Organic Lab Reactions 185

Organic Lab Reactions 185 - pound The exact mechanism of...

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180 REDUCTION WITH ALUMINUM ALKOXIDES with unsaturated aldehydes and ketones. Its general value, however, was not fully apparent until 1937, when Lund B applied the method to a variety of aldehydes and ketones and studied the scope and limitations of the reaction. He also developed a simple method for determining completeness of reduction, which consisted in testing the distillate for acetone with 2,4-dinitrophenylhydrazine. The Nature of the Reaction The reduction of aldehydes and ketones is carried out very easily. The carbonyl compound and aluminum isopropoxide, prepared from aluminum and isopropyl alcohol, are heated in boiling isopropyl alcohol solution with provision for slow distillation until no more acetone is formed. The general equation may be represented as follows. R' H CH 3 R' H CH 3 \ \ / \ / / c=o + c <=± c + o=c / A l / \ / \ Al \ R ^-0 CH 3 R 0 - ^ CH 3 The reaction involves the transfer of one valence bond of the aluminum atom and one hydrogen atom from the alkoxide to the carbonyl com-
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Unformatted text preview: pound. The exact mechanism of this transfer is unknown, although an intermediate aluminum derivative of a hemiacetal (I) has been postu-lated. 1-2> s When isopropyl alcohol is the solvent the aluminum iso-R' O—A1(OC 3 H 7 ) 2 H CH 3 \ / \ / C 0 C R CH 3 i propoxide may be considered a catalyst, since an exchange reaction of the solvent with the aluminum derivative of the reduction product regener-ates the aluminum isopropoxide. However, it is "usually preferable to employ enough of the reagent to accomplish the reduction directly. It has been shown that under the influence of light 6 or at temperatures of 200 to 300° 3 the equilibrium between the alcohol and the carbonyl com-pound is brought about slowly without a catalyst. At these high tem-6 Lund, Ber,, 70, 1520" (1937); also Kern. Maanedsblad, 17, 169 (1936) [Chem. Zentr. (I), 3480 (1937)]. 6 Ciamician and Suber, Ber , 33, 2911 (1900); 34, 1530 (1901)....
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