215F07-notes-11-26-07 - Chem 215 F07 Notes Dr Masato...

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Chem 215 F07 Notes – Dr. Masato Koreeda - Page 1 of 8. Date: November 26, 2007 Chapter 16-2: Acidity and Basicity of Polyfunctional Compounds I. Polyprotic acids Diprotic acid O O H O O H oxalic acid (found in spinach) pKa 1 1.27; pKa 2 4.27 O O H O O O O O O H inductively electrons are pushed toward the C(=O)OH? stabilized by an intra- molecular H-bonding less acidic than the first C(=O)O H also, considerably more acidic than usual R-C(=O)O H H O O H O O succinic acid (found in amber, fossil, and fungi) pKa 1 4.21; pKa 2 5.64 O O H H OH H O O OH H (+)-tartaric acid (a byproduct in wine making) pKa 1 2.93; pKa 2 4.23 O O H O H O O OH H H low pKa 1 due to the H-bonding of the conjugate base? Acidity Constant (pK a ) AH + H 2 O A - + H 3 O + K e K e = (a A " )(a H 3O + ) (a AH )(a H 2O ) (a X ) : activity of each species; K e : equilibrium constant At low concentrations, (a H 2 O ) constant. Therefore, K a = K e • (a H 2 O ) = (a A " )(a H 3O + ) (a AH ) = [A " ][H 3 O + ] [AH] where K a : (thermodynamic) acidity constant [X] : concentration of each species; f : activity coefficient, i.e., (a X ) = f X • [X] This equation can be approximated as follows in a dilute solution: K a [A " ][H 3 O + ] [AH] = K a K a = acidity constant pK a = - log K a thus, smaller pK a stronger acid pH vs. pK a Since pH = - log [H + ] = log 1 [H + ] , note: [H + ] = [H 3 O + ] K a = [A " ][H 3 O + ] [AH] can be transformed into pK a = log [AH] [A " ] + pH Note: The pH of a solution can be adjusted by adding an acid or base.
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Chem 215 F07 Notes – Dr. Masato Koreeda - Page 4 of 8. Date: November 26, 2007 15.8, 23.6: Chemical Synthesis of Peptides Peptides : Any polymer of amino acids linked by amide bonds between the amino group of each amino acid and the carbonyl group of the neighboring amino acid. Polypeptide: A peptide containing many amino acid residues, but usually having a molecular weight of less than about 5,000. Proteins: Mw 6,000-40,000,000 Oligopeptides: Peptides containg about 4 – 10 amino acid residues.
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