Monday 23 December 2013

Basicity of Amines, Acidity of Ammonium Ions


                                                                                                                                               

                      Basicity of Amines, Acidity of Ammonium Ions                                                                                                                                                                                              
  • N lone pair relatively easily protonated
  • note that Kb x Ka = [H+] [OH-] = Kw = 10-14
    or pKa + pKb = 14
  • recall that when pH = pKa , there are equal concentrations of the conjugate acid and conjugate base present (i.e., RNH2 and RNH3+ )
  • for typical aliphatic amines, pKb = 3 - 4
    so pKa = 10 - 11 for their ammonium ions
    so at around pH 10 - 11 , RNH2 and RNH3+ are both present
  • for typical aromatic amines, pKb = 9 - 10
    so pKa = 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

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