September 5

September 5 - Lecture#5 September 5 2008 Acid-base chemistry(continued Factors affecting Brnsted acid strength(continued 3 Comparing two acids

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Acid-base chemistry (continued) Factors affecting Brönsted acid strength (continued) 3) Comparing two acids where the protons to be donated are attached to the same element: the compound with the positive charge is more acidic The pK a of the ammonium ion, NH 4 + , is 9.3 while the pK a of NH 3 is 36; NH 4 + is a stronger acid than NH 3 (and NH 2 - is a stronger base than NH 3 ). 4) The more acceptable resonance contributors a conjugate base has, the stronger the acid CH 3 COOH (pK a = 4.7), a carboxylic acid, is a stronger acid than CH 3 CH 2 OH (pK a = 16), an alcohol. The conjugate base of CH 3 COOH is the acetate ion, CH 3 COO - , and it is stabilized by resonance. The ethoxide ion, CH 3 CH 2 O - , that is the conjugate base of ethanol, is not stabilized by resonance. 5) Inductive effects: electron-withdrawing groups (EWG) near the acidic proton increase acid strength Acid pK a Cl 3 C-CO 2 H 0.6 Cl 2 CHCO 2 H 1.3 ClCH 2 CO 2 H 2.9 CH 3 COOH 4.7 The inductive effect decreases with increasing distance. Acid
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This note was uploaded on 02/25/2010 for the course CHEM 3311 at Colorado.

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September 5 - Lecture#5 September 5 2008 Acid-base chemistry(continued Factors affecting Brnsted acid strength(continued 3 Comparing two acids

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