of the C == O group to disperse the negative charge.
Because of this, delocalization of charge is more effective in acetylacetonate anion than in ethyl acetoacetate anion, i.e., the former conjugate base is more stabilized by resonance than the latter conjugate base. The difference is stability between the neutral molecule and the conjugate base is, therefore, greater in the case of ethyl acetoacetate (CH3COCH2CO2Et) than in the case of acetylacetone (CH3COCH2COCH3) and hence the equilibrium involved in the case of aceylacetone is relatively more favourable than that involved in the case of ethyl acetoacetate. That is, acetylacetone is more acidic than ethyl acetoacetate.
(2) Acidic character of imides Imides are often suffi ciently acidic to form alkali metal salts. The acidity of an imide, e.g., phthalimide, is due to —I and —R effects of the two C == O groups. The unshared pair of electrons on nitrogen is suffi ciently delocalized and as a consequence, it becomes considerably positively polarized. Because of this, the polarity of the N—H bond increases and hence the tendency of N—H bond fi ssion to release proton increases. For this, imides exhibit acidic character. The acidity of phthalimide can also be explained by considering phthalimide-phthalimide anion equilibria. Both phthalimide and its conjugate base can be represented as a hybrid of three resonance structures such as follows:
In phthalimde, the unshared pair of electrons on nitrogen is delocalized by the two adjacent C == O groups. However, this resonance is less effective and less stabilizing because it involves charge separation. On the other hand, resonance is more effective and more stabilizing in phthalimide anion because there occurs no charge separation. Resonance thus lowers the energy difference between the imide and its conjugate base. Because of this, the ionization becomes much favourable, i.e., the compound exhibits considerable acidity and dissolves readily in alkali metal hydroxide (e.g., NaOH) solutionto form salts.
(3) Acidic Character of Nitroform [CH(NO2)3], Chloroform (CHCl3) and Fluoroform (CHF3) Nitroform is more acidic than chloroform which in turn is more acidic than fl uroform. These observations can be well explained by resonance. Any thing that stabilizes a conjugate base A①: makes the starting acid H—A more acidic. The conjugate base of nitroform, i.e., C(NO ) , @ 2 3 is stabilized by —I and —R effect (p-p p bonding) of the three —NO2 groups.
The conjugate base of chloroform, i.e., CCl , @ 3 on the other hand, is stabilized by —I effect and d-orbital resonance (p-d p bonding) involving three Cl atoms. Because of longer bond length and the difference in size between the 2p and 3d orbitals, the p-d p bonding is far less signifi cant than p-p p bonding. Also, a —NO2 group exerts stronger —I effect than a Cl atom. For these reasons, C(NO ) @ 2 3 is very much stable than CCl @ 3 and therefore, nitroform is a much stronger acid than chloroform.
Fluorine has no d orbital. So, unlike CCl @ 3 there is no question of resonance stabilization of CF . @ 3 It is only stabilized by the —I effect of fl uorine. Thus, CCl @ 3 is relatively more stable than CF @ 3 and for this reason, chloroform (CHCl3) is more acidic than fl uoroform, even though fl uorine is more electronegative than chlorine.