Boiling Points overall dipole moment R H R CH3 1°or2°amine: 3°amine hydrogen bond donor and acceptor hydrogen bond acceptor only Copyright 2010 Pearson Prentice Hall.Inc. N-H less polar than O-H. Weaker hydrogen bonds,so amines will have a lower boiling point than the corresponding alcohol. Tertiary amines cannot hydrogen-bond,so they have lower boiling points than primary and secondary amines. Chapter 19 16
Chapter 19 16 Boiling Points ▪ N—H less polar than O—H. ▪ Weaker hydrogen bonds, so amines will have a lower boiling point than the corresponding alcohol. ▪ Tertiary amines cannot hydrogen-bond, so they have lower boiling points than primary and secondary amines
Solubility and Odor Small amines (6 Cs)are soluble in water. All amines accept hydrogen bonds from water and alcohol. Branching increases solubility. Most amines smell like rotting fish. NH2CH2CH2CH2CH2CH2NH2 1,5-pentanediamine or cadaverine Chapter 19 17
Chapter 19 17 Solubility and Odor ▪ Small amines (< 6 Cs) are soluble in water. ▪ All amines accept hydrogen bonds from water and alcohol. ▪ Branching increases solubility. ▪ Most amines smell like rotting fish. 1,5-pentanediamine or cadaverine NH2CH2CH2CH2CH2CH2NH2
Basicity of Amines Lone pair of electrons on nitrogen can accept a proton from an acid. Aqueous solutions are basic to litmus. Ammonia pKp 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. Chapter 19
Chapter 19 18 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
Reactivity of Amines Reaction of an amine as a nucleophile H R- N-CH3 H nucleophile electrophile new N-C bond formed Reaction of an amine as a proton base H 一H H base proton acid protonated Copyright2010 Pearson Prentice Hall,Inc. Chapter 19 19
Chapter 19 19 Reactivity of Amines
Base-Dissociation Constant of Amines H +H一O一H ← R一NH+ OH H H [RNH3][OH] [RNH2] pKb=-l0g10Kb Copyright2010 Pearson Prentice Hall,Inc. An amine can abstract a proton from water,giving an ammonium ion and a hydroxide ion. The equilibrium constant for this reaction is called the base-dissociation constant for the amine, symbolized by Kb. Chapter 19 20
Chapter 19 20 Base-Dissociation Constant of Amines ▪ An amine can abstract a proton from water, giving an ammonium ion and a hydroxide ion. ▪ The equilibrium constant for this reaction is called the base-dissociation constant for the amine, symbolized by Kb