S20-Week05-Lecture20-23

S20-Week05-Lecture20-23 - Carboxylic Acids & Derivatives...

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GP20-01 Alternatives to Fischer Esterification - Using S N 2 Reactions: + Overall Reaction: O H O R R O O R X R Base R O O R X + X • The carboxylate ion is a weak nucleophile - so this reaction ONLY works for reactive alkyl halides - Me–X, Et–X, allylic halides, benzylic halides are best (the • This reaction does NOT work with secondary, tertiary, or neopentyl alkyl halides. Typical examples: + O H O R R O O H 3 C I CH 3 K 2 CO 3 (the base) acetone (the solvent) intermolecular reaction acetone intramolecular reaction K 2 CO 3 H 2 C O O CH 2 H O O Br Alternatives to Fischer Esterification - Using Diazomethane: Diazomethane ( CH 2 N 2 ) is a particularly useful method for forming methyl esters. It behaves first as a base, and the resulting conjugate acid is exceedingly reactive in S N 2 reactions: O O R H 2 C N N H 2 C N N diazomethane H H 2 C N N O O R C N N H H H O O R H H H C + N N
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GP20-02 Alternatives to Fischer Esterification - Using S N 1 Reactions: + Overall Reaction: O O R R O O This method is usually only used to make tert -butyl esters. As in all S N 1 reactions the key is the carbocation. In this case it must be generated in the absence of any nucleophiles, apart from the carboxylic acid. Most other nucleophiles compatible with the reaction conditions (catalytic amount of strong acid, e.g. H 2 SO 4 ) would react more rapidly with the carbocation than the carboxylic acid: H 3 C CH 3 CH 2 H + catalyst dry conditions H 3 C CH 3 CH 2 H Mechanism: O O R CH 3 CH 3 CH 2 H The equilibria are 'driven' to the right by using a large excess of the alkene. Alternatives to Ester Hydrolysis - Using S N 1 Reactions: The above reaction can be reversed. If we start with a tert -butyl ester and treat it with a moderately strong acid in dilute solution, the equilibria are 'driven' to the right (as written below) because the alkene will be present in low concentration, and also as it's a gas at room temperature so it is lost (irreversibly) from the reaction solution. H H H O O R CH 3 CH 3 CH 2 H O O R CH 3 CH 3 CH 2 H O O R CH 3 CH 3 CH 2 H H R O O H 3 C CH 3 CH 2 H H + R O O H + H 3 C CH 3 CH 2 Overall Reaction: Mechanism: MeO OCMe 3 O O R MeO OH O O R F 3 CCO 2 H (pK a 0.2), CH 2 Cl 2 (solvent) Example: tert -Butyl esters are stable to mild acids such as acetic acid (pK a 4.8) and mild acid work-up –H + +H + –H + +H +
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GP20-03 H 3 C O CH 3 O H 3 C O O CH 3 This O has 2 lone pairs, one in an sp 2 orbital and one in a p orbital The O lone pair in the p orbital overlaps strongly with the C=O ! * (it's the one involved in resonance). This interaction (stabilizing) is the
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This note was uploaded on 01/25/2011 for the course CHEM S20ab taught by Professor Mccarty during the Summer '10 term at Harvard.

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S20-Week05-Lecture20-23 - Carboxylic Acids & Derivatives...

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