Later the tetrahedral intermediate is protonated and

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Unformatted text preview: this error. Suggested solution ■ This mechanism is described in detail on p. 208 of the textbook. The direct reaction works in acid solution as the carboxylic acid is protonated (at the carbonyl group, note) and becomes a good electrophile. Later the tetrahedral intermediate is protonated and can lose a molecule of water. H O R1 OH R1 OH HO OH HOR2 R1 OH O R2 H OH H2O OH R1 R1 OR2 O –H OR2 R1 OR2 In basic solution, the first thing that happens is the removal of the proton from the carboxylic acid to form a stable delocalised anion. Nucleophiles cannot attack this anion and no further reaction occurs. O R1 O base OH R1 O no further reaction Acid anhydrides and acid chlorides do not have this acidic hydrogen so the alcohol attacks them readily and the base is helpful in removing the acidic proton from the intermediate. The weak base pyridine (pKa of the conjugate acid 5.5) is ideal. The product from the uncatalysed reaction would be HCl from the acid chloride and the base also removes that. Solutions for Chapter 10 – Nucleophilic Substitution at the Carbonyl Group HOR2 O R1 O Cl R1 Cl O R2 O Cl base ester R1 OR2 H PROBLEM 3 Predict the success or failure of these attempted substitutions at the carbonyl group. You should use estimated pKa values in your answer and, of course, draw mechanisms. ? n-PrOH O Me O...
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This document was uploaded on 02/10/2014.

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