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L_8_09 - • At this point we compare the structures of...

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318N – Krische, Lecture 8: Th - 02/12/09, Acyl Derivatives Carboxylic acids react with diazomethane to give methyl esters. The first step is an acid base reaction between the carboxylic acid and diazomethane. The second step is an S N 2 reaction of the resulting carboxylate and H 3 C-N 2 . Carboxylic acids can react with sodium hydroxide and the resulting sodium carboxylate can engage in S N 2 reactions with primary bromides, iodides or tosylates to provide the corresponding esters. The LiAlH 4 reduction of carboxylic acids: This mechanism has three main parts: The first part of the mechanism is an acid-base reaction between LiAlH 4 and the carboxylic acid, the second part of the reaction is an acyl substitution of the resulting aluminum carboxylate to furnish the aldehyde, and the third part is simply an aldehyde reduction, as discussed earlier in the course. Finally, we examine the conversion of carboxylic acids to acid chlorides using SOCl 2 . The mechanism of this transformation was described and is closely related to
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Unformatted text preview: • At this point, we compare the structures of acid chlorides (a.k.a. acyl halides), esters and amides. Heteroatoms bound to the carbonyl perturb the structure of the carbonyl group in two ways: through resonance via pi-bonds and through electron withdrawing effects (inductive effects) via sigma bonds. comparisons of bond length, we were able to infer the extent of resonance effects for acyl halides, esters and amides: for acid chlorides resonance was not important, for esters resonance was somewhat important, and for amides resonance was very significant. These results agree with the barrier to rotation about the heteroatom-carbonyl carbon bond. For esters, this barrier to rotation is about 13 Kcal/mol and for amides this barrier to rotation was about 22 Kcal/mol. We compared this to the strength of a C=C pi-bond: 66 Kcal/mol. • Hydrolysis of esters and other acyl derivatives was discussed....
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