Organic 2 Review Problem 350

Organic 2 Review Problem 350 - = R'OH R CO'NH'OH(I NH-O...

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334 PRACTICAL ORGANIC CHEMISTRY Section 8. Reaction with 2,4-Dinitrophenylhydrazine. This is a test for the >C=O group. Most aldehydes and ketones readily condense with this reagent* giving yellow- or orange-coloured precipitates. To a few drops of formalin solution add a few drops of dinitro- phenylhydrazine reagent A (p. 263): a yellow precipitate is produced in the cold. Acetaldehyde and acetone give orange-coloured precipitates. Dissolve water-insoluble compounds (e.g., benzaldehyde, salicylalde- hyde, acetophenone and benzophenone) in a small volume of methanol before adding reagent B. With benzophenone the precipitate forms slowly. Section 9. Reaction of Esters with Hydroxylamine. As esters are usually difficult to detect, this test is of considerable value. In general esters react when heated with hydroxylamine to give a hydroxamic acid (I). The latter gives a coloured complex (II) with ferric salts in acid solution. R-COOR' + H-NHOH
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Unformatted text preview: = R'OH + R CO'NH'OH (I) NH-O R-CO-NH-OH + Fe+++ = R-C/ (II) O . ... Fe/3 Hydroxamic acid formation resembles amide formation (pp. 117-119) and therefore certain other classes of substances will respond to the test, e.g., acid chlorides and acid anhydrides, but these substances are readily distinguished by other reactions. R-CO yo + H-NHOH = R-CO-NH-OH + R-COOH R-CO Amides themselves (e.g., acetamide) also often respond to the reaction: NH-OH R-C^ +H-NHOH -> R-C-OH-> R-CO-NH-OH+ NH 8 k Some anilides may give the reaction. To a few drops of the ester, add 0-2 g. of hydroxylamine hydro-chloride and about 5 ml. of io% NaOH solution and gently boil the mixture for 1-2 minutes. Cool and acidify with HCl, cool again and then add a few drops of FeCl 8 solution. A violet or deep red-brown colour develops immediately. * Details of the preparation of the reagent are given on p. 263....
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This note was uploaded on 09/19/2011 for the course CHM 2211 taught by Professor Castalleano during the Fall '06 term at University of Florida.

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