Organic 2 Review Problem 295

Organic 2 Review Problem 295 - with the reactive methylene...

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PREPARATIONS 279 two adjacent carbonyl groups. The reactivity is shown by the following reactions: (i) If a warm aqueous solution is treated with aqueous sodium hypoiodite solution, a reaction analogous to the iodoform reaction occurs, with the formation of p p-dimethylglutaric acid CH 2 CO Me 2 C< )CH 2 + NaIO -»- Me 2 C(CH 2 COONa) 2 H-CHI, CH 5 CO and iodoform. (2) The "Dimedone" (V) in ethanolic solution reacts rapidly with formaldehyde to give "methylene-dimedone" (VI) which, CH 2 -CO CH 2 -CO /COCH 2 2Me 2 C<; >CH 2 + HCHO -*- Me 2 C( )CH CH 2 -C^ ) CH 2 -CO CH 2 -(^O Vo- (VI) having a low solubility, readily crystallises out, and has a sharp m.p. ( c /. p. 146). Many aldehydes can be identified by their analogous "alkylidene-dimedone" derivatives, which also serve to distinguish aldehydes from ketones. Sorbic Acid. CH 3 -(CH:CH-) 2 COOH. (The Knoevenagel Condensation.) The term Knoevenagel Condensation was originally applied to the base- catalysed condensation of the carbonyl (:CO) group of aldehydes and ketones
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Unformatted text preview: with the reactive methylene group of malonic acid, with loss of water: RCHO + H 1 C(COOH) 2- R-CH:CH-COOH + H 8 O + CO 2 It is now applied more widely to include malonic acid derivatives, such as diethyl monoethyl-malonate, ethyl cyanacetate, etc. Various amines may be used as catalysts, and usually the most effective is piperidine (hexahydro-pyridine): a mixture of piperidine and pyridine, or pyridine alone, is also often used. The role of the base is apparently primarily that of a proton remover from the reactive methylene group: thus if B represents the base, reaction (i) gives the carbanion, which then combines with the positive carbon of the carbonyl group (reaction ii): the product regains a proton from the piperidinium ion, and then by loss of water followed by mono-decarboxvlation of the malonic acid residue gives the final acid....
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