Chapter 21 Amines

1. Chapter 21Amines

2. Amine Nomenclature

3. Classification of Amines • Alkylamine • N attached to alkyl group • Arylamine • N attached to aryl group • Primary, secondary, or tertiary • determined by number of carbon atoms directly attached to nitrogen

4. Nomenclature of Primary Alkylamines (RNH2) • Two IUPAC styles • 1) analogous to alcohols: replace -e ending by -amine • 2) name alkyl group and attach -amine as a suffix

5. NH2 CH3CHCH2CH2CH3 NH2 Examples: some primary alkylamines (RNH2: one carbon directly attached to N) ethylamine or ethanamine CH3CH2NH2 cyclohexylamine orcyclohexanamine 1-methylbutylamine or2-pentanamine

6. NH2 Br CH2CH3 F NH2 Nomenclature of Primary Arylamines (ArNH2) • Name as derivatives of aniline. p-fluoroaniline 5-bromo-2-ethylaniline

7. O HC NH2 Amino groups as substituents • amino groups rank below OH groups and higher oxidation states of carbon • in such cases name the amino group as a substituent HOCH2CH2NH2 2-aminoethanol p-aminobenzaldehyde

8. Secondary and Tertiary Amines • Name as N-substituted derivatives of parent primary amine. • (N is a locant-it is not alphabetized, but is treated the same way as a numerical locant) • Parent amine is one with longest carbonchain.

9. NHCH2CH3 NO2 Cl CH3 N CH3 Examples CH3NHCH2CH3 N-methylethylamine 4-chloro-N-ethyl-3-nitroaniline N,N-dimethylcycloheptylamine

10. CH3 – + – Cl N CF3CO2 CH2CH3 H Ammonium Salts • A nitrogen with four substituents is positivelycharged and is named as a derivative of ammonium ion (NH4+). + CH3NH3 methylammoniumchloride N-ethyl-N-methylcyclopentylammoniumtrifluoroacetate

11. CH3 + – CH3 CH2 I N CH3 Ammonium Salts • When all four atoms attached to N are carbon,the ion is called a quaternary ammonium ion andsalts that contain it are called quaternaryammonium salts. benzyltrimethylammonium iodide

12. Structure and Bonding

13. Alkylamines 147 pm 112° 106°

14. Alkylamines Most prominent feature is high electrostaticpotential at nitrogen. Reactivity of nitrogen lonepair dominates properties of amines.

15. Geometry at N Compare geometry at N of methylamine, aniline,and formamide. • Pyramidal geometry at sp3-hybridized N in methylamine. • Planar geometry at sp2-hybridized N in formamide. H H H sp3 sp2 NH2 C NH2 C O H

16. Geometry at N Compare geometry at N of methylamine, aniline,and formamide. • Pyramidal geometry at sp3-hybridized N in methylamine. • Planar geometry at sp2-hybridized N in formamide. sp3 sp2

17. Geometry at N Angle that the C—N bond makes with bisector ofH—N—H angle is a measure of geometry at N. • Note: this is not the same as the H—N—H bond angle sp3 sp2 180° ~125°

18. Geometry at N Angle that the C—N bond makes with bisector ofH—N—H angle is a measure of geometry at N. sp3 sp2 180° ~125° 142.5°

19. Geometry at N Geometry at N in aniline is pyramidal; closer tomethylamine than to formamide. 142.5°

20. Geometry at N • Hybridization of N in aniline lies between sp3 and sp2. • Lone pair of N can be delocalized into ring best if N is sp2 and lone pair is in a p orbital. • Lone pair bound most strongly by N if pair is in an sp3 orbital of N, rather than p. • Actual hybridization is a compromise that maximizesbinding of lone pair. 142.5°

21. Electrostatic Potential Maps of Aniline Nonplanar geometry at N. Region of highestnegative potential is at N. Planar geometry at N. High negative potential shared by N and ring.

22. Physical Properties

23. Physical Properties • Amines are more polar and have higher boiling points than alkanes; but are less polar andhave lower boiling points than alcohols. CH3CH2CH3 CH3CH2NH2 CH3CH2OH dipolemoment (): 0 D 1.2 D 1.7 D boiling point: -42°C 17°C 78°C

24. Physical Properties CH3CH2CH2NH2 CH3CH2NHCH3 (CH3)3N boilingpoint: 50°C 34°C 3°C • Boiling points of isomeric amines decrease ingoing from primary to secondary to tertiary amines. • Primary amines have two hydrogens on N capable of being involved in intermolecular hydrogen bonding. Secondary amines have one. Tertiary amines cannot be involved in intermolecular hydrogen bonds.

25. Basicity of Amines

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

27. Table 22.1 (page 920)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.

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

29. Table 22.1 (page 920)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.

30. 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.

31. Table 22.1 (page 920)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

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

33. 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

34. 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.

35. 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

36. 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

37. 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

38. – •• •• 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.

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

40. •• 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

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

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

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

44. Tetraalkylammonium Saltsas Phase-Transfer Catalysts

45. 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.

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

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

48. 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

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

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