1. 22.4 Measures of Amine Basicity3

2. The basicity of amines may be measured by: 1) Kb 2) pKb
Measures of Basicity
The basicity of amines may be measured by:
1) Kb
2) pKb
3) Ka of conjugate acid
4) pKa of conjugate acid 6

3. Basicity Constant (Kb) and pKb
Kb is the equilibrium constant for the reaction:
R3N + H H OH •• –
• • OH ••
R3N
• • + +
[R3NH+][HO–]
Kb =
[R3N]
and
pKb =
- log Kb 2

4. Ka and pKa of Conjugate Acid
Ka is the equilibrium constant for the dissociation of the conjugate acid of the amine:
R3N
• • + H H+
[R3N][H+]
Ka =
[R3NH+]
and
pKa =
- log Ka 3

5. Relationships between acidity and basicity constants
Ka Kb =
pKa + pKb = 14 3

6. 22.5 Basicity of Amines 3

7. Effect of Structure on Basicity
1. Alkylamines are slightly stronger bases than ammonia. 5

8. Table 22.1 (page 866) Basicity of Amines in Aqueous Solution
Amine Conj. Acid pKa
NH3 NH4+ 9.3
CH3CH2NH2 CH3CH2NH 6

9. Table 22.1 (page 866) Basicity of Amines in Aqueous Solution
Amine Conj. Acid pKa
NH3 NH4+ 9.3
CH3CH2NH2 CH3CH2NH
CH3CH2NH3+ is a weaker acid than NH4+; therefore, CH3CH2NH2 is a stronger base than NH3. 6

10. Effect of Structure on Basicity
1. Alkylamines are slightly stronger bases than ammonia.
2. Alkylamines differ very little in basicity. 5

11. Table 22.1 (page 866) Basicity of Amines in Aqueous Solution
Amine Conj. Acid pKa
NH3 NH4+ 9.3
CH3CH2NH2 CH3CH2NH
(CH3CH2)2NH (CH3CH2)2NH
(CH3CH2)3N (CH3CH2)3NH+ 10.8
Notice that the difference separating a primary, secondary, and tertiary amine is only 0.3 pK units. 6

12. Effect of Structure on Basicity
1. Alkylamines are slightly stronger bases than ammonia.
2. Alkylamines differ very little in basicity.
3. Arylamines are much weaker bases than ammonia. 5

13. Table 22.1 (page 866) Basicity of Amines in Aqueous Solution
Amine Conj. Acid pKa
NH3 NH4+ 9.3
CH3CH2NH2 CH3CH2NH
(CH3CH2)2NH (CH3CH2)2NH
(CH3CH2)3N (CH3CH2)3NH+ 10.8
C6H5NH2 C6H5NH3+ 4.6 6

14. Decreased basicity of arylamines
NH2 •• H OH •• +
NH3 + –
• • OH •• + 2

15. Decreased basicity of arylamines
NH2 •• H OH •• +
Aniline (reactant) is stabilized by conjugation of nitrogen lone pair with ring p system.
This stabilization is lost on protonation.
NH3 + –
• • OH •• + 2

16. Decreased basicity of arylamines
Increasing delocalization makes diphenylamine a weaker base than aniline, and triphenylamine a weaker base than diphenylamine.
C6H5NH2
(C6H5)2NH
(C6H5)3N Kb
3.8 x 10-10
6 x 10-14
~10-19 2

17. Effect of Substituents on Basicity of Arylamines
1. Alkyl groups on the ring increase basicity, but only slightly (less than 1 pK unit). 5

18. Basicity of ArylaminesX
NH2 X
NH3+
X pKb pKa H CH 7

19. Effect of Substituents on Basicity of Arylamines
1. Alkyl groups on the ring increase basicity, but only slightly (less than 1 pK unit).
2. Electron withdrawing groups, especially ortho and/or para to amine group, decrease basicity and can have a large effect. 5

20. Basicity of ArylaminesX
NH2 X
NH3+
X pKb pKa H CH CF
O2N 7

21. p-Nitroaniline
NH2 O N – ••
• • + 2

22. p-Nitroaniline NH2 O N – + O N – NH2 + +••
• • + O N – ••
• •
NH2 + +
Lone pair on amine nitrogen is conjugated with p-nitro group—more delocalized than in aniline itself. Delocalization lost on protonation. 2

23. Aniline is 3800 times more basic than p-nitroaniline.
Effect is Cumulative
Aniline is 3800 times more basic than p-nitroaniline.
Aniline is ~1,000,000,000 times more basic than 2,4-dinitroaniline. 2

24. (resembles an arylamine in basicity)
Heterocyclic Amines N •• N H ••
is more basic than
piperidine
pyridine
Kb = 1.6 x 10-3
Kb = 1.4 x 10-9
(an alkylamine)
(resembles an arylamine in basicity) 9

25. Heterocyclic Amines N N H is more basic than imidazole pyridine•• N H
• •
is more basic than
imidazole
pyridine
Kb = 1 x 10-7
Kb = 1.4 x 10-9 9

26. Which nitrogen is protonated in imidazole?
• • H+ H+ N H
• • N H
• • + + 11

27. Which nitrogen is protonated in imidazole?
• • H+ N H
• • + 11

28. Protonation in the direction shown gives a stabilized ion.
Imidazole
Protonation in the direction shown gives a stabilized ion. N H
• • H+ N H
• • + + N H
• • 11

29. 22.6 Tetraalkylammonium Salts as Phase-Transfer Catalysts3

30. Phase-Transfer Catalysis
Phase-transfer agents promote the solubility of ionic substances in nonpolar solvents. They transfer the ionic substance from an aqueous phase to a non-aqueous one.
Phase-transfer agents increase the rates of reactions involving anions. The anion is relatively unsolvated and very reactive in nonpolar media compared to water or alcohols. 5

31. Phase-Transfer Catalysis
Quaternary ammonium salts are phase-transfer catalysts. They are soluble in nonpolar solvents. N
H3C
CH2CH2CH2CH2CH2CH2CH2CH3 +
Cl–
Methyltrioctylammonium chloride 5

32. Phase-Transfer Catalysis
Quaternary ammonium salts are phase-transfer catalysts. They are soluble in nonpolar solvents. N
CH2CH3 +
Cl–
Benzyltriethylammonium chloride 5

33. Example
The SN2 reaction of sodium cyanide with butyl bromide occurs much faster when benzyl- triethylammonium chloride is present than when it is not.
CH3CH2CH2CH2Br +
NaCN
benzyltriethylammonium chloride
CH3CH2CH2CH2CN +
NaBr 5

34. Mechanism N
CH2CH3 +
Cl– +
CN–
(aqueous)
(aqueous) 5

35. Mechanism N CH2CH3 + Cl– + CN– (aqueous) (aqueous) N CH2CH3 + CN– +5

36. Mechanism N
CH2CH3 +
CN–
(aqueous) 5

37. Mechanism N
CH2CH3 +
CN–
(in butyl bromide) N
CH2CH3 +
CN–
(aqueous) 5

38. Mechanism N
CH2CH3 + +
CN–
CH3CH2CH2CH2Br
(in butyl bromide) 5

39. Mechanism CH2CH3 + + N CN– CH3CH2CH2CH2Br (in butyl bromide) CH2CH3 +
CH3CH2CH2CH2CN
(in butyl bromide) 5