Ch_322b_18.03

Ch_322b_18.03 - inductive effects because direct resonance...

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Effect of Substituents on Acidity Any factor that stabilizes the anion more than it stabilizes the acid should increase acidity (increase the magnitude of K a ). Any factor that destabilizes the anion relative to the acid should decrease acidity . K a = [RCO 2 - ] [H 3 O + ] [RCOOH] Electronic Influences The electronic effect of a substituent ( G ) operates more strongly on the anion (charged species) than on the carboxylic acid (neutral species).
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Some Examples of Substituent Effects H-COOH CH 3 -COOH Cl CH 2 -COOH K a 17.7 x 10 -5 1.75 x 10 -5 136 x 10 -5 Cl 2 CH-COOH Cl 3 C-COOH K a 5530 x 10 -5 23200 x 10 -5 Cl CH 2 CH 2 CH 2 -COOH 2.96 x 10 -5
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Substituent Effects in Benzoic Acids Substituents introduced into the para position of a benzoic acid affect the acidity as expected for the electron influence on the stability of the benzoate anion. The substituent influence is, at best, a combination of resonance and
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Unformatted text preview: inductive effects because direct resonance interaction between the benzene ring and the carboxylate anion is not important. COOH NO 2 COOH Cl COOH COOH CH 3 K a 6.3 x 10-5 electron withdrawing electron releasing 4.2 x 10-5 36 x 10-5 10.3 x 10-5 The Nitro Group The adjacent electropositive ring carbon stabilizes the carboxylate ion by the inductive effect. C :O: = O: ::-N :O: :O :-+ The electron influence of the nitro group is transmitted by !-electron delocalization to the carbons shown in the resonance structures. C :O: O:-N :O-+ :O:-+ C :O: O:-N :O-+ :O:-+ Quiz 18.03 In each pair of compounds below, circle the stronger carboxylic acid. Cl 3 CCOOH CH 3 COOH Cl 2 CHCH 2 COOH CH 3 CCl 2 COOH COOH COOH COOH NO 2 COOH NO 2 CH 3...
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Ch_322b_18.03 - inductive effects because direct resonance...

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