Basicity of Amines Lone pair of electrons on nitrogen can accept a proton from an acid Aqueous solutions are basic to litmus. ·Ammonia pK,=4.74 Alkyl amines are usually stronger bases than ammonia.Increasing the number of alkyl groups decreases solvation of ion,so 2°and3°amines are similar to1°amines in basicity. 二> Chapter 19 16
Chapter 19 16 Basicity of Amines • Lone pair of electrons on nitrogen can accept a proton from an acid • Aqueous solutions are basic to litmus. • Ammonia pKb = 4.74 • Alkyl amines are usually stronger bases than ammonia. Increasing the number of alkyl groups decreases solvation of ion, so 2 and 3 amines are similar to 1 amines in basicity. =>
Energy Diagram Alkyl groups are electron-donating and stabilize the cation. H N H amine ammonium ion R-NH3 +OH R-NH2 +H2O Copyright 2005 Pearson Prentice Hall,Inc. => Chapter 19 17
Chapter 19 17 Energy Diagram Alkyl groups are electron-donating and stabilize the cation. =>
Resonance Effects Any delocalization of the electron pair weakens the base. H aniline anilinium ion stabilized by overlap with the ring no overlap is possible aliphatic amine R一iH2+H R-NH,○-NH stabilized by more endothermic overlap ○-iH2 is less basic 三> ○-NH2+H aromatic amine 18 Copyright2005 Pearson Prentice Hall.Inc
Chapter 19 18 Resonance Effects Any delocalization of the electron pair weakens the base. =>
Hybridization Effects Electrons are held more tightly in orbitals with more s character,so those compounds are weaker bases. H sphybridized sp2hybridized (more basic) (less basic) pyridine,pKp=8.75 piperidine,p=2.88 Copyright2005 Pearson Prentice Hall,Inc. => Chapter 19 19
Chapter 19 19 Hybridization Effects Electrons are held more tightly in orbitals with more s character, so those compounds are weaker bases. =>