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23.5 Basicity and Acidity of Amines

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23.5 Basicity and Acidity of Amines 23_BRCLoudon_pgs5-0.qxd 12/8/08 1:22 PM Page 1122 1122 CHAPTER 23 • THE CHEMISTRY OF AMINES alkylamines, this resonance occurs at rather small chemical shift—typically around d 1. In aromatic amines, this resonance is at greater chemical shift, as in the second of the preceding examples. Like the OH protons of alcohols, phenols, and carboxylic acids, the NH protons of amines under most conditions undergo rapid exchange (Secs. 13.6 and 13.7D). For this reason, split- ting between the amine N H and adjacent C H groups is usually not observed. Thus, in the NMR spectrum of diethylamine,L the N H resonanceL is a singlet rather than the triplet ex- pected from splitting by the adjacent CHL2 protons. In some amine samples, the N H resonance is broadened and, like the OL H protonL of alcohols, it can be obliterated fromL the spectrum by exchange with D2O (the “DL2O shake,” p. 611). The characteristic 13C NMR absorptions of amines are those of the a-carbons—the carbons attached directly to the nitrogen. These absorptions occur in the d 30–50 chemical-shift range. As expected from the relative electronegativities of oxygen and nitrogen, these shifts are somewhat less than the a-carbon shifts of ethers. PROBLEMS 23.4 Identify the compound that has an M 1 ion at mÜz 136 in its CI mass spectrum, an IR 1 + = absorption at 3279 cm_ , and the following NMR spectrum: d 0.91 (1H, s), d 1.07 (3H, t, J 7Hz), d 2.60 (2H, q, J 7Hz), d 3.70 (2H, s), d 7.18 (5H, apparent s). = = 1 23.5 A compound has IR absorptions at 3400–3500 cm_ and the following NMR spectrum: d 2.07 (6H, s), d 2.16 (3H, s), d 3.19 (broad, exchanges with D2O), d 6.63 (2H, s). To which one of the following compounds do these spectra belong? Explain. (1) 2,4-dimethylbenzylamine (2) 2,4,6-trimethylaniline (3) N,N-dimethyl-p-methylaniline (4) 3,5-dimethyl-N-methylaniline (5) 4-ethyl-2,6-dimethylaniline 23.6 Explain how you could distinguish between the two compounds in each of the following sets using only 13C NMR spectroscopy. (a) 2,2-dimethyl-1-propanamine and 2-methyl-2-butanamine (b) trans-1,2-cyclohexanediamine and trans-1,4-cyclohexanediamine 23.5 BASICITY AND ACIDITY OF AMINES A. Basicity of Amines Amines, like ammonia, are strong enough bases that they are completely protonated in dilute acid solutions. 1 | (23.4) H3C NH2 H Cl H3CNH3 Cl_ H2O L ++L L methylamine methylammonium chloride The salts of protonated amines are called ammonium salts. The ammonium salts of simple alkylamines are named as substituted derivatives of the ammonium ion. Other ammonium salts are named by replacing the final e in the name of the amine with the suffix ium. O S | | (CH3)2NH2 Cl_ NH3 _OCPh cL L L dimethylammonium chloride anilinium benzoate (aniline ium) + 23_BRCLoudon_pgs5-0.qxd 12/8/08 1:22 PM Page 1123 23.5 BASICITY AND ACIDITY OF AMINES 1123 TABLE 23.1 Basicities of Some Amines (Each pKa value is for the dissociation of the corresponding conjugate-acid ammonium ion.) Amine pKa Amine pKa Amine pKa CH3NH2 10.62 (CH3)2NH 10.64 (CH3)3N9.76 C2H5NH2 10.63 (C2H5)2NH 10.98 (C2H5)3N10.65 PhCH2NH2 9.34 PhNH2 4.62 PhNHCH3 4.85 PhN(CH3)2 5.06 O2N $ O2N NH2 ≈1.0NH2 2.45 L L L Cl Cl $ ) Cl NH2 3.81NH2 3.32NH2 2.62 L L L L H3C CH3 $ ) H3C NH2 5.07NH2 4.67NH2 4.38 L L L L Always remember that ammonium salts are fully ionic compounds. Although ammonium chloride is often written as NH4Cl, the structure is more properly represented as |NH4 Cl_. Although the N H bonds are covalent, there is no covalent bond between the nitrogen and the chlorine. (A Lcovalent bond would violate the octet rule.) Recall that the basicity of any compound, including an amine, is expressed in terms of the pKa of its conjugate acid (Sec. 3.4D). The higher the pKa of an ammonium ion, the more basic is its conjugate-base amine. (A discussion of the relationship between basicity constants Kb and dissociation constants Ka is given in Study Guide Link 3.5.) B. Substituent Effects on Amine Basicity The pKa values for the conjugate acids of some representative amines are given in Table 23.1. As this table shows, the exact basicity of an amine depends on its structure. Four factors influ- ence the basicity of amines; these are the same effects that influence the acid–base properties of other compounds. They are: 1. the effect of alkyl substitution 2. the polar effect 3. the resonance effect 4. the effect of charge Recall that the pKa of an ammonium ion, like that of any other acid, is directly related to the standard free-energy difference DGa between it and its conjugate base by the following equa- tion (Eq. 3.37, p. 112 ). ° DGa 2.3RT(pKa) (23.5) °= 23_BRCLoudon_pgs5-0.qxd 12/8/08 1:22 PM Page 1124 1124 CHAPTER 23 • THE CHEMISTRY OF AMINES The effect of a substituent group on pKa can be analyzed in terms of how it affects the energy of either an ammonium ion or its conjugate-base amine, as shown in Fig. 23.2. For example, if a substituent stabilizes an amine more than it stabilizes the conjugate-acid ammonium ion (Fig. 23.2a), the standard free energy of the amine is lowered, DGa is decreased, and the pKa of the ammonium ion is reduced; that is, the amine is less basic than° the amine without the substituent. If a substituent stabilizes the ammonium ion more than its conjugate-base amine (Fig. 23.2b), the opposite effect is observed: the pKa is increased, and the amine basicity is also increased. Consider first the effect of alkyl substitution. Most common alkylamines are somewhat more basic than ammonia in aqueous solution: pKa in aqueous solution: NH| 4 MeNH| 3 Me2NH| 2 Me3NH| (23.6) 9.21 10.62 10.64 9.76 However, the increase in basicity that results from substitution of one hydrogen of ammonia by a methyl group is reversed as the number of alkyl substituents is increased to three. How can we explain this “turnaround” in amine basicity? Two opposing factors are actually at work here. The first is the tendency of alkyl groups to stabilize charge through a polarization effect. The electron clouds of the alkyl groups distort so as to create a net attraction between them and the positive charge of the ammonium ion: d+ the electron clouds of the methyl groups CH3 are distorted an attractive d– (stabilizing) interaction d– NH CH3 d– d+ CH3 d+ Because the ammonium ion is stabilized by this effect, its pKa is increased (Fig. 23.2b). This effect is evident in the gas-phase basicities of amines. In the gas phase, the acidity of ammo- nium ions decreases regularly with increasing alkyl substitution: Gas-phase acidity: NH4 >>MeNH3 Me2NH2 > Me3NH (23.7) The same polarization effect also operates in the gas-phase acidity of alcohols (Sec. 8.6C), ex- cept in the opposite direction. In other words, the polarization of alkyl groups can act to stabi- lize either positive or negative charge. In the presence of a positive charge, as in ammonium ions, electrons polarize toward the charge; in the presence of a negative charge, as in alkoxide Further Exploration 23.1 Alkyl Group ions, they polarize away from the charge. We might say that electron clouds are like some Polarization in politicians: they polarize in whatever way is necessary to create the most favorable situation. Ionization Reactions (See Further Exploration 23.1 for a more extensive discussion.) The second factor involved in the effect of alkyl substitution on amine basicity must be a solvent effect, because the basicity order of amines in the gas phase (Eq. 23.7) is different from 23_BRCLoudon_pgs5-0.qxd 12/8/08 1:22 PM Page 1125 23.5 BASICITY AND ACIDITY OF AMINES 1125 reducing the energy of the amine decreases the pKa RNH2 H3O| RNH2 H3O| of the ammonium ion + + (decreases amine basicity) RNH2 H3O| + RNH2 H3O| + ∆G°a 2.3RT (pKa) = ∆G°a 2.3RT (pKa) = lower ∆G°a 2.3RT (pKa) higher = a ∆G°a 2.3RT (pKa) = higher lower RNH| 3 H2O STANDARD FREE ENERGY STANDARD FREE ENERGY STANDARD + RNH| 3 H2O RNH| 3 H2O RNH| 3 H2O + + + reducing the energy of the ammonium ion increases its pKa (increases amine basicity) (a) (b) Figure 23.2 Effect of the relative free energies of ammonium ions and amines on the pKa values of the ammo- nium ions. (a) Reducing the energy of an amine decreases the pKa of its conjugate-acid ammonium ion and thus reduces the basicity of the amine. (b) Reducing the energy of an ammonium ion increases its pKa and thus in- creases the basicity of its conjugate-base amine. that in aqueous solution (Eq. 23.6). In other words, the solvent water must play an important role in the solution basicity of amines. An explanation of this solvent effect is that ammonium ions in solution are stabilized not only by alkyl groups, but also by hydrogen-bond donation to the solvent: H OH2 3 2 H3C"N| H OH2 LL 3 2 "H OH2 3 2 Primary ammonium salts have three hydrogens that can be donated to form hydrogen bonds, but a tertiary ammonium salt has only one.
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