Organic Chemistry

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26 Apr 2012 09:26:04 IST

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please explain the trends in basicity of amines

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please explain the trends in basicity of amines

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26 Apr 2012 10:32:46 IST

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Basicity of Amines, Acidity of Ammonium Ions

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

aromatic amines are less basic due to resonance 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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