Basicity of Amines, Acidity of Ammonium Ions

                                                                                                                                               
                      Basicity of Amines, Acidity of Ammonium Ions                                                                                                                                                                                              

• N lone pair relatively easily protonated

• note that K_b x K_a = [H⁺] [OH^-] = Kw = 10^-14
  or pK_a + pK_b = 14
• recall that when pH = pK_a , there are equal concentrations of the conjugate acid and conjugate base present (i.e., RNH₂ and RNH₃⁺ )
• for typical aliphatic amines, pK_b = 3 - 4
  so pK_a = 10 - 11 for their ammonium ions
  so at around pH 10 - 11 , RNH₂ and RNH₃⁺ are both present
• for typical aromatic amines, pK_b = 9 - 10
  so pK_a = 4 - 5 for their ammonium ions
  so at around pH 4 - 5 , ArNH₂ and ArNH₃⁺ are both present
• water solubility of amines can be easily changed with pH
  aromatic amines are water-soluble (protonated) below pH 4
  aliphatic amines are water-soluble (protonated) below pH 9

Basicity Trends

Amines as Bases

• deciding the stronger of two acids, e.g. H-A₁ and H-A₂

• the stronger acid has the more stable anion

• the stronger acid corresponds to the more favorable "reaction", left to right

• deciding the stronger of two bases, e.g. B₁ and B₂

• there are no anions here, but the stronger base STILL corresponds to the more favorable reaction

Example

• measure basicity in terms of acidity (pKa) of conjugate acid  (we will not use the pKb scale)

• compare (and don't confuse!!)

Trends in Basicity ??

• no clear trends among the aliphatic amines.....

But

• (minor) resonance delocalization (i.e. partial bonding) stabilizes (lowers energy) of the non-bonding electrons on N, these electrons are less reactive

• aromatic amines are thus less basic than aliphatic amines

• clear decrease in basicity with decreasing energy of non-bonding electrons as the hybridization of the orbitals gains more s character and less p character

• the protonated nitrile is about the strongest acid that we discuss this entire course

why aromatic amines are less basic ?

• aromatic amines are less basic due to resonance delocalization of the N lone pair

important conclusion : conjugation reduces basic nature  .

• amides are nonbasic due to strong delocalization of the N lone pair

• electron withdrawing effects decrease basicity
  because the N lone pair is less available for bonding to a proton