Chemistry 2302 Monday, July 7 In-Class Exercise: Electron Pushing in Carboxylic Acid Derivative Exchange In this exercise, you will be practicing drawing mechanisms for the general reaction: Wade shows many examples of mechanisms for this general class of reactions in Chapters 20 & 21, but I feel that it is possible to come up with all of those mechanisms on your own, as long as you follow a few simple rules. 1. As we’ve discussed before, under acid catalysis, try not to create anionic bases in your mechanism; if you can, keep everything either neutral or positively charged. For example, in an acid-catalyzed ester hydrolysis reaction, the -OH and -OCH3 groups exchange, and we need to draw a mechanism in which the water oxygen adds and the methanol oxygen leaves. But we can’t just start with water attack; instead of drawing we can avoid the unstable intermediate by protonating the oxygen first: ROXHY+ ROYHX+ H3O+ or –OH catalyzed H3COOCH3OHHH3COOOCH3HHcreates negatively charged alkoxide, won’t happen in acid. HOHH3COOCH3+ H3COOH+ HOCH3H3O+In the space below, finish the mechanism for the acid-catalyzed hydrolysis of an ester. H3COOCH3OHHHH3COOCH3HOHHH3COOHOCH3HHOHHH3COHOHOCH32. By the same token, under base catalysis, try not to create cationic acids in your mechanism; if you can, keep everything either neutral or negatively charged. In other words, if your electron pushing creates a positively charged atom, ask yourself whether that atom could be deprotonated first before doing what you are doing. For example, the base-catalyzed transesterification reaction, starts differently from the acid-catalyzed reaction. How? Draw the mechanism for this reaction. H3COOCH2CH3H3COOCH3+ + HOCH3-OH (just a catalytic amount)
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