L_7_09 - intermediate, which (c) eliminates to reform the...

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318N – Krische, Lecture 7: Tu - 02/10/09, Carboxylic Acids and Esters In this lecture, we begin carboxylic acids. We’ve previously encountered several methods for the formation of carboxylic acids: (a) the reaction of Grignard reagents with carbon dioxide, (b) the reaction of primary alcohols with chromic acid, (c) the reaction aldehydes with Tollen’s reagent, and (d) the haloform reaction. In the Fischer Esterification, a carboxylic acid and a primary (or secondary) alcohol may be condensed using an acid catalyst to provide an ester. The first part of this mechanism is analogous to the mechanism for hemi-ketal formation. The second part of the mechanism is similar to the reverse of acid catalyzed ketone hydration. Note: The Fischer Esterification reaction is an example of an “acyl substitution. ” Acyl substitutions mechanism all involve 3 fundamental steps: (a) carbonyl addition, which leads to (b) a tetrahedral
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Unformatted text preview: intermediate, which (c) eliminates to reform the carbonyl moiety. Basic catalyzed esterification of carboxylic alcohols using alcohols does not work due to acid-base chemistry, i.e. deprotonation of the carboxylic acid. The conversion of esters to carboxylic acids is known as ester hydrolysis. Under acidic conditions, the mechanism is simply the reverse of the Fischer esterification. Ester hydrolysis can also be conducted using aqueous base. The mechanism for basic hydrolysis or saponification was discussed in class. It is another example of an acyl substitution. Accordingly, the mechanism involves the same 3 fundamental steps described above. Using methanol, compare and contrast the mechanisms for acid catalyzed ketal formation and acid catalyzed esterification....
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