US2294849A - Manufacture of nitric acid esters - Google Patents

Abstract

540,050. Alkyl nitrates. STEVENS, A. H. (Sharples Solvents Corporation). March 29, 1940, No. 5722. Drawings to Specification. [Class 2 (iii)] Alkyl nitrates are produced by reacting in alcohol with nitric acid of 35-68 per cent. strength under a pressure of 20-650 mm., the nitrate being removed by azeotropicdistillation, water (or in the case of ethyl or methyl nitrate, the ester) being returned from the distillate or otherwise to the reaction vessel to prevent accumulation of ester or water. The invention is applicable to the esterification of dihydric alcohols. Urea is preferably added during the process, and the water to be returned may be added in the form of solvent or diluent of the nitric acid, urea or alcohol. Examples 1-11 describe the esterification of n-butyl, 2-chlorethyl, mixed amyl, secondary amyl, iso-amyl, n-amyl, n-hexyl, secondary hexyl, 2-ethoxyethyl, cyclohexyl, and octyl nitrates, using the reagents in the liquid phase. The alcohol may be supplied in the liquid or vapour phase. Another example describes the production of isopropyl nitrate, the alcohol being supplied in the form of vapour above the surface of the nitric acid. Specification 379,312 is referred to.

Description

Sept. Il, 1942.

MANUFACTRE oF J. F. OLIN El AL NITRIC ACID ESTERS Filed Aug. 20, 1938 To Vncuur l FREDERICK PR1Tscu BY JOSEPH J. SCHQEFER.

iowa/afa. a. OWMTTORNEY Patented Sept. l, 1942 MANUFACTURE 0F NITRIC ACID ESTERS John F. Olin, Grosse Ile, and Frederick P. Fritsch, Wyandotte, Mich., and Joseph J. Schaefer, Philadelphia, Pa., assignors to Sharples Chemicals Inc., a. corporation of Delaware Application August 20, 1938, Serial No. 225,908

(Cl. 26o-467) 7 Claims.

This invention relates to a process for the preparation of nitrate esters from aliphatic alcohols. The nitration of aliphatic alcohols to obtain esters of nitric acid has long presented a problem of considerable commercial importance. The potential importance of such esters is very great since they are useful as intermediates in the preparation of a wide variety of organic chemicals in addition to possessing properties that render them suitable for many purposes as ultimate products. Among the elds of usefulness in which these compounds show greatI promise, that of ignition promotion in Diesel engines is believed to be highly important. For example, a relatively small quantity of an alkyl nitrate improves the combustion characteristics of Diesel fuel to a remarkable degree.

Heretofore, the development of uses for compounds of the alkyl nitrate type has been hampered by the lack of a process for producing such materials on a commercial scale at a reasonable price. Although it is well known that nitric acid reacts with the alcohols to form nitrates, no really satisfactory method for conducting the reaction has been heretofore developed. Under present practice the yields of nitrated products, with very few exceptions, are loW and the operations are dangerous. l

Two procedures for the preparation of nitrates from methyl, ethyl, propyl, butyl and amyl alcohols have been described. One method, which has been used to prepare nitrates from monohydric alcohols containing less than three carbon atoms, consists essentially in the distillation of a mixture of nitric acid and alcohol in the presence of urea. The nitrate formed is readily volatilized and may be distilled from the reaction zone to prevent accumulation of the ester in the nitrating vessel which might otherwise result in explosions. Since the presence of even small amounts of nitrous acid tends to initiate a vigorous auto-oxidation of explosive violence, urea is added to decompose nitrous acid as formed in accordance with the following equation:

By the above process the methyl and ethyl nitrates may be prepared with fairly good results, but the method is not suitable for the nitration of alcohols containing three or more carbon atoms. In the case of the nitration of propyl or butyl alcohol or alcohols of higher carbon content than butyl alcohol, higher temperatures are required to obtain the esterifcation reaction than when the lower alcohols are nitrated. Higher reaction occurring incident to nitration of the alcohol and therefore preventing the successful practice of the nitration process. In such cases the oxidation of the alcohol can be to some eX- tent prevented by the addition of urea to the reaction mixture, but even when this precaution is taken the yield of the desired nitrate is still very low. In the course of the oxidation reaction occurring when attempts are made to nitrate the higher alcohols, the nitric acid intended for esterication is reduced to nitrous acid. The oxidation of the alcohol is therefore a deleterious reaction both from the standpoint of producing undesired -by-products and from the standpoint of reducing the esterifying acid. Even with the lowest alcohols the yieldl is rarely in excess of 75% ("Systematic Organic Chemistry, Cumming, Hopper and Wheeler, page 254) A process of nitrating isoamyl alcohol Vwhich has been successful from a laboratory viewpoint involves the addition of isoamyl alcohol-or a mixture of the alcohol with concentrated sulfuric acid to an ice-cold bath of nitric and concentrated sulfuric acid in the Volume ratio of 1:3. This process has not achieved commercial success because of the diiliculties of recovering unused nitric acid and of reconcentrating the sulfuric acid. A more serious defect of this method lies in the frequent explosions occurring unless the alcohol is very pure.

Neither of the above processes gives satisfactory results in the nitration of monohydric alcohols other than primary alcohols and no satisfactory method has been heretofore devised for the preparation of secondary alkyl nitrates. The yields obtained by treating a secondary or tertiary aliphatic alcohol by either of the methods described above are negligible. Further, ithas been found that the rst of these methods is unsuitable for the treatment of alcohols higher than ethyl.

It has been found that the use of dilute nitric acid avoids the objections to prior processes to some extent. Again, using amyl alcohol as an example, it was found that moderate conversion of alcohol to nitrate was obtained by treatment with 35% to 60% nitric acid. Concentrations below 30% gave very low-grade esters while the use of acid having a nitric acid content in excess of 60% resulted in substantial oxidation with consequent decrease in yield. It has now been found that the objectionable features of prior art processes may be avoided to a large extent by lowering the pressure under which the nitration is accomplished and keeping the nitric acid dilute, and of substantially uniform concentration by the process hereinafter described.

An object of this invention has been the provision of a process for reacting an aliphaticl Experimental determinations indicate that amyl nitrate and water form a constant boiling mixture at 20 mm. absolute pressure containing 1.8'parts of water to 1 part of ester and that mixed amyl alcohols distilled with water at the same pressure give a constant boiling mixture in which Water and alcohol are present in the approxi mate ratio of 125:1. On this basis it was calculated that -a 40% nitric acid solution should give satisfactory results at 20 mm. of mercury absolute pressure. In actual practice under such conditions it was found that the ratio of water to ester in the distillate was unexpectedly high.

The present invention resulted from the form;- ulation of a theory that appears to explain the facts and has led to the solution of the problem, although it is to be understood that the invention is not limited by these theoretical concepts, which have yet to be conclusively proven.

It may be logically inferred that the esterication of an aliphatic alcohol proceeds through the formation of an oxonium derivative of very low vapor pressure. From that intermediate compound, the ester is formed by loss of water, but in the presence of concentrated nitric acid, the oxonium derivative is oxidized to form undesired by-products. These reactions may be represented as follows:

l. CsHiiOH+HONO2=C5H11OH.HONO2 amyl alcohol oxonium derivative 2. C5H11OH.HONO2=C5H11ONO2+H2O oxonium derivative amyl nitrate water or 3. C5H11OH.HONO2=C4H9CHO+H`NO2+H2O oxouium derivative valeraldenitrous hyde acid In the above equations the reactions represented by Equations 1 and 2 are the desirable reactions, while the reaction represented by Equation 3 is the undesirable reaction, since it results in the formation of undesired valeraldehyde and reduction of the nitric acid which might otherwise be converted into amyl nitrate.

When concentrated nitric acid isv used in the practice of the process of nitration as indicated by Equations 1 to 3 above, the reactions of Equations 1 and 3- predominate, and the desired reaction of Equation 2 is depressed. 1

When concentrated nitric acid is used, the concentrated nitric acid also causes auto-oxida- Cil tion of the alkyl nitrate formed in accordance with Equation 2 above, with the result that the contents of the reaction vessel frequently explode.

On the other hand, when very dilute nitric acid is employed in this reaction, the reaction is extremely slow and the yields of the desired products are small.

The operator, desiring to produce amyl nitrate, for example, is on .the horns of a dilemma when attempting to operate in acocrdance with the prior art procedure for producing methyl and ethyl nitrates. If he initiates the reaction with the use of a concentrated nitric acid, the initial reaction involves undesired production of valeraldehyde and reduction of the nitric acid as indicated in Equation 3. If, on the other hand, he initiates the reactionby the use of nitric acid of the concentration found most suitable for the accomplishment of the reactions of Equations 1 and 2, water formed incident to the esterication reaction as indicated in Equation 2 rapidly dilutes the reaction mixture and reduces the nitric acid to a concentration at which it will not efficiently perform its nitrating function.

Reasoning from the above premises, the applicants conceived the present invention and solved the difficulties of the prior art procedures by developing nitration technique whereby the concentration of the nitric acid in the esterication vessel may be maintained substantially constant, and may be maintained at such point as to effectively accomplish the nitration function while minimizing the occurrence of the undesired reaction of Equation 3, and minimizing the occurrence of auto-oxidation of the alkyl nitrate formed in accordance with Equation 2.

In accordance with therpresent invention, autooxidation in the esterication vessel is prevented by removing the alkyl nitrate from the esterication vessel promptly after the formation thereof and water is removed with the alkyl nitrate in the form of an azeotropic mixture. The amount of water necessary to maintain the desired dilution of the nitric acid in the esterication vessel may be added to the esterication vessel, when necessary, in the form of a diluent for the nitric acid and/or alcohol added to the esterication vessel from vtime to time and a. part of the water required for such dilution may be returned to the esterication vessel from the azeo- Vtropic mixture removed from the vessel by condensing and decanting this azeotropic mixture and returning the water so decanted to the esterification vessel.

The azeotropic distillation of the mixture of water, alkyl nitrate, alcohol and nitric acid from the esterication vessel is accomplished in accordance with the preferred embodiment of the invention by maintaining a sub-atmospheric pressurein that vessel. The maintenance of such sub-atmospheric pressure enables the operator to remove the azeotropic mixture from the vessel without the application of excessive heat, and thus to maintain'the reaction mixture at a lower temperature than could be maintained if distillation were attempted at atmospheric pressure. y As the result of the maintenance of this lower temperature, the reaction of Equations 1 and '2 is favored, the reaction of Equation 3 is suppressed, and auto-oxidation of the alkyl nitrate is suppressed.

Since there is usually a tendency toward formation of nitrous acid in the reaction mass which will initiate oxidation if permitted to accumuiate, it is preferred that a small proportion of urea be added to the vessel in which the present process is practiced. As a general rule the proportion of urea added is less than that employed in the process of the prior art, but as a precaution against decreased yields, inferior grade ester, and explosion it is always well to follow the practice of adding 'at least a small amount of urea. The fact that only small amounts of nitrous acid are formed in the reaction here described when practiced in accordance with the present invention is attested by the presence of urea nitrate crystals remaining in the reaction vessel following the nitration of alcohol by the process of this invention when the quantities of urea are those prescribed by the prior art.

In its preferred embodiment the invention may be advantageously practiced in the apparatus shown diagrammatically in the accompanying drawing wherein I, 2 and 3 designate storage tanks for alcohol, 50% urea solution and 70% nitric acid, respectively, each connected by a valved pipe to reaction vessel 4. The reaction vessel is equipped with a condenser 6. A pipe to convey distillate connects condenser 6 to a decanter l, adapted to separate the distillate into two layers of substantially immiscible liquids of different specific gravity. The draw-oir lines for the decanter are preferably adapted to be transposed in order that either the lighter or heavier liquid can be passed to storage vessel 8, While the other layer is recycled, at least in part, to reaction vessel 4, through a line having an outlet controlled by valve 9 to permit withdrawal of water from the system.

In operation of the described apparatus for continuous nitration, nitric acid of the desired concentration is supplied to reaction vessel 5, together with a. small quantity of vurea, the pressure reduced in the system to the desired point and heat applied to vessel until rapid boiling of the acid is begun. Molecular equivalents of aliphatic alcohol and nitric acid from tanks l and 3 are slowly run into vessel 0 at a xed rate and enough urea solution is added from tank 2 continuously or intermittently to substantially inhibit oxidation due to the presence of nitrous acid. The vapors rising from the reaction mass pass through the column 5 to condenser 6 from which a distillate containing alkyl nitrate, alcohol, water and a little nitric acid is conducted to decanter 1. In the decanter, the distillate separates into two layers. The upper layer in most instances contains alkyl nitrate and alcohol which is passed continuously to storage vessel 8, from which it is Withdrawn for purification. The water layer may be used in part to dilute the 70% nitric acid to the desired degree, but since Water is obtained as a result of the reaction, it is necessary to continuously withdraw a part thereof from the system through valve 9 in order to maintain the quantity of water in the vessel i substantially constant. Ester is withdrawn from storage vessel 8 and purified.

While the ester-alcohol layer in the decanter is generally lighter than the water layer, it must be borne in mind that some esters such as 2- chloroethyl nitrate, are heavier than water, and the decanter connections must be interchanged in such esterication operations.

Using the apparatus described in the manner set forth, a large number of different aliphatic nitrates have been prepared with uniformly good results.

EXAMPLE 1.-n-Butyl nitrate-Using the apparatus described above n-butyl nitrate was prepared by the reaction of nitric acid on n-butanol. The reactor was initially charged with 10 mols of a 50% aqueous solution of nitric acid which was then vigorously boiled at a pressure of about 400 mm. of mercury absolute, which pressure was maintained substantially constant throughout the run. 20 mols of n-butyl alcohol and 22 mols of nitric acid in the form of .a 70% solution were added during the course of the experiment. The average temperature in the reaction vessel was approximately 97 C. Small quantities of 50% urea solution were added from time to time in order to inhibit oxidation. The crude ester was washed, neutralized, dried and fractionated at reduced pressure to obtain 17.3 mols of butyl nltrate and 1.23 mols of butyl alcohol. Approximately grams of material was lost in the process of purifying the crude ester. The normal butyl nitrate was obtained in a state of high purity as a clear water-white liquid of mild ethereal odor, having a specific gravity of 1.032 at 20 C. and a boiling point of 133 C. at a pressure of 29.62 inches of mercury. The conversion of butanol to butyl nitrate was 86.5% of the original charge With a yield of 92.2% based on the alcohol consumed during the process. It is significant that only 20 grams (0.33 mol) of urea were needed to prevent oxidation throughout the run. This is in the ratio of 0.019 mol of urea consumed to each mol of ester formed. At the conclusion of the run 9 mols of nitric acid remained in the reaction vessel and 4.2 mols in the form of weak acid had accumulated as withdrawals through valve 8 making a total of 13.2 mols of acid recovered.

In the present example as in each of the following examples water decanted from decanter l was returned to the vessel d in suiiicient quantity to maintain the water content of the vessel t substantially constant. Similarly, the alcohol and acid were continuously passed from the tanks l and 3 at a rate substantially equal to the rate at which these compounds were reacted together in the esterication vessel Il. Thus, the esteriflcation conditions were maintained substantially constant during the entire course of the reaction by removal of an azeotropic mixture of ester and Water from the esterification vessel and feed of water and acid and alcohol to the esterication vessel.

EXAMPLE 2 2-chloro ethyl nitrate.-In a similar manner, nitric acid was reacted with a 38% aqueous solution of ethylene chlorohydrin. A pressure of 260 mm. of mercury was maintained. In this case the decanter connections had to be reversed by reason of the fact that 2-chloro ethyl nitrate is heavier than the weak acid condensed with' it. The latter, after purification, was obtained as a colorless liquid with a specic gravity of 1.388 at 20 C. and boiled at 46 C. under a pressure of 8 mm. of mercury. At atmospheric pressure the boiling point is approximately 148 C. The product appeared to be contaminated by a trace of ethylene nitrate, since upon continuous distillation the small residue remaining in the still at 149 C. caused mild explosion.

EXAMPLE 3.-Mi:red amg/l nitrates-A mixture of isomeric primary and secondary amyl' alcohols was nitrated. The original charge was 15.5 mols of nitric acid as a 50% solution under an absolute pressure of 300 mm., 48 mols of the mixed amyl alcohols were added to the boiling acid in the reactor together With 40 mols of nitric and 154.2o c.r

' isoamyl alcohol WBS nitrated at 350mm. of mer- C. with 50% vaporizing below 163 C.

gravity at 20'? C. was 0.976.

aiA

ity of 0.990 at C. and was a colorless liquid of fpleasant odor. .An Engler distillation showed that 95% yof the mixture boiled between 145.8

l'lirxlimme 4.-*-Secondaru amy! ynitrates-4 mols of methyl propyl carbinol was reacted with nitric V acid 'in the manner described above at an absolute pressure of 300-mm.y of mercury and a temperature'of 94.5' C. rThe reaction showed a rstrong f tendency toward oxidation, particularly in the j initial stages, which was readily controlled by f the 'addition of urea.v Onthebasls of alcohol consumed, thefyield was 87.2% rof water-white liquid having a mild ethereal odor with a.y specic y'gravityy of 0.992'fat 20 Gand boiling at 144 C. f

'Exmrrsr-Iso-amvl nitrate-A good cut of cury, employing the technique'of this invention with a yield of 92.6%. 0.043'mol' of yurea were used per mol of ester produced. i f f f Exmrra Gif-Normal amyl nitrat.e.---Peritanol-1y f was nitrated in ythe manner described togiv'er a 1.108 atj20 C. -Thepleasantodor is very muchY like that ot benzaldehyde. The product boiled l betweeny 68 and '12 C. at 18 mm. oi mercury.

cinc gravity of 0.954 at 20 C., boilingv between 46 and 56 C. at 4 mm. of mercury. The reaction proceeded quite .smoothly butk a. little kmore f slowly than rin the preceding examples. 0.25

mol of 'urea per mol of alcohol were yused 'to ,i

inhibit oxidation.y y n y The examples given above are chosen as repf Lresentativ'e of the operation of the process in its ypreferred form, but a large number of tests have f given results which indicate' that the inventionv e ymay be'g'enerally applied in the manufacture of alkyl nitrates. v f

acid ysho d be ymaintained practically constant Forbes yresults the concentration of the nitric ata value found to beA satisfactory in'connection with the particular alcohol'being esteriiled. The

yield of 91.2% yunder ay pressure' of 250mm. ofy mercury. The low proportion of urea'used (6.086l

other primary alcohols indicates :clearly that they f f primary alcohols show less tendency yto oxidation' during nitration than the secondaryy alcohols.y

The milicia was a colorless liquid boiling at 52 C. under 2 mm. 'pressure and atfl'lfi C. under rpressure vwith 50% nitricr acid gave a good yield mol per-,mol of ester) wheny considered withl thev vsame data in 'connection' with 'the nitration of -oi the .present type. y acid as used in this disclosure and in the claims y -use of'concentrated acids, thatis. nitric acid conf taining substantiallyA more than y'10%' HNO:y is possible under very closely controlled conditions but should be avoided in most nltration reactions f The lterm dilute nitric appended hereto isto be understood as referring vto an acid containing HNOinot substantially in excess of ,70%. f It has beeninoted that extremely nizraieQ-Nirrauon of ynormal hexyl nitrate yas a light colored liquid: 40

oi' mild pleasant odor boiling at about 171 C. and having a. specific gravity' of 0.983 at 20 C.

Exnlrra 8.-Secondary hexyl nitrate- The nitration of methyl normal butyl carbinol emphasized again the greater tendency of secondary alcohols to oxidize. The control of that tendency required somewhat more urea than was used in connection with primary hexyl alcohol. The nnished ester was obtained as a light yellow liquid, 90% ofwhlch boiled between 161 C. and 165.8 Specific EXAMPLE 9.-2-Ethoxy ethyl nitrate.-Ethoxy ethyl alcohol (Cellosolve) was nitrated in accordance with the present invention with nitric acid and the pressure employed was 260 mm. The temperature of the reactor was 93 C. To prevent oxidation 0.07 mol of urea was used for each mol of Cellosolve passed to the reactor. The finished product was a cut obtained boiling between 59-65 C. at 15 mm. pressure, and was a colorless liquid. The Engler distillation showed 100% to dlstill between 154.8160.8 C. As was the case with 2-chloro ethyl nitrate a mild explosion occurred at the end of the distillation. Specinc gravity of the ester was 1.121 at 20 C.

Exner.: 10.-C'yclohexul nitrate.-Cyclohexyl alcohol was treated with 45% nitric acid under an average pressure of 300 nun. absolute. Because of the strong tendency of this secondary alcohol to oxidize, 0.42 mol of urea. were used per mol of alcohol admitted to the reaction zone. After purification the ester was observed to be a water-white liquid with a specific gravity of dilute acids generally give yslow reactions `and poor conversions and althoughsome of the ybenf f ents of this invention may be obtainedv with acid of ylow concentration itr is yusually found that better results are obtained when using rnitric f f acid yhaving an HNCh-'contentin excess 01,30%;

i. e., in'which; the total watercontent of the f esterlilca.tionr vessel does notbear ra. ratio to ther f total HNO: content of greaterv than 70:30. While acids having a concentration between about 30% and 65% give highly satisfactory results as compared with prior processes. the preferred range is 45% to 55% of nitric acid since a reagent within the latter limits has been found generally suitable to the nitration of alcohols esteriiied in accordance with the present invention. A fairly good general rule is that lower acid concentrations should be used in connection with alcohols having a greater number of substituents on the carbinol group; e. g., that lower acid concentrations should be used in esterifying secondary alcohols than primary alcohols.

In connection with most of the tests run, the acid in storage tank 3 has contained 30% of water (specic gravity of 1.42). As discussed above, water, preferably that decanted from the raw product in order to conserve the acid content thereof, is added to the 70% acid in such ratio as to dilute the strong acid to the desired concentration. By the preferred continuous process described above, a nitric acid solution is placed in the reaction vessel and heated to vigcrous boiling under reduced pressure. Alcohol and nitric acid are then continuously added to the heated mass in substantially molecular proportions. As the ester distills over, a portion of the water in the distillate (containing some nitric acid) is decanted therefrom and used to dilute the acid supplied to the reaction. As the reaction proceeds, water equivalent to that formed by the nitration plus the amount used to dilute the acid as added is Withdrawn from the system as a lay-product inthe form of dilute nitric acid.

'I'he contents of the reaction flask should be maintained in an active state of ebullition and heat supplied at such a rate as to strip the alcohol and ester from the reaction mass. It is extremely important that large quantities of these materials should not be -permitted to accumulate in contact with the hot acid. If this precaution is ignored,-excessive oxidation occurs, resulting in loss of yield, low grade product and explosions. i, The use of urea is recommended to inhibit oxidation. The process here described represents la striking improvement over the prior art by4 de- .pressing side reactions to.the very minimuml and the amount of urea required is therefore very small. The low cost of this reagent in the amounts used in the preferred embodiment of this invention more than justies the additional safety and improved yield obtained thereby. In connection with the nitration of primary alcohols, such as butanol-1, very little of the oxidation inhibitor is normally used, While more highly substituted :carbinols generally require appreciable amounts of this reagent.

The absolute pressure permissible may be any subatmospheric pressure although it is highly desirable to operate at pressures above 20 mm. of mercury. In general, it has been determined that a-bsolute pressures within the range of 50 mm. to 650 mm. of mercury give good results although much better results are obtained by maintaining the pressure at 250-400 mm. abso` lute. As the pressure is reduced below 250 mm. the conversion of alcohol to nitrate is decreased. Pressures above the preferred range result in low yields, due to the increased tendency toward oxidation.

It is to be understood that the specific exa-mples given above are not exha-ustive, but only illustrative of the practice of the process.

As used in the description of the invention and in the following claims, the term aliphatic hydroxy compound is to be understood as `contemplating those compounds in which an hydroxy radical is attached to a carbon atom of an aliphatic compound or group. Thus the invention includes nitration of alkyl, alicyclic and heterocyclic alcohols, except in the case of heterocyclic compounds, those in which an hydroxyl radical is joined to an aryl grou-p.

In the above description of the invention, special emphasis has -been placed upon the esteriiication of alcohols containing between three and eight carbon atoms. While the invention has particular merit -when applied to the nitration of alcohols Within this range, it is also applicable to alcohols containing more than eight carbon atoms and to methyl and ethyl alcohols. In

cases in which the ester and water are removed :from the esterication vessel in the iorm of an azeotropic mixture containing less water than that formed during esterification, the invention -may be practiced by continuously or intermittently returning ester from the decanter to the esteriiication vessel instead of returning water to the esterni-cation vessel as described in the albove illustrative examples. Thus, in the esterication of methyl alcohol, it will be desirable to return methyl nitrate to the esterification vessel in order that the ratio of methyl nitrate to water in the esteriiication vessel may be suihcient to eiect removal of the water fromthe esterication vessel in the form of an azeotropic mixture. Such return of ester to carry off the water as formed in the form of an azeotropic mixture enables the operator to remove the Water as formed,

thereby maintaining the acid in the esterifcation vessel at the desired degree of dilution. Alternatively, in cases in which the azeotropic mixture of the ester with water normally contains less water tha-n that formed incident to esterification, an entraimng liquid, such as benzene, can be intermittently or continuously added to the esterification vessel to increase the ratio of Water to ester in the distillate.

While the invention has been described in connection with operations involving addition of the esterifying acid and alcohol in the liquid phase, the alcohol may, in an alternative embodiment of the invention, be reacted with the acid in the vapor phase. Thus, for example, alcohol from the tank I may be vaporized and passed into the space above the .boiling nitric acid in the reactor 4, instead of adding this alcohol in the liquid phase. The following is an example of such an operation:

EXAMPLE l2.-Isopropyl nitrate.l0 mols of nitric acid in the form of a 50% aqueous solution are brought to boiling temperature in the reaction vessel under an absolute pressure of 275 mm. of mercury. To the contents of the reaction vessel nitric acid and iso-propyl alcohol are then added in molecular equivalents, the isopropyl alcohol ibeing preheated and -passed in the vapor phase into the space above the yboiling nitric acid in the reactor 4. During the course oaf the reaction the liquid in the esteriication vessel 4 is at a temperature of about 92 C., while the temperature at the top of the column 5 is slightly higher, indicating that reaction has occurred in the vapor phase. From time to time, urea solution is added in quantities sucient to inhibit oxidation. Upon 4washing with water and sodium carbonate solution, and then drying and distilling, iso-propyl nitrate is obtained as a :colorless liquid with a specic gravity of 1.043 at 20 C. boiling at 102 C.

While the invention has been described above in connection with operations in which the ratio of water to HNOa in the esterication Vessel is substantially constant, it will be evident that the advantages of the invention may be at least partially realized in connection with operations in which such ratio uctuates materially within the limits above indicated. Thus, it is not vital tothe practice of the invention that water be returned to the esterication vessel in the amounts necessary to keep the ratio offwater to HNO; exactly constant.

Similarly, the, invention is not necessarily limited to the practice of esterication by a continuous process. tion can be partially realized by returning water or ester to the esterication vessel in such proportions as to reduce the degree of concentration or dilution of the acid in the esterication vessel and thereby avoid the alternative defects of the prior art of over-concentration or overdilution of the acid.

The advantages of the invention may also be partially realized in the esterication of some alcohols without the use of vacuum in the practice of the azeotropic distillation. Thus, the features of reducing the temperature in the esteriiication vessel by the employment of azeotropic distillation and avoiding or reducing the tendency of the nitric acid to become more dilute or more concentrated as the case may be, as the esterication reaction proceeds, have an advantage independent of the advantage obtained by the use of sub-atmospheric pressures in the The advantages of the invenesterication vessel, and these features may be used per se to obtain such advantage.

In the above description, considerable emphasis has been placed upon return of water from the decanter 'l to the esterication vessel 4. We wish it to be understood that such return is not vital to the practiceA ofthe invention. There is a certain advantage in 4returning water from the decanter 1 because of the fact that this water contains unreacted nitric acid which may be utilized upon return of such water. However, the amount of water necessary to maintain the contents of the esteriiication vessel 4 at the desired degree of dilution may be added to the esterification vessel in the form of diluent for the nitric acid in the container 3, or such water may be separately added to the esterication vessel.V

Thus, in its broadest aspects, the invention includes the addition of water whether in the form of diluent for the nitric acid, alcohol or urea added during the course of the reaction, in the form of dilute acid decanted from the decanter 1 or in the form of separately added water.

Still further modiiications will be obvious to those skilled in the art and we do not therefore wish to be limited except by the scope of the sub-joined claims.

We claim:

1. The process of preparing an alkyl nitrate containing between 3 and 8 c rbon atoms in its alkyl radical comprising conta ting boiling dilute nitric acid with a monohydric aliphatic alcohol containing the alkyl radical of the desired alkyl nitrate under a pressure between 20 and 650 mm. of mercury, continuously removing from the esteriiication zone water and the desired alkyl nitrate by azeotropic distillation during the course of the esterincation reaction. and adding water to the esterication zone during the course of the esteriiication reaction to maintain the ratio of water to nitric acid in the esterincation zone at a concentration between and 68% during the course of said reaction.

2. The process of preparing an alkyl nitrate containing between 3 and 8 carbon atoms in its alkyl radical comprising contacting boiling dilute nitric acid with a monohydric aliphatic alcohol containing the alkyl radical of the desired alkyl nitrate under a pressure between 20 and 650 mm. of mercury, continuously removing from the esterication zone water and the desired alkyl nitrate by azeotropic distillation during the course of the esterication reaction, decanting the azeotropic mixture removed from the esterification zone to obtain a decanted fraction containing the water and a second decanted fraction containing the desired alkyl nitrate, and returning said decanted water fraction to the esteriiication zone to maintain the ratio of water to nitric acid in the esteriilcation zone at a concentration between 30 and 68% during the course of said reaction.

3. 'Ihe process of preparing an aliphatic nitrate containing at least three carbon atoms in the molecule which comprises treating a monohydric aliphatic alcohol containing between 3 and 8 carbon atoms in the molecule with nitric acid having a concentration between 35% land 68% and removing the resulting aliphatic nitrate by azeotropic distillation under a reduced pressure between 20 and 650 mn. of mercury, and adding to the reaction mixture water in sufilcient amount to prevent an accumulation of ester in the reaction mixture during the course of the esteriflcation reaction and maintain the nitric acid in the esterification reaction at a concentration between 35% and 68%.

4. The process of preparing an alkyl nitrate containing between 3 and 8 carbon atoms in its alkyl radical comprisingcontacting boiling dilute nitric acid with a monohydric aliphatic alcohol containing the alkyl radical of the desired alkyl nitrate, and continuously removing from the esterication zone water and the desired alkyl nitrate by azeotropic distillation during the course of the esteriiication reaction while maintaining the concentration of the nitric acid between 35 iand 60% by adding water to the reaction mixure.

5. The process of preparing an alkyl nitrate containing between 3 and 8 carbon atoms in its alkyl radical comprising contacting boiling dilute nitric acid with a monohydric aliphatic alcohol containing the alkyl radical of the desired alkyl nitrate. continuously removing from the esterilication zone water and the desired alkyl nitrate by azeotropic distillation during the course of the esteriiication reaction, and returning water of the esterification reaction to maintain the nitric acid in the esteriilcation zone at a degree of concentration between 35 and 60%.

6. The process of preparing an aliphatic nitrate containing at least three carbon atoms in the molecule which comprises treating a monohydric aliphatic alcohol containing between 3 and 8 carbon atoms in the molecule with nitric acid having a concentration between 45% and 55% and removing the resulting aliphatic nitrate by azeotropic distillation under a reduced pressure between 20 and 650 mm. of mercury, and adding to the reaction mixture water in suilicient amount to prevent an accumulation of ester in the reaction mixture during the course of the esteriiication reaction and maintain the nitric acid in the esterification reaction at a concentration between 45% and 55%.

- 7. The process of preparing an aliphatic nitrate containing at least three carbon atoms in the molecule which comprises treating a monohydric aliphatic alcohol containing between 3 and 8 carbon atoms in the molecule with nitric acid having a concentration between 45% and 55% and removing the resulting aliphatic nitrate by azeotropic distillation under a reduced pressure be- -tween 250 and 400 mm. of mercury, and adding to the reaction mixture water in sufficient amount to prevent an accumulation of ester in the reaction mixture during the course of the esteriiication reaction and maintain the nitric acid in the esterification reaction at a concentration between 45% and 55%.

JOHN F. OLIN.

FREDERICK P. FRITSCH.

JOSEPH J. SCHAEFER.

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