I - ORGANIC DEUTERIUM COMPOUNDS XIII. THE MECHANISM OF THE NEF REACTION SYNTHESIS OF ETHANAL-1-dl

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1 ORGANC DEUTERUM COMPOUNDS X THE MECHANSM OF THE NEF REACTON SYNTHESS OF ETHANAL-1-dl ABSTRACT The mechanism of the Nef reaction was investigated with nitroethane-1,l-d2 The isolation of 95 mole % ethanal-1-d in 70% yield renders a mechanism proposed by Mahler untenable, but supports others submitted by van Tamelen and Thiede and by Nametkin NTRODUCTON The conversion of a primary nitroparaffin into an aldehyde or of a secondary nitroparaffin into a ketone is known, after its discoverer, as the Nef reaction (9) ts scope was later investigated by Johnson and Degering (4) when nitroparaffins became readily accessible from the nitration of aliphatic hydrocarbons Mahler (7) defined the optimum conditions for the preparation of propionaldehyde from 1-nitropropane and suggested "the highly tentative mechanism" shown below R-C-NHOH - R-CH +NH(OH): lafanz~script received October 65, 1964 Contribution from the Division of Pure Chemistry, National Research Laboratories, Ottawa, Canada ssued as NRC No 3489 Presented at the 126th meeting of the American Chemical Society, New York City, September,

2 LETCH: ORGANC DEUTERUM COMPOUNDS 40 1 This mechanism hinges on the addition of a inole of water in one direction, its elimination in another, then addition of a second mole of water, followed by hydrolysis The addition of water in the manner indicated is improbable on account of the charge distribution between carbon and nitrogen Also, this mechanism cannot account for the formation of ketones from secondary nitroparaffins An alternative mechanism which is shown below was recently advanced by van Tamelen ancl Thiede (11) These authors point out a parallel between the Nef reaction and the hydrolysis of oximes and present evidence strongly in favor of this scheme i The correctness or otherwise of these mechanisms may be readily tested using deuterated nitroethane as a tracer; for, if the reaction proceeds as \ proposed by Mahler, nitroethane-1,l-dz (CH3CD2N02) should give normal acetaldehyde because the hydrogen at C-1 is lost as water in step (c) and acetaldehyde is formed by hydrolysis in step (e) On the other hand acetaldehyde deuterated in the functional group should result if the reaction follows the course suggested by van Tamelen and Thiede (11) Nitroethane-1,l-d2 was prepared by exchanging nitroethane with deuterium oxide as described for nitromethane in a previous paper (5) but in the presence 1 of a weak base After two exchanges the bands at 1330, 1255, 1132, 994, and 811 cm- in the infrared spectrum of CH3CH2N02 were reduced in intensity, while new ban'ds corresponding to CH3CDzN02 appeared at 1305, 1289, and 1025 cm- The mass spectrum of nitroethane shows no parent pealc, but measurement of the pealc intensity due to CH3CD2 indicated that the com- pound contained about 45 mole % CCH3CD2N02 and unknown amounts of CH3CHDN02 and CH3CH2NOZ There was no peak corresponding to three deuterium atoms in the molecule so deuteration therefore does not extend to the H in the methyl group The deuterated nitroethane was subjected to the Nef reaction essentially as described by Johnson and Degering (4) except that the acetaldehyde itself was isolated instead of its semicarbazone The mass spectrum of the acetaldehyde indicated it was largely CH3CD0 The mechanism of the Nef reaction as proposed by Mahler (7) is therefore incorrect On the other hand, the present work supports the alternative andsimpler mechanism of van Tamelen and Thiede (11) +

3 402 CANAD4N JOURNAL OF CHEMSTRY VOL 33 A minor shortcoming of their interpretation of the Nef reaction is its failure to account for the appearance of a transient blue color in the reaction mixture prior to the evolution of nitrous oxide Nametkin (8) attributed this color to the formation of a nitroso intermediate* t is possible to modify the mechanism of van Tamelen and Thiede (11) to include a nitroso intermediate as shown below R OH R \ -+/ \ /OH - H?O C=N --f C-N + /T \ R' 1 0- /v H H ( \ R OH OH The Nef reaction is well adapted to the syilthesis of ethanal-1-d and other aldehydes deuterated only in the formyl group on account of its simplicity and the high isotopic purity of the product, which is over 95y0 if we base our results on the theoretical calculation of Brinton and Blacet (3) for the mass ratio CDO/CHO obtained by mass spectrometry The apparent discrepancy between the deuterium coiltent of the nitroethane and that of the acetaldehyde prepared from it inay be readily reconciled by reference to data in a paper by Wynne-Jones (12) According to this author the relative rates of ionization of the hydrogen and deuterium atoms in nitroethane are in the ratio 10: 1 Therefore, assuming the deuterated nitroethane contained 50% of the isotopic species, CH3CHDN02, its ion, CH3CD= NO1 will be present to the extent of 45y0; this, added to the 45y0 from CH3CD2N02, would lead to acetaldehyde containing goyo CH3CDO Ethanal-1-d has been synthesized by Blacet and Brinton (2) in five steps starting from butyne-2 Another synthesis from diacetyl was reported recently by Loewus et al (6) The first is tedious and leads to a product of low isotopic purity, while the second requires lithium aluminum deuteride which is both difficult to obtain and very expensive Moreover, these methods are not suitable for the synthesis of other aldehydes deuterated in the functional group because the starting materials are not readily accessible *This information was kindly supplied by Dr W E Noland, Department of Chemistry, University of Minnesota, Minneapolis, iminn

4 LETCH: ORGAXC DEUTERUM COMPOUNDS 103 The ethanal-l-d reported in this work has since been used by others in these laboratories in spectroscopic and photocl~emical studies (1, 10) EXPERMENTAL Nitroethane-,-&, CHsCD2N02 A mixture of nitroethane (25 ml) and deuterium oxide (25 ml) containing 10 mgm of dissolved anhydrous sodium acetate was heated overnight in a sealed tube in a rocking furnace at 90 C The tube containing the deep-yellow reaction mixture was opened, attached to a vacuum line, frozen, and evacuated The supernatant layer of deuterated nitroethane was distilled off through a U-tube containing Drierite and condensed in a trap at -78OC yield of colorless liquid was 190 ml The loss is due to the formation of a water-soluble product by a side reaction which was not further investigated The exchange was repeated with an equal volume of fresh deuterium oxide and sodium acetate The yield of nitroethane-1,l-dz analyzing 45 mole % CH3CD2N02 was 110 ml Nitroethane-1,l-d2 (30 ml) was dissolved in 20 ml of ice-cold 10yo sodium hydroxide in a small separatory funnel The solution was added slo\vly dropwise to one of 6 ml sulphuric acid dissolved in 40 ml of water, which was kept at 0 to 5OC and stirred continuously The addition of the sodium nitroethane solution produces a characteristic blue color in the sulphuric acid which fades as nitrous oxide is evolved Any acetaldehyde entrained in the nitrous oxide was condensed in a U-tube coolecl to -40 C ~vitll dry ice and acetone Stirring was continued for 15 min after all the sodium salt solution had been added When the reaction mixture mas heated the acetaldehyde distilled and condensed in the U-tube The product was then distilled from the U-tube into a graduated trap on the vacuum line Yield: 08 ml The mass spectrum gave the following principal peaks: 46 (CH2D CDO) (CH3CDO) (;DO) (6~0) 2483 Evidently, ethanal-l-d is large1 y produced in this reaction rather than normal acetaldehyde There is no advantage in dissolving the deuterated nitroethane in sodium deuteroxide n fact, this leads to a higher peak at 46 due to CH2DCD0 produced by the exchange: ACKNOWLEDGMENTS The author is grateful to Dr D A Ramsay for the interpretation of the infrared absorption spectra and to Miss F Gauthier for the mass analyses

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