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Approach:
- Recall: pKa = – logKa [stronger acid = larger Ka/ smaller pKa]
- R–COOH is a stronger acid than R–NH3+
- –COOH closer to –NH3+ is the stronger acid
- –NH3+ closer to –COOH is the stronger acid.
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Example 1 :
- R–COOH is a stronger acid than R–NH3+ → pKa3 must correspond to the only –NH3+
- –COOH closer to –NH3+ is the stronger acid
Explanation:
- Recall factors which affects the acidity of organic compounds i.e. stability of conjugate base
- –NH3+ is an electron withdrawing group; –COOH closer to it will form a more stable conjugate base (– COOH–) since the –ve charge the O atom will be more dispersed.
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Example 2 :
- R–COOH is a stronger acid than R–NH3+ → pKa1 must correspond to the only –COOH
- –NH3+ closer to –COOH is the stronger acid
Explanation:
- –COOH is an electron withdrawing group; –NH2 closer to –COOH will be a weaker base since its lone pairs are less available for dative bonding to H+ → corresponding –NH3+ is the stronger acid
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