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3_Acid_Base - Chem 267 Basic Organic Chemistry 2 Prof Dr...

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Prof. Dr. Xiangdong Fang University of Waterloo Chem 267 Basic Organic Chemistry 2 1
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Brønsted-Lowry Acids and Bases 1) Brønsted Acid: a proton donor (e.g. H-A); Brønsted Base: a proton acceptor (e.g. B); 2) the strength of an acid is measured by its acid dissociation constant or equivalently its p K a values (p K a=-log K a) 3) strong acid: pH , [H + ] µ , K a µ , p K a ; weak acid: pH µ , [H + ] , K a , p K a µ ; 2
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Acidity Increases with Increasing Anion Stability “an acid is stronger if its conjugate base is weaker (e.g. more stable anion or lower basicity)” H A + H + What makes the conjugate base weaker? Key factors: 1) electronegativity; 2) induction; 3) atomic size; 4) resonance stabilization; 5) hybridization; 3
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Electronegativity “the more electronegative the anionic atom in the conjugate base, the greater its ability to carry the negative charge” C H 4 H 2 O N H 3 H F within the same row of the periodic table H + H + H + H + + + + + CH 3 NH 2 OH F increasing electronegativity increasing anion stability increasing acidity 4
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