Oxygen-Induced Vapor-Phase Nitration of Paraffins H. B. HASSl AND L. G. ALEXANDER2 Purdue University and Purdue Research Foundation, Lufayette, Ind.
I.
'i'he eiyect. on the nitration of methane with nitric acid, of adding separately nitrogen, nitric oxide, and ox>gen to
Tlic first step in this process is known to occur (j), but thrrr r i o evidence for the second. I t has also been proposed that nitration proceeds by the uriioii of nitric oside with an alkoxy free radical (possibly present under oxidizing conditions), follovied by the isomerization of the result ing nitrite t o the corresponding nitro compound. The isomerization of nitrites in very poor yields and conversions has been reported ( 1 5 ) but not confirmcd, and there is no evidence for tht, first step in this process. of either pri~posal,it n-ould be predicted that i l l creasing the nitric oxide concentration should increase the conversion. On the other hand, nitric oxide is oxidized to nitroget: llioxitle by oxides and 0x3- acids of nitrogen that are in a higher atnte of oxitintion than nitrogen dioxide. Since nitric acid give$ better conversions than does nitrogen dioxide ( 7 ) ,the addition of nitric oside should, from these considerations, decrease the conversion i n nitrations employing nitric acid as the agent of nitriitioi,. 1g:rin: if free alkyl radicals are involved in the nitration reaction, then nitric oxide (if it is not the agent of nitration) migh: decrease the conversion by uniting with them and preventing their ui.iion with the nitrating agent.
the reaction mixture, and the effect of adding oxygen on the nitration of propane with both nitric acid and nitrogen dioxide, were studied. Nitrogen did not affect the reaction in concentrations up to 307'; nitric oxide reduced the conversionof nitric acid to nitromethane and incrcased the acidity of the product; oxjgen increased the conversion nioderatclj, increased the acidity of the product, increased the amount of formaldeb) de and other methaneoxidation products, and decreased the \olume of the exit gases bj one tenth. When propane was nitrated with nitric acid in the presence of excess ox)gen at 410" C., the conversion was increased from 28 to 6 2 7 ~the , aciditj of the product was not changed much, the amount of nonnitro compounds boiling below 80" C. was increased, and the volume of the exit gakes was reduced by one fifth. I t 395' C. the conFersion was increased from 20 to 767' by the addition of 3.8 moles of oxygen per mole of nitric acid, rrhile the \olunie of the exit gases was reduced b\ one third. When propane was nitrated with nitrogen dioxide at 410" C., the addition of 3.1 moles of oxygen per mole of nitrogen dioxide increased the conversion from 14 to 2.557~.
TARLF. 11.
HE purpose of this investigation was
study the effec.t O I I T t h e \'a p or-phase nitration of paraffms of adding sepnrn:el? nitrogen, nitric oxide, and oxygen to the reaction mixture. Thr process for nitrating paraffins in the vapor phase was disclosed by FIass, Hodge, and Vanderbilt in 1936 (8). T h e nitratiori 01' methane was first reported by Landon (12, 1 3 ) , and \ m s subseRideout (16), anti 1% quently studied by Boyd and Hass (4), Shechter, Alexander, and Hatcher ( 1 1 ) . The mechanism of the nitration reaction was studied by McCleary and Degering (1.; j , and Hass and Shechter enumerated thirteen rules regnrtiing t!ie react,ion (10). Rideout (16) disclosed a process for nitrating methane iri r l i e presence of excess nitrogen, and gives the data shown in Table I. .4n increase in the conversion of nitric acid to nitromethane was cobtained Tvhen 2.5 moles of nitrogen were added per mole of nitri. acid; however, in these experimentfi the ratio of methane t o nitrir;) cid was also varied.
10 0 22.0
7.5 2.5 24.3
5.0
5.0 18.8
.;
2 7.7 12.S
: 1)
6.4
It has been postulated that the mechanism of nitration involves the interaction of nitric oxide (which is a by-product of the reaction) with free alkyl radicals (which are almost certainly present in the reaction mixture, according to Degering and McCleary, I d ) , and t h a t the resulting nitroso compound is oxidized to a nitro compound before i t can tautomerize to the oxime. 1
Present address, General Aniltne and Film Corporation. 230 Park .iw
New York, N. Y . 2
MIXTURESA T 435' C.
10
TABLE1. DATAOF R.IDEOL-T (16) Lloles C:Hii mole HKOs 3ioles r\z/niole "Os Conversion of CHI t o hZeSOz, %
DECO\fPOSITION O F S I T R I C A C I D IS S I T R A T I O K
Present address, Unirer.ity o f Iilinoip, I'rhans. Ill.
Fraction of "Os - _ _ NO
_
Sloies of 1
2.5 1 0
01
0.3s 0.50 0.57 0.63
Present as _ ~
_
.
KO1 0.62 0.50 0.43
0.37
Hass predicted early that addition of oxygen to the reactioil mixture might increase the conversion of nitric acid t o nitro psrafins ( 8 ) . T h e process of nitration in the presence of excess oxygen was disclosed by Hass, Hodge, and Vanderbilt (9) in H patent which claimed: "13. The process of nitrating a saturated aliphatic hydrocarbon having from 3 to 8 carbon atoms inclusive Khich consists in producing contact between such saturated aliphatic 1iydroc:irbon and nitric acid in the presence of free oxygeii in excess of that formed by the decomposition of nitric acid." Hass ( 8 ) pointed out later t h a t nitric acid has negligible thermoiijnamic stability at nitration temperatures (400 O to 450 O C.), and a-ould not be formed t o any significant extent from oxygen, watrr. and oyides of nitrogen in the reaction zone. Hence, oxygen could not act to increase the conversion by reoxidizing nitric oxide t o nitrir acid. Sitrogen dioxide, on the other hand, is relative]! StatJk at, nitration temperatures. Table 11 shows the fraction of nitrogen present as nitrogen dioxide a t equilibrium (in the acsunied absence of nitration and oxidation reactions) in mixturclformed by heating 1 mole of nitric acid, 15 moles of methane, 2 moles of water, and the number of moles of oxygeri 4iown in the left-hand column. Thus, the addition of 5 moles of oxygen per mole of nitric: acid would result, if equilibrium were attained, in an increase in the effective concentration of nitrogen dioxide by a factor of 1.6 However, before predictions based on these considerations concerning the effect of oxygen are made, the rate of appros.rh t o
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October 1949
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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equilibrium as vie11 as the effectiveness of nitrogen (dioxideas an agent of nitration must be considered. Seither the rate nor the primary products of che decomposition of nitric acid are known. At nitration tempwatures the half life of nitric acid must be of the same order of magnitude as the
