22.3 Physical Properties an extended T system than is an sp-hybridized orbital. As a result of these two oppos- ing forces, nitrogen adopts an orbital hybridization that is between sp' and sp The corresponding resonance description shows the delocalization of the nitrogen lone-pair electrons in terms of contributions from dipolar structures NH2 NH2 NH2 JHa H < H H Lewis structure The orbital and resonance models for bonding in arylamines are simply alternative ways of describing the same phenomenon. Delocalization of the nitrogen lone pair decreases the electron density at nitrogen while increasing it in the Tr system of the aro- matic ring. Weve already seen one chemical consequence of this in the high level of reactivity of aniline in electrophilic aromatic substitution reactions( Section 12. 12). Other ways in which electron delocalization affects the properties of arylamines are described in later sections of this chapter PROBLEM 22. 4 As the extent of electron delocalization into the ring increases the geometry at nitrogen flattens p-Nitroaniline, for example, is planar write a resonance form for p-nitroaniline that shows how the nitro group increases elec- tron delocalization. Examine the electrostatic potential of the p-nitroaniline model on Learning By Modeling. Where is the greatest concentration of negative charge 22.3 PHYSICAL PROPERTIES We have often seen that the polar nature of a substance can affect physical properties such as boiling point. This is true for amines, which are more polar than alkanes but less polar than alcohols. For similarly constituted compounds, alkylamines have boiling points higher than those of alkanes but lower than those of alcohols ollection of physical prop- CH3CH,CH3 CH3 CH2NH, CH3 CH,OH Propane Ethylamine Ethanol OD u=1.7D ith unpleasant,“ fishy bp-42°C bp 17C bp78°C Dipole-dipole interactions, especially hydrogen bonding, are present in amines but absent in alkanes. The less polar nature of amines as compared with alcohols, however, makes these intermolecular forces weaker in amines than in alcohols Among isomeric amines, primary amines have the highest boiling points, and ter- tiary amines the lowest CH3 CH,CH,NH CH=,NHCH3 (CH3)3N (a primary amin a tertiary amine) bps0°c bp3° Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
an extended system than is an sp3 -hybridized orbital. As a result of these two opposing forces, nitrogen adopts an orbital hybridization that is between sp3 and sp2 . The corresponding resonance description shows the delocalization of the nitrogen lone-pair electrons in terms of contributions from dipolar structures. The orbital and resonance models for bonding in arylamines are simply alternative ways of describing the same phenomenon. Delocalization of the nitrogen lone pair decreases the electron density at nitrogen while increasing it in the system of the aromatic ring. We’ve already seen one chemical consequence of this in the high level of reactivity of aniline in electrophilic aromatic substitution reactions (Section 12.12). Other ways in which electron delocalization affects the properties of arylamines are described in later sections of this chapter. PROBLEM 22.4 As the extent of electron delocalization into the ring increases, the geometry at nitrogen flattens. p-Nitroaniline, for example, is planar. Write a resonance form for p-nitroaniline that shows how the nitro group increases electron delocalization. Examine the electrostatic potential of the p-nitroaniline model on Learning By Modeling. Where is the greatest concentration of negative charge? 22.3 PHYSICAL PROPERTIES We have often seen that the polar nature of a substance can affect physical properties such as boiling point. This is true for amines, which are more polar than alkanes but less polar than alcohols. For similarly constituted compounds, alkylamines have boiling points higher than those of alkanes but lower than those of alcohols. Dipole–dipole interactions, especially hydrogen bonding, are present in amines but absent in alkanes. The less polar nature of amines as compared with alcohols, however, makes these intermolecular forces weaker in amines than in alcohols. Among isomeric amines, primary amines have the highest boiling points, and tertiary amines the lowest. CH3CH2CH2NH2 Propylamine (a primary amine) bp 50°C CH3CH2NHCH3 N-Methylethylamine (a secondary amine) bp 34°C (CH3)3N Trimethylamine (a tertiary amine) bp 3°C CH3CH2CH3 Propane 0 D bp 42°C CH3CH2NH2 Ethylamine 1.2 D bp 17°C CH3CH2OH Ethanol 1.7 D bp 78°C H H H H H NH2 Most stable Lewis structure for aniline H H H H H NH2 H H H H H NH2 H H H H H NH2 Dipolar resonance forms of aniline 22.3 Physical Properties 863 A collection of physical properties of some representative amines is given in Appendix 1. Most commonly encountered alkylamines are liquids with unpleasant, “fishy” odors. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-TWo Amines Primary and secondary amines can participate in intermolecular hydrogen bonding, but tertiary amines cannot. Amines that have fewer than six or seven carbon atoms are soluble in water. all amines, even tertiary amines, can act as proton acceptors in hydrogen bonding to water molecules The simplest arylamine, aniline, is a liquid at room temperature and has a boiling point of 184C. Almost all other arylamines have higher boiling points. Aniline is only slightly soluble in water (3 g/100 mL) Substituted derivatives of aniline tend to be even 22.4 MEASURES OF AMINE BASICITY Two conventions are used to measure the basicity of amines. One of them defines a basicity constant Kb for the amine acting as a proton acceptor from water: RN: /+H-OH RN—H+:OH IR3NHTJHO IR,N and pkb =-log Kb For ammonia, Kb=1.8 10(pKb=4.7). A typical amine such as methylamine (CH3 NH2) is a stronger base than ammonia and has Kb =4.4 X 10(pKb=3.3) The other convention relates the basicity of an amine(r3N) to the acid dissocic tion constant Ka of its conjugate acid(r3NH R N-HFH++R,N where Ka and pKa have their usual meaning HR,N RNH’] log Ka The conjugate acid of ammonia is ammonium ion(NH), which has K=5.6X 10- (pKa=9.3). The conjugate acid of methylamine is methylammonium ion(CH3NH3) which has Ka=2 X 10(pKa =10.7). The more basic the amine, the weaker is its conjugate acid Methy lamine is a stronger base than ammonia; methylammonium ion is a weaker acid than ammonium ion The relationship between the equilibrium constant Kb for an amine(R3N)and Ka KnKb =10 and pka t pkb= 14 PROBLEM 22.5 A chemistry handbook lists Kb for quinine as 1 x 10. What is pk for quinine? What are the values of Ka and pka for the conjugate acid of qui nine Citing amine basicity according to the acidity of the conjugate acid permits acid-base reactions involving amines to be analyzed according to the usual Bronsted relationships. By comparing the acidity of an acid with the conjugate acid of an amine, for example, we see that amines are converted to ammonium ions by acids even as weak as acetic acid Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Primary and secondary amines can participate in intermolecular hydrogen bonding, but tertiary amines cannot. Amines that have fewer than six or seven carbon atoms are soluble in water. All amines, even tertiary amines, can act as proton acceptors in hydrogen bonding to water molecules. The simplest arylamine, aniline, is a liquid at room temperature and has a boiling point of 184°C. Almost all other arylamines have higher boiling points. Aniline is only slightly soluble in water (3 g/100 mL). Substituted derivatives of aniline tend to be even less water-soluble. 22.4 MEASURES OF AMINE BASICITY Two conventions are used to measure the basicity of amines. One of them defines a basicity constant Kb for the amine acting as a proton acceptor from water: Kb and pKb log Kb For ammonia, Kb 1.8 105 (pKb 4.7). A typical amine such as methylamine (CH3NH2) is a stronger base than ammonia and has Kb 4.4 104 (pKb 3.3). The other convention relates the basicity of an amine (R3N) to the acid dissociation constant Ka of its conjugate acid (R3NH): where Ka and pKa have their usual meaning: Ka and pKa log Ka The conjugate acid of ammonia is ammonium ion (NH4 ), which has Ka 5.6 1010 (pKa 9.3). The conjugate acid of methylamine is methylammonium ion (CH3NH3 ), which has Ka 2 1011 (pKa 10.7). The more basic the amine, the weaker is its conjugate acid. Methylamine is a stronger base than ammonia; methylammonium ion is a weaker acid than ammonium ion. The relationship between the equilibrium constant Kb for an amine (R3N) and Ka for its conjugate acid (R3NH) is: KaKb 1014 and pKa pKb 14 PROBLEM 22.5 A chemistry handbook lists Kb for quinine as 1 106 . What is pKb for quinine? What are the values of Ka and pKa for the conjugate acid of quinine? Citing amine basicity according to the acidity of the conjugate acid permits acid–base reactions involving amines to be analyzed according to the usual Brønsted relationships. By comparing the acidity of an acid with the conjugate acid of an amine, for example, we see that amines are converted to ammonium ions by acids even as weak as acetic acid: [H][R3N] [R3NH] R3N H R3N H [R3NH][HO] [R3N] R3N H OH R3N H OH 864 CHAPTER TWENTY-TWO Amines Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
22.5 Basicity of Amines Recall from Section 4.6 that acid-base reactions are char. H—OCCH3 OCCH cterized by equilibrium con Methylamine Acetate the left side of the equation (stronger acid; pKa= 4.7) eaker acid; pK=10.7) ind the weaker acid on the Conversely, adding sodium hydroxide to an ammonium salt converts it to the free CH →CH3NH2+ H-OH Methylammor Hydroxide ion Methylamine (stronger acid; pKa =10.7) ply the Henderson-Hasselbalch equation (see"Quantitative Relationships Involving Carboxylic Acids, the box accompanying Section 19.4)to calculate the CHaNH3 T/CH3 NH, ratio in water buffered at pH 7. Their basicity provides a means by which amines may be separated from neutral organic compounds. A mixture containing an amine is dissolved in diethyl ether and shaken with dilute hydrochloric acid to convert the amine to an ammonium salt. The ammonium salt, being ionic, dissolves in the aqueous phase, which is separated from the ether layer. Adding sodium hydroxide to the aqueous layer converts the ammonium salt back to the free amine, which is then removed from the aqueous phase by extraction with a fresh portion of ether. 22.5 BASICITY OF AMINES Amines are weak bases, but as a class, amines are the strongest bases of all neutral mol- ecules. Table 22 1 lists basicity data for a number of amines. The most important rela- tionships to be drawn from the data are 1. Alkylamines are slightly stronger bases than ammonia. 2. Alkylamines differ very little among themselves in basicity. Their basicities cover a range of less than 10 in equilibrium constant (1 pK unit). 3. Arylamines are much weaker bases than ammonia and alkylamines. Their basicity constants are on the order of 10 smaller than those of alkylamines(6 pK units) The differences in basicity between ammonia, and primary, secondary, and tertiary alkylamines result from the interplay between steric and electronic effects on the mole- cules themselves and on the solvation of their conjugate acids. In total, the effects are mall, and most alkylamines are very similar in basicity Arylamines are a different story, however; most are about a million times weaker as bases than ammonia and alkylamines As unfavorable as the equilibrium is for cyclohexylamine acting as a base in nh h,o NH3 HO (Kb44×10;pKb3.4) Cyclohexylamine Water Cyclohexylammonium Hydroxide Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
Conversely, adding sodium hydroxide to an ammonium salt converts it to the free amine: PROBLEM 22.6 Apply the Henderson–Hasselbalch equation (see “Quantitative Relationships Involving Carboxylic Acids,” the box accompanying Section 19.4) to calculate the CH3NH3 /CH3NH2 ratio in water buffered at pH 7. Their basicity provides a means by which amines may be separated from neutral organic compounds. A mixture containing an amine is dissolved in diethyl ether and shaken with dilute hydrochloric acid to convert the amine to an ammonium salt. The ammonium salt, being ionic, dissolves in the aqueous phase, which is separated from the ether layer. Adding sodium hydroxide to the aqueous layer converts the ammonium salt back to the free amine, which is then removed from the aqueous phase by extraction with a fresh portion of ether. 22.5 BASICITY OF AMINES Amines are weak bases, but as a class, amines are the strongest bases of all neutral molecules. Table 22.1 lists basicity data for a number of amines. The most important relationships to be drawn from the data are 1. Alkylamines are slightly stronger bases than ammonia. 2. Alkylamines differ very little among themselves in basicity. Their basicities cover a range of less than 10 in equilibrium constant (1 pK unit). 3. Arylamines are much weaker bases than ammonia and alkylamines. Their basicity constants are on the order of 106 smaller than those of alkylamines (6 pK units). The differences in basicity between ammonia, and primary, secondary, and tertiary alkylamines result from the interplay between steric and electronic effects on the molecules themselves and on the solvation of their conjugate acids. In total, the effects are small, and most alkylamines are very similar in basicity. Arylamines are a different story, however; most are about a million times weaker as bases than ammonia and alkylamines. As unfavorable as the equilibrium is for cyclohexylamine acting as a base in water, CH3N H H H Methylammonium ion (stronger acid; pKa 10.7) OH Hydroxide ion CH3NH2 Methylamine H OH Water (weaker acid; pKa 15.7) CH3NH2 Methylamine H OCCH3 O Acetic acid (stronger acid; pKa 4.7) CH3NH3 Methylammonium ion (weaker acid; pKa 10.7) OCCH3 O Acetate ion 22.5 Basicity of Amines 865 NH2 Cyclohexylamine H2O Water NH3 Cyclohexylammonium ion HO Hydroxide ion (Kb 4.4 104 ; pKb 3.4) Recall from Section 4.6 that acid–base reactions are characterized by equilibrium constants greater than unity when the stronger acid is on the left side of the equation and the weaker acid on the right. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CHAPTER TWENTY-TWo Amines TABLE 22.1 Base Strength of Amines As Measured by Their Basicity Constants and the Dissociation Basic Acidity of conjugate acid Compound Structure 1.8×10-5 5.5×10-10 4.4×10 2.3×10-11 10.6 Ethylamine 5.6×10 1.8×10 sopropylamine 4.3×10-4 2.3 10.6 tert-Butylamine (CH3)3CNH2 2.8×10-4 3.6 3.6×10 10.4 Aniline 3.8×10-10 2.6×10 Secondary amines Dimethylamine 5.1×10-4 2.0×10-1 10 Diethylamine (CH3 CH2)2NH 1.3×10-3 7.7×10-1 N-Methylaniline HsNHCH 6.1×10-10 1.6×10-5 Tertiary amines amine CHaN 5.3×10-5 4.3 1.9×10-10 Triethylamine (CH3 CH2)3N 5.6×10-4 1.8×10 N, N-Dimethylaniline CHsN(CH3)2 1.2×10-9 8.9 8.3×10 *In water at25°C it is far less favorable for aniline Ha HO NHa Ho (K3.8×10-10;pk94) Anilinium ion Hydroxid lamine and aniline on Learning Aniline is a much weaker base because its delocalized lone pair is more strongly held than the nitrogen lone pair in cyclohexylamine. The more strongly held the electron pair, the less able it is to abstract a proton +hop HO M Aniline is stabilized by delocalization of lone pair into T system of ring, decreasing the electron When the proton donor is a strong acid, arylamines can be completely protor Aniline is extracted from an ether solution into 1 M hydrochloric acid because it is erted to a water-soluble anilinium ion salt under these condition Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
it is far less favorable for aniline. Aniline is a much weaker base because its delocalized lone pair is more strongly held than the nitrogen lone pair in cyclohexylamine. The more strongly held the electron pair, the less able it is to abstract a proton. When the proton donor is a strong acid, arylamines can be completely protonated. Aniline is extracted from an ether solution into 1 M hydrochloric acid because it is converted to a water-soluble anilinium ion salt under these conditions. N H H Aniline is stabilized by delocalization of lone pair into system of ring, decreasing the electron density at nitrogen. H2O N H H H HO 866 CHAPTER TWENTY-TWO Amines TABLE 22.1 Base Strength of Amines As Measured by Their Basicity Constants and the Dissociation Constants of Their Conjugate Acids* Compound *In water at 25°C. Ammonia Primary amines Methylamine Ethylamine Isopropylamine tert-Butylamine Aniline Secondary amines Dimethylamine Diethylamine N-Methylaniline Kb 1.8 105 4.4 104 5.6 104 4.3 104 2.8 104 3.8 1010 5.1 104 1.3 103 6.1 1010 5.3 105 5.6 104 1.2 109 pKb 4.7 3.4 3.2 3.4 3.6 9.4 3.3 2.9 9.2 4.3 3.2 8.9 Ka 5.5 1010 2.3 1011 1.8 1011 2.3 1011 3.6 1011 2.6 105 2.0 1011 7.7 1012 1.6 105 1.9 1010 1.8 1011 8.3 106 pKa 9.3 10.6 10.8 10.6 10.4 4.6 10.7 11.1 4.8 9.7 10.8 5.1 Structure NH3 CH3NH2 CH3CH2NH2 (CH3)2CHNH2 (CH3)3CNH2 C6H5NH2 (CH3)2NH (CH3CH2)2NH C6H5NHCH3 (CH3)3N (CH3CH2)3N C6H5N(CH3)2 Tertiary amines Trimethylamine Triethylamine N,N-Dimethylaniline Basicity Acidity of conjugate acid NH2 Aniline H2O Water NH3 Anilinium ion HO Hydroxide ion (Kb 3.8 1010; pKb 9.4) Compare the calculated charge on nitrogen in cyclohexylamine and aniline on Learning By Modeling. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
22.5 Basicity of Amines PROBLEM 22. 7 The two amines a factor of 40.000 in their values. Which is the stronger base heir structures on Learning 6 Modeling. What are the calculated ne two nitro Tetrahydroquinoline Tetrahydroisoquinoline Conjugation of the amino group of an arylamine with a second aromatic ring, then a third, reduces its basicity even further. Diphenylamine is 6300 times less basic than aniline, whereas triphenylamine is scarcely a base at all, being estimated as 10 times less basic than aniline and 10 4 times less basic than ammonia. C6HSNH3 (ChS)2NH (C6H5)3N Aniline Diphenyl Triphenylamine (Kb3.8×10-10;(K16×1 pKb.4) pKb =19) In general, electron-donating substituents on the aromatic ring increase the basic ity of arylamines slightly. Thus, as shown in Table 22.2, an electron-donating methyl group in the para position increases the basicity of aniline by a factor of only 5-6 (less than I pk unit). Electron-withdrawing groups are base-weakening and exert larger effects. A p-trifluoromethyl group decreases the basicity of aniline by a factor of 200 ind a p-nitro group by a factor of 3800. In the case of p-nitroaniline a resonance inter action of the type shown provides for extensive delocalization of the unshared electron pair of the amine grou Just as aniline is much less basic than alkylamines because the unshared electron pair of nitrogen is delocalized into the T system of the ring, p-nitroaniline is even less basic because the extent of this delocalization is greater and involves the oxygens of the nitro TABLE 22.2 Effect of Substituents on the Basicity of Aniline H 4×10 9.4 CHa 2×10 8.7 2×10-12 11.5 O2N 1×10-13 13.0 Back Forward Main MenuToc Study Guide ToC Student o MHHE Website
PROBLEM 22.7 The two amines shown differ by a factor of 40,000 in their Kb values. Which is the stronger base? Why? View their structures on Learning By Modeling. What are the calculated charges on the two nitrogens? Conjugation of the amino group of an arylamine with a second aromatic ring, then a third, reduces its basicity even further. Diphenylamine is 6300 times less basic than aniline, whereas triphenylamine is scarcely a base at all, being estimated as 108 times less basic than aniline and 1014 times less basic than ammonia. In general, electron-donating substituents on the aromatic ring increase the basicity of arylamines slightly. Thus, as shown in Table 22.2, an electron-donating methyl group in the para position increases the basicity of aniline by a factor of only 5–6 (less than 1 pK unit). Electron-withdrawing groups are base-weakening and exert larger effects. A p-trifluoromethyl group decreases the basicity of aniline by a factor of 200 and a p-nitro group by a factor of 3800. In the case of p-nitroaniline a resonance interaction of the type shown provides for extensive delocalization of the unshared electron pair of the amine group. Just as aniline is much less basic than alkylamines because the unshared electron pair of nitrogen is delocalized into the system of the ring, p-nitroaniline is even less basic because the extent of this delocalization is greater and involves the oxygens of the nitro group. N NH2 O O N NH2 O O Electron delocalization in p-nitroaniline C6H5NH2 Aniline (Kb 3.8 1010; pKb 9.4) (C6H5)2NH Diphenylamine (Kb 6 1014; pKb 13.2) (C6H5)3N Triphenylamine (Kb 1019; pKb 19) N H Tetrahydroquinoline NH Tetrahydroisoquinoline 22.5 Basicity of Amines 867 TABLE 22.2 Effect of Substituents on the Basicity of Aniline X H CH3 CF3 O2N 4 1010 2 109 2 1012 1 1013 Kb 9.4 8.7 11.5 13.0 pKb X NH2 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website