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22.4 Basicity of Amines

22.4 Basicity of Amines. Effect of Structure on Basicity. 1. Alkylamines are slightly stronger bases than ammonia. Table 22.1 (page 950) Basicity of Amines in Aqueous Solution. Amine Conj. Acid pK a NH 3 NH 4 + 9.3 CH 3 CH 2 NH 2 CH 3 CH 2 NH 3 + 10.8.

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22.4 Basicity of Amines

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  1. 22.4Basicity of Amines

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

  3. Table 22.1 (page 950)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.

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

  5. Table 22.1 (page 950)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.

  6. 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.

  7. Table 22.1 (page 950)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

  8. H •• + N H + H2N Strongeracid Strongerbase pKa = 4.6 H + H3N •• + NH2 Weakerbase Weakeracid pKa =10.6 Decreased basicity of arylamines

  9. H N H H + H3N •• + NH2 Decreased basicity of arylamines •• + + H2N Strongeracid When anilinium ion loses a proton, theresulting lone pair is delocalized into the ring. Weakeracid

  10. H N H Strongerbase H + H3N •• + NH2 Weakerbase Decreased basicity of arylamines •• + + H2N Aniline is a weaker base because its lone pair is more strongly held.

  11. pKa of conjugate acid: 4.6 0.8 ~-5 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

  12. X NH2 Effect of Substituents on Basicity of Arylamines • 1. Alkyl groups on the ring increase basicity, but only slightly (less than 1 pK unit). X pKa of conjugate acid H 4.6 CH3 5.3

  13. X NH2 Effect of Substituents on Basicity of Arylamines • 2. Electron withdrawing groups, especially ortho and/or para to amine group, decrease basicity and can have a large effect. X pKa of conjugate acid H 4.6 CF3 3.5O2N 1.0

  14. •• •• 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.

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

  16. •• N N •• H piperidine pyridine pKa of conjugate acid: 11.2 pKa of conjugate acid: 5.2 (resembles anarylamine inbasicity) (an alkylamine) Heterocyclic Amines is more basic than

  17. N H •• N •• N •• Heterocyclic Amines is more basic than imidazole pyridine pKa of conjugate acid: 7.0 pKa of conjugate acid: 5.2

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

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

  20. 22.5Tetraalkylammonium Saltsas Phase-Transfer Catalysts

  21. 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.

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

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

  24. 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

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

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

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

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