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Ch17_Types_4 - More About Acids and Bases • Classes of...

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Unformatted text preview: More About Acids and Bases • Classes of acids— • Molecular acids, CH3CO2H • Cationic acids, NH4+ • Anionic acids, H2PO4- 1 2 3 Molecular Acids Carboxylic acids are a major class —acetic, citric, malic, acetylsalicyclic, etc. Molecular Acids Acidic proton Lactic Citric acid Carboxylic acid group Acetylsalicylic, aspirin Tartaric Oxoacids Note that all have H attached to an O atom that is attached to N, P, S, etc. The N, P, S, etc. is also has a double bond to another O atom 4 5 6 Cationic Acids Ammonium ion, NH 4+ NH 4+ + H2O ----> NH3 + H3 O+ NH acid base base acid acid NH4+ ion is a moderate acid because its ion conjugate base is weak. Ammonium Salts • Many natural materials have the -NR 2 group. • –NR2 is a base and forms –NR2H + Ka = 5.6 x 10-10 5.6 Caffeine Cadaverine Page 1 7 Novocaine —NH3+ Anionic Acids • Examples: H 2PO4-, HPO42-, HSO4-, HCO3 - 8 9 Molecular Bases • Many natural materials have the -NR 2 group. • –NR2 is a base and forms –NR2H + H2PO4- H+ + H+ HPO42- Nicotine These anions are all AMPHIPROTICthey are both acids and bases and bases 10 11 12 Anions as Bases • Carbonate CO 32-(aq) + H2O(liq) ¸ aq) O(liq) -(aq) + OH- (aq) HCO3 aq) HCO aq) • Phosphate, PO 43• Sulfate, SO42- Anions as Bases Anions as Bases Lactate Acetate All are conjugate bases of weak weak acids • Note: All are conjugate bases of weak acids weak acids Benzoate Page 2 13 14 Amino Acids Internal proton transfer Acid Strength: pKa H3PO4 Ka = 7.5 x 10 -3 7.5 pKa = -log Ka -log pKa = 2.12 2.12 pKa = 4.74 4.74 Carboxylic acid group Basic amine group Substitutent on alpha carbon + CH3CO2H Ka = 1.8 x 10 -5 1.8 Stronger acids have smaller pK a values Page 3 ...
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