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Leila Section Week 6b

Leila Section Week 6b - Chemistry 35 Organic Monofunctional...

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Chemistry 35: Organic Monofunctional Compounds Section 6: November 3, 2008 Prof. Huestis, Fall 2008 TA: Leila Yeh Beach Amides, Acid Halides, Nitriles 1] Weak Nucleophile Reactions So, we’ve gone over weak nucleophile reactions for aldehydes, ketones, carboxylic acids, esters, amides, acid halides, and nitriles. It’s worth looking at all the mechanisms side-by- side so that we can identify similarity (there’s lots of them) and differences (there aren’t many of them). This way, you don’t have to memorize seven different mechanisms—you can just understand the fundamentals and tweak your mechanism depending on the situation. Amides Acid Catalyzed Overall Reaction Mechanism Base Mediated Overall Reaction Mechanism R NR'' 2 O H + R NR'' 2 OH R NR'' 2 OH R NR'' 2 OH H 2 O R NR 2 OH 2 + HO H 2 O R NR 2 OH HO H + R NHR 2 OH HO R OH OH R OH O HNR 2 H 2 NR 2 R OH O R NR'' 2 O H + , H 2 O R OH O R NR'' 2 O OH - , H 2 O H 2 O OH - OH - R NR'' 2 O R NR'' 2 O HO R'' 2 N R OH O R 2 NH R O O
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Important Notes o Amides are not very reactive (less reactive than esters) so, both of these reactions require stringent heating in strong acid/base. o The first step in both of these reactions is meant to increase the willingness of the nucleophile to attack the carbonyl carbon of the amide. For the acid catalyzed reaction, this is accomplished by increasing the delta positive (or electrophilicity) of the carbonyl carbon. For the base mediated reaction, this is accomplished by increasing the strength of the nucelophile. This is exactly the same as the first step in acid/base catalyzed hemiacetal/hemiketal from aldehydes/ketones. o All weak nucleophile addition/eleminiations are fundamentally the same. Something is added to the carbonyl carbon and something else eliminates. In the case of ester hydrolysis to a carboxylic acid, water adds and an alcohol is eliminated. In both of the reactions above, water (or, in the case of the base mediated reaction, deprotonated water) adds and an amine leaves. Acid Halides Overall Reaction Mechanism Important Notes o What’s different about this mechanism with the amide ones given above? Acid halides are super reactive. They have a huge partial positive charge on the carbonyl carbon. So, acid halides don’t require acid/base catalysis to react with weak nucleophiles. The first step in this reaction, then, is not an acid/base
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