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22.4 Measures of Amine Basicity

22.4 Measures of Amine Basicity. Measures of Basicity. The basicity of amines may be measured by: 1) K b 2) p K b 3) K a of conjugate acid 4) p K a of conjugate acid. +. –. ••. ••. R 3 N. H. OH. OH. R 3 N. H. • •. • •. ••. ••. Basicity Constant (K b ) and pK b.

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22.4 Measures of Amine Basicity

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  1. 22.4Measures of Amine Basicity

  2. 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

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

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

  5. Relationships between acidity and basicity constants Ka Kb = 10-14 pKa + pKb = 14

  6. 22.5Basicity of Amines

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

  8. Table 22.1 (page 866)Basicity of Amines in Aqueous Solution • Amine Conj. Acid pKa • NH3 NH4+ 9.3 • CH3CH2NH2 CH3CH2NH3+ 10.8

  9. Table 22.1 (page 866)Basicity of Amines in Aqueous Solution • Amine Conj. Acid pKa • NH3 NH4+ 9.3 • CH3CH2NH2 CH3CH2NH3+ 10.8 CH3CH2NH3+ is a weaker acid than NH4+;therefore, CH3CH2NH2 is a stronger base than NH3.

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

  11. Table 22.1 (page 866)Basicity of Amines in Aqueous Solution • Amine Conj. Acid pKa • NH3 NH4+ 9.3 • CH3CH2NH2 CH3CH2NH3+ 10.8 • (CH3CH2)2NH (CH3CH2)2NH2+ 11.1 • (CH3CH2)3N (CH3CH2)3NH+ 10.8 Notice that the difference separating a primary,secondary, and tertiary amine is only 0.3 pK units.

  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.

  13. Table 22.1 (page 866)Basicity of Amines in Aqueous Solution • Amine Conj. Acid pKa • NH3 NH4+ 9.3 • CH3CH2NH2 CH3CH2NH3+ 10.8 • (CH3CH2)2NH (CH3CH2)2NH2+ 11.1 • (CH3CH2)3N (CH3CH2)3NH+ 10.8 • C6H5NH2 C6H5NH3+ 4.6

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

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

  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 3.8 x 10-10 6 x 10-14 ~10-19 Kb

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

  18. X NH2 X NH3+ Basicity of Arylamines • X pKb pKa • H 9.4 4.6 • CH3 8.7 5.3

  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.

  20. X NH2 X NH3+ Basicity of Arylamines • X pKb pKa • H 9.4 4.6 • CH3 8.7 5.3 • CF3 11.5 2.5 • O2N 13.0 1.0

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

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

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

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

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

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

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

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

  29. 22.6Tetraalkylammonium Saltsas Phase-Transfer Catalysts

  30. Phase-Transfer Catalysis • Phase-transfer agents promote the solubility ofionic substances in nonpolar solvents. Theytransfer the ionic substance from an aqueousphase to a non-aqueous one. • Phase-transfer agents increase the rates ofreactions involving anions. The anion is relativelyunsolvated and very reactive in nonpolar mediacompared to water or alcohols.

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

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

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

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

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

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

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

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

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

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