970
GUENTHER VON ELBEAND BERNARD LEWIS
Vol. 59
u. s.
[CONTRIBU'IION F R O M THE PIT'l'SBUR(;H EXPERIMENT STATION, B U R ~ ~ OF A UM i N E s , AND THE C O A L R E S E A R C H LABORATORY, CARNECIE INSTITUTE OF TECHNOLOGY ]
The Combustion of ParafEn Hydrocarbons' BY
GUENTHER VON ELBE*AND BERNARDLEWIS'
Norrish' has proposed a chain mechanism of the oxidation of hydrocarbons which describes the slow and explosive reaction of methane in agreement with a number of observations; i t does not describe the phenomenon of the lowtemperature explosioii peiiirisula arid other associated effects ("cool" flames, maximum i n the reaction rate found in the neighborhood of ;430' for all paraffin hydrocarbons above methane) ; it also does iiot explaiii the formation of methyl alcohol i i i large quantities in the oxidation of rich mixtures of propaneb a t low temperatures, a fact which, according to €'raw, caii best be explained by a chaiii mechanism tiiivolving motiovalent (alkyl aiid alkoxyl) radicals. Pease's mechanism applies exclusively to this observation and does i i o t describe the experimental facts accounted for by Norrish's scheme. A more extensive mechanism was proposed by I!bbelohde" but was not treated kiiietically. In the preseiit paper a mechanism of the oxidation of paraffin hydrocarbons is proposed which is believed to allow a more comprehensive iriterpretation of chemical and kinetic facts.
quent chain reactions in the gas phase. Thus, the chemistry of hydrocarbon and aldehyde oxidation being intimately related, it is advantageous to consider first the aldehyde oxidatioii. Oxidation of Aldehydes.-Two oxidation reactions of aldehydes have beeii reported. One is a catalytic surface reactiong which is particularly proiiounced at low temperatures (not much above room temperature) but decreases toward higher temperatures. 'This reaction passes through a peracid stage to carbon dioxide as the chief oxide of carbon, the process being strongly accelerated by coating the surface with potassium ~ h l o r i d e . ' ~ The other reaction is a gas-phase reaction unquestionably of the chain it proceeds through a step-wise degradation to lower aldehydesIUYby the intermediate formation of peracid^,^,''' carbon monoxide being the only oxide of carbon formed. Formaldehyde cannot degrade further but is oxidized to carbon monoxide and water. The chain character of the gas-phase reaction is demonstrated by the inhibitory action of packing the vessel and by the large quantum yield in the photochemical oxidation of acetaldehyde. I ' The primary products in the photochemical reaction consist largely of monovalent radicals arising from the photodissociation1* CHsCHO + hr == CH, HCO (1)
Chemistry of the Oxidation I t is generally recognized that the oxidatioii of hydrocarbons passes through the aldehyde stage to the ultimate products carbon monoxide, carbon dioxide arid water. Sirice aldehyde added ill small amounts to a hydrocarboii-qxygeii mixture very markedly decreases the induction pe- In the absence of oxygen this would be followed riodI7the latter is interpreted as the period dur- by CH, + CHaCHO = CH, CHsCO (2) ing which the aldehyde gradually accumulate^.^^^ Its preseiice appears to be necessary for the crea- In the presence of oxygen i t is plausible that tion of chaiii ceiiters responsible for the subse- 0, adds directly to the C&CO forming a peroxidic ( I ) Published by perirlis2:1. Measurements show that the rate is still increasing for a 2 : l
Summary It is pointed out that there are certain shortcomings in existing theories which are overcome in the proposed theory. The concept of chain initiation v i a aldehyde is seen to require discussion of aldehyde oxidation. A chain mechanism consistent with experimental facts and chemical plausibility is described. The chain mechanisms of aldehyde and hydrocarbon oxidation are linked to each other and combined with a modified form of Pease’s mechanism for higher hydrocarbons. Plausible relations between the high pressure oxidation of methane and ethane and the chemistry of peroxides are discussed. From this, a branching reaction involving alkyl peroxides and aldehydes which is operative predominantly a t low temperatures is shown to be plausible chemically. It is called “peroxide branching” to distinguish it from “peracid branching” operative predominantly a t high temperatures. The mechanisms of primary aldehyde forming reactions and secondary non-chain reactions are discussed. It is shown that kinetic facts concerning the slow reaction and explosion limits of methane are consistent with the scheme; that certain phenomena exhibited by higher hydrocarbons a t relatively low temperatures (ex-
984
RICHARD SALATHIEL, J. MARVINBURCHAND R. M. HIXON
plosion peninsula, maximum in the reaction rate, "cool" flames) can be described by peroxide branching; and that peracid branching can de-
[CONTRIBUTION FROM
THE
Vol. 59
scribe the explosion limits of higher hydrocarbons a t high temperatures. PITTSBURGH, PA.
RECEIVED SEPTEMBER 11, 1930
DEPARTMENT O F CHEMISTRY, IOWA STATE COLLEGE]
Synthesis of Some New Alpha-Substituted Tetrahydropyridines and Piperidines' BY RICHARD SALATHIEL, J. MARVINBURCHAND R. M. HIXON A general method for the preparation of a-substituted tetrahydropyridines has been developed, a preliminary report of which was presented by Starr and Hixon,2 who reported the preparation of a-phenyltetrahydropyridine. This preparation followed the general plan of the method used by Craig, Bulbrook and H i ~ o n who , ~ reported the preparation of a-substituted pyrrolines by a modification of Cloke's r e a ~ t i o n . ~This paper extends this synthesis .to the preparation of other compounds in this series, and to the preparation of the corresponding piperidines. The reactions involved are first the reaction of a Grignard reagent with 6-chlorovaleronitrile to form the mixed magnesium bromide salt of a 6-chlorobutylketimine, Cl(CH2)dCM RMgBr -+ ClCH2CHzCH2CH2-CR=NMgBr, and second the closing of the ring which is accomplished by replacing the ether in the reaction mixture with xylene and raising the temperature. Expressing this reaction by equations completely analogous to those proposed by Craig, Bulbrook and H i ~ o ntwo , ~ possible mechanisms are shown.
as the reaction mixture waq stirred and refluxed The addition product, a heavy viscous colorless liquid, separated from the solvent. The ether was removed by di5tillation from an oil-bath, the volume in the flask being kept constant by the addition of xylene. When the temperature in the flask reached 130-135', the liquid addition product quickly changed to a solid, semi-crystalline, yellow mass. Heating just sufficient to reflux the xylene was continued for an hour. The material was cooled and hydrolyzed, using 130 cc. of hydrochloric acid (sp. gr. 1.10). The xylene layer was separated and extracted again with hydrochloric acid. The acid solution was extracted with 20 cc. of petroleum ether to remove residual xylene, then made strongly basic with sodium hydroxide and extracted with ether. The ether extract was dried and fractionated. There was collected a t 142-150" (20 mm.) 21 g. of a nearly colorless oil, which turns brown quickly on exposure to air. The yield is 66% of the theoretical. The hydrochloride, precipitated from aqueous acetone, melted a t 86-87' as reported by Gabriel6 but when dried in a curtent of warm air the melting point becomes 152-153". Other derivatives, the picrate, m. p. 181.5", and the mercuric chloride-hydrochloride double salt, m. p. 165-166', were prepared. a-(p-Chloropheny1)-tetrahydr0pyridine.-A Grignard reagent prepared from 2.43 g. of magnesium, 21.1 g. of p chlorobromobenzene and 125 cc. of ether, was added to a solution of 11.8 g. of 6-chlorovaleronitrile. A CICH~CHLCH~CH~-C-R CICH2CH2CH2CH~C--K procedure similar to that for a-phenyltetra/I hydropyridine yielded 9.5 g. of a colorless base HN-MgBr boiling at 165' (13 mm.) which crystallized in N-MgBr . C the receiver, m. p. 54'. The yield is 49% of c/ \o e/' C"C the theoretical. I I f MgBrCl + I/ A picrate, m. p. 177-178', was crystallized C\ /C -K C\ /C--R N/ NI from an alcohol solution. A mercuric chloridehydrochloride double salt, m. p. 133-135', a H chloroplatinate, m. p. 192', and a hydrochloride, m. p. 215-217', were prepared from aqueous solutions. Experimental Part 6-ChloroPa]eronitrile, b p, 118-121" (25 mm,), was preAnal. Calcd. for C11H1KlN.HCI: N, 6.09. Found: N, 6.19, 6.15. pared as described by Star; and Hixom2 a-Pheny1tetrahydropyridine.-A Grignard reagent was CY-( +Tolyl)-tetrahydropyridine.-A Grignard reagent prepared from 4.86 g. of magnesium, 34.5 g. of bromoben- prepared from 2.5 g. of magnesium turnings, 21 g. of pzene and 160 cc. of ether. A solution of 23.5 g. of 6- bromotoluene and 120 cc. of ether was treated with 11.7 chlorovaleronitrile in 25 cc. of ether was added dropwise, g of 6-chlorovaleronitrile. The product was obtained by a procedure similar to that for a-phenyltetrahydropyidine. (1) Supported in part by grants from the Rockefeller Fluid ReThe major portion Of the g g. Of the product Over search Fund and from the Industrial Science Research Funds of the a t 145' (15 mm.). The yield is 52% of the theoretical. Iowa State Colleae. (2) Starr and Hixoo, THISJOURNAL, 66, 1595 (1934). The following derivatives were prepared : chloroplati-
+
.1.
'
(3) Craig, Bulbrook and Hixon, ibid., 68, 1831 (1931). (4) Cloke. ibid., 61, 1174 (1929).
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15) Gabriel, Bcr.. 41, 2012 (1908).
