胺碱度测量可通过以下方法columbia有机化学课件06 html.pptx

22.4

MeasuresofAmineBasicity

MeasuresofBasicityThebasicityofaminesmaybemeasuredby: 1) Kb 2) pKb 3) Kaofconjugateacid 4) pKaofconjugateacid

BasicityConstant(Kb)andpKbKbistheequilibriumconstantforthereaction:R3N?

?+HOH????R3N+H–?

?OH????+Kb=[R3NH+][HO–][R3N]pKb=-logKband

KaandpKaofConjugateAcidKa=[R3N][H+][R3NH+]pKa=-logKaandR3N?

?R3N+H+H+ Kaistheequilibriumconstantforthedissociationoftheconjugateacidoftheamine:

RelationshipsbetweenacidityandbasicityconstantspKa+pKb=14KaKb=10-14

22.5

BasicityofAmines

EffectofStructureonBasicity1.Alkylaminesareslightlystrongerbasesthan

ammonia.

Amine Conj.Acid pKaNH3 NH4+ 9.3CH3CH2NH2 CH3CH2NH3+ 10.8Table22.1(page866)

BasicityofAminesinAqueousSolution

Amine Conj.Acid pKaNH3 NH4+ 9.3CH3CH2NH2 CH3CH2NH3+ 10.8Table22.1(page866)

BasicityofAminesinAqueousSolutionCH3CH2NH3+isaweakeracidthanNH4+;

therefore,CH3CH2NH2isastrongerbase

thanNH3.

EffectofStructureonBasicity1.Alkylaminesareslightlystrongerbasesthan

ammonia.2.Alkylaminesdifferverylittleinbasicity.

Amine Conj.Acid pKaNH3 NH4+ 9.3CH3CH2NH2 CH3CH2NH3+ 10.8(CH3CH2)2NH (CH3CH2)2NH2+ 11.1(CH3CH2)3N (CH3CH2)3NH+ 10.8Table22.1(page866)

BasicityofAminesinAqueousSolutionNoticethatthedifferenceseparatingaprimary,

secondary,andtertiaryamineisonly0.3pKunits.

EffectofStructureonBasicity1.Alkylaminesareslightlystrongerbasesthan

ammonia.2.Alkylaminesdifferverylittleinbasicity.3.Arylaminesaremuchweakerbasesthan

ammonia.

Amine Conj.Acid pKaNH3 NH4+ 9.3CH3CH2NH2 CH3CH2NH3+ 10.8(CH3CH2)2NH (CH3CH2)2NH2+ 11.1(CH3CH2)3N (CH3CH2)3NH+ 10.8C6H5NH2 C6H5NH3+ 4.6Table22.1(page866)

BasicityofAminesinAqueousSolution

Decreasedbasicityofarylamines+HOH????NH2??+NH3+–?

?OH????

+HOH????NH2??+NH3+–?

?OH???? Aniline(reactant)isstabilizedbyconjugationofnitrogenlonepairwithringpsystem. Thisstabilizationis