Ch17_Types_4 - More About Acids and Bases 1 2 Molecular...

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