Analvsis of Mixtures of the MonoJ chlorides of n-Pentane and Isopentane H. B. HASS AND PAUL WEBER,' Purdue University, Lafayette, Ind.
D
URING a recent investigation of the chlorination of the pentanes, carried out in this laboratory, it was necessary to analyze mixtures of the three monochlorides of n-pentane and mixtures of the four monochlorides of isopentane. All the theoretically possible monochlorides derivable without carbon skeleton rearrangement are always formed upon chlorination of either normal pentane or isopentane. Polychlorides are aIso formed. The monochlorides can readily be separated from the polychlorides and from excess unreacted pentane by rectification. Not all the monochlorides can be separated from one another, however, by means of fractional distillation; therefore supplementary methods of analysis had to be worked out or applied. It is felt that these methods might prove applicable to other similar mixtures of aliphatic halides. They are described in this paper separately from the data obtained during the chlorination work proper.
Analysis of Mixtures of Monochlorides Derived from Isopentane The monochlorides obtained upon chlorination of isopentane are easily separated from the polychlorides and from unchanged isopentane by fractional distillation. It is possible through[ repeated efficient rectification to separate the two primary chlorides as a mixture from the 2-chloro-3-methylbutane (b. p. 93.0"/760 mm.) and the 2-chloro-2-methylbutane (b. p. 85.7 "/760 mm.). Separation of 1-chloro-3-methylbutane (b. p. 98.8"/760 mm.) from 1-chloro-2-methylbutane (b. p. 99.9"/760 mm.) cannot be effected with reasonable facility by fractional distillation. The amount of 2-chloro-2-methylbutane present in the mixture of monochlorides derived from isopentane was determined by repeated hydrolysis with water a t temperatures below 35" C. Whitmore and Johnston (8) used this method to analyze a mixture of tertiary and secondary isoamyl chlorides. The tertiary chloride undergoes hydrolysis with water st this temperature, liberating hydrochloric acid. From the amount of acid formed as determined by titration of the squeous layer with standard alkali, the amount of tertiary amyl chloride formed was known. The secondary chloride and the two primary isoamyl chlorides are not subject to hydrolysis under these conditions. Secondary isoamyl chloride was separated practically quantitatively from the higher boiling chlorides (isoamyl and polychlorides) by careful fractional distillation. The amount of secondary chlorides was also determined by subjecting the chloride mixture (after removal of the tertiary chloride by hydrolysis with water) to hydrolysis with 0.1 N silver nitrate. Whitmore and Johnston (8) observed that secondary isoamyl chloride is practically completely hydrolyzed in 60 hours by an exceBs of 0.1 N silver nitrate solution, whereas under the same conditions isoamyl chloride (a primary chloride) reacts only to the extent of 3 to 4 per cent. The other primary isomer which was present-namely, 1-chloro-2-methylbutane-was shown by a special experiment to be even more resistant than isoamyl chloride to hydrolysis under these conditions. The identification of the secondary isoamyl chloride by conversion to the Grigiiard 1
Present address, Georgia School of Technology, Atlanta, Ga.
reagent, followed by oxygenation, hydrolysis to 3-methylbutanol-2, formation of the alpha-naphthyl urethane, and 3,5-dinitrobenzoate derivatives and taking mixed melting points with authentic samples, is described by Hass, McBee, and Weber (3). After removal of the tertiary and secondary chlorides, the remaining organic chloride mixture was carefully fractionated. The fraction boiling between 98.8" and 99.9" C., representing the mixture of the two primary monochlorides of isopentane, was completely removed from the higher boiling polychlorides. This mixture of 1-chloro-3-methylbutane and l-chloro-2methylbutane was then analyzed according to the method described below.
Primary Monochloride Mixtures of Isopentane The densities and refractive indices of 1-chloro-2-methylbutane and 1-chloro-3-methylbutane are not sufficiently different to serve as the basis of a method for analyzing mixtures of these two chlorides, since a mere trace of pentane or dichlorides would seriously affect the results. I n synthesizing these chlorides from the corresponding alcohols it was observed that the rates of the reaction CIHIIOH
+ HCl+
C6HnCl
+ HzO
differed considerably. It was suspected that the difference in reaction rates of these chlorides with potassium iodide might be sufficient to form the basis of a method of analysis. Conant and Kirner (9) worked out an excellent method of determining the reaction rate constants for the reactions of a number of aliphatic halides with potassium iodide (dissolved in anhydrous acetone). With a few minor modifications their technic was used to determine the reaction rate constants of each of the two primary monochlorides of isopentane with potassium iodide (dissolved in acetone) a t 60" C. Especially pure samples of these chlorides were used, prepared from the corresponding methylbutanols, which were synthesized as described below. These reaction rate constants were found to be appreciably different. Three different known mixtures of these chlorides were prepared and their reaction rate constants with potassium iodide were determined in the same manner as those for the pure chlorides. These constants were found to be a linear function of the composition, thereby making it possible to analyze unknown mixtures with an accuracy of *2 per cent.
Determination of Reaction Rate Constants Test tubes 18 cm. long, made from 18-mm. Pyrex tubing, were used as the reaction vessels. A thin-walled bulb containing a known wei ht of the organic chloride (1-chloro-2-methylbutane or l-c%loro-3-methylbutane or a mixture of the two) was inserted into each reaction tube. Four or five small glass beads were also introduced, to aid in breaking the bulb when the reaction was to be started. Then the open end of the reaction tube was drawn down to a diameter of about 5 mm. (just large enough for a pipet t o be inserted). Ten milliliters of the standard potassium iodide-acetone solution (0.04 M in potassium iodide) were finally introduced, and the tube was immediately sealed off. The tubes were placed in a thermostat kept at 60" C. (k0.05") for an hour. The reaction was started by vigorously shaking the tube, the inner bulb was thus broken, and the reactants 231
INDUSTRIAL AND ENGINEERING CHEMISTRY
232
were intimately mixed. At the end of a given period of time, a tube was withdrawn from the thermostat, the end broken, and the contents were quickly poured into a 250-ml. glassstoppered, wide-mouth bottle containing a mixture of 20 grams of ice, 20 ml. of concentrated hydrochloric acid, and 5 ml. of chloroform. The bottle was immediately stoppered and shaken and the amount of potassium iodide still unreacted was determined by immediate titration with potassium iodate, according t o the directions of Conant and Kirner (I).
Calculation of Reaction Rate Constants
+
+
The reaction C6HI1Cl K I = C5HJ KCl proceeds as a simple metathesis and, for the two primary rnonochlorides of isopentane, is not accompanied, under the conditions of the experiment, by any side reactions. The regular equation for bimolecular reactions
was used in making the calculations for the reaction velocity constant K . The original concentration in moles per liter of the organic chloride is represented by a; b represents the original concentration of potassium iodide in moles per liter; 1: represents the number of moles per liter of organic chloride which reacts in t hours. The data for the two pure chlorides and the three known mixtures are given in Table I. The average values of the reaction rate constants recorded in Table I were plotted against compositions to yield the curve shown in Figure 1. Thus, with the aid of this curve it is possible t o determine the composition of any unknown mixture of the two primary monochlorides of isopentane.
TABLEI. DETERMINATION OF REACTION RATECONSTANTS IN METATHESES BETWEEN POTASSIUM IODIDEAND PRIMARY MONOCHLORIDES OF ISOPENTANE [33.65 ml. of 0.00602 M potassium iodate required for 10 ml. of potassium iodide-acetone solution (blank). Thus, the latter,was 0.0405 M i n potassium iodide. Titration: ml,. of 0.00602 M potassium iodate for unreacted potassium iodide. Composition of mixture 1: 67.4 per cent 1-chloro-3-methylbutane; mixture 2: 33.6 per cent 1-chloro-3-methylbutane; mixture 3: 49.9 oer cent 1-chloro-3-methylbutane.] Wei.ght of Original Potassium Reaction Chloride Iodate Time Velocity Cram Ml. Hours K Reaction between 1-ohloro-3-methylbutane and potassium iodide at 60' C. 0.2352 17.2 21.0 0.0664 19.1 24.0 0.0646 0.1797 0.1676 18.2 30.0 0.0648 0.1619 25.0 12.0 0,0734 0.1630 14.0 43.0 0.0635 13.3 50.0 0.0626 0.1514 Av. 0.0660 Reaction between 1-ohloro-2-methylbutane and potassium iodide at 60' C. 30.6 20.5 0.0153 0.1807 29.5 24.0 0.0123 0.2071 27.7 45.0 0.0120 0.1725 27.9 35.0 0.0124 0.2033 24.7 58.5 0.0131 0.1917 Av. 0.0130 ~otassiumiodide at 60' c. Reaction C. _.__. . between mixture 1 and potassium 26.1 15.5 0.0475 0.1644 21.4 23.0 0.0478 0.2004 25.0 20.0 0.0474 0.1527 16.2 40.0 0,0483 0.1838 20.6 30.0 0.0486 0.1668 Av. 0.0482 Reaction between mixture 2 and potassium iodide at 60° C. 0.1513 22.4 42.0 0.0314 0.1436 29.0 15.5 0,0322 0.2331 22.9 25.0 0.0315 0,2127 23.0 30.0 0,0288 0.1822 26.5 20.5 0.0308 Av. 0.0316 Reaction between mixture 3 and uotassium iodide at 60' C. 0.1851 19.0 40.0 0,0380 0.1752 26.5 16.0 0.0410 0.1773 21.6 30.0 0.0408 0.1860 23.1 25.0 0.0393 0.1872 24.8 20.5 0.0382 Av. 0.0395
VOL. 7, NO. 4
Analysis of Monochloride Mixtures of n-Pentane By careful fractionation of the chlorinated products of n-pentane, 1-chloropentane (b. p. 108.2"/760 mm.) can be separated practically quantitatively from a mixture of 2chloropentane (b. p. 96.7"/760 mm.) and 3-chloropentane (b. p. 97.2"/760 mm.) and from the polychlorides. This procedure was followed. A method of analysis for the mixtures of the two secondary chlorides which did not involve separation had to be found. An effort was made to determine the reaction rate constants of these chlorides with potassium iodide, in the hope that a method of analysis similar to the one used with the mixtures of primary monochlorides of isopent a n e could be w o r k e d out. The results were not sati sf a c t o r y because t h e r e a c t i o n was very slow a t 60" C. and the amyl iodide formed underwent considerable reduction, with formation of free iodine. The difference in the ref r a c t i v e indices of the two chlorides is /-C/-J-me-bufone,$I not s u f f i c i e n t f o r analytical purposes, FIGURE1. DIAGRAM FOR ANALYSIS OF MIXTURES OF PRIMARY ISOAMYL since a mere trace of CHLORIDES Dentane or dichlo;ides would seriously affect the analysis by this method. This also holds true for the densities. Lauer and Stodola (4) developed a method of analysis for mixtures of 2-bromopentane and 3-bromopentane. After preparing the pure bromides they developed a carefully standardized procedure for the conversion of the bromides to their anilides and a large number of bromide mixtures of known composition were subjected to this procedure. The melting points of the anilide-mixtures were determined and plotted a g a i n s t t h e c o m p o s i t i o n of the bromides from which they were obtained. A melting point-composition diagram of the simple e u t e c t i c type was o b t a i n e d w h i c h w a s used in analyzing u n k n o w n mixtures. This method was found to be equally applicable to the corresponding chloride mixtures and FIGURE2. DIAGRAM FOR ANALYSIS OF MIXTURES OF SECONDARY CHLOwas used. RIDES OF R-PENTANE T h e experimental p r o c e d u r e used by Lauer and Stodola for converting the bromides into the anilides was used without modification in converting the chlorides into the anilides. Six known mixtures of 2-chloropentane and 3-chloropentane were prepared and five samples of each subjected to this method of analysis. The results are contained in Table 11. The diagram obtained by
JULY 1.5, 1935
ANALYTICAL EDITION
233
theory. The procedure for converting this alco$ol into the chloride was identical with the one used to prepare l-chloro-3methylbutane. Three hundred grams of pure l-chloro-2methylbutane (b. p. 99.8' to 100.0"/760 mm.) were obtained, a yield of 49 per cent based upon the alcohol. 2-CHLOROPENTANE. For the synthesis Of pentanol-2, the method of Sherrill, Baldwin, and Haas was used (6). The alcohol boiled at 118.5' to 119.5"/760 mm. Sherrill ( 7 )observed in the preparation of 3-bromopentane and 2-bromopentane from TABLE11. MELTINGPOINTS OF ANILIDEMIXTURES OBTAINED the corresponding alcohols, that if the reaction temperature be FROM MIXTURES O F 2-CHLOROPENTANE AND 3-CHLOROPENTANE permitted to go higher than 60' C., a mixture of the two isomers OF KNOWNCOMPOSITION was obtained instead of the individual bromide. In preparing 2-Chloropentane, Per Cent r. 2-chloropentane (and also 3-chloropentane) from the corre100 90.0 69.6 59.7 50.0 29.3 9.9 0 sponding alcohol, high temperatures were avoided in order to 3-Chloropentane, Per Cent prevent, if possible, any such rearrangement during formation 40.3 70.7 90.0 100 0 10.0 50.0 30.4 of the chlorides. The pentanol-2 was saturated with hydrogen chloride a t 0" C. OC. OC. OC. OC. OC. O C . OC. O C . and twice the volume of concentrated hydrochloric acid added. 87.6 94.4 107.2 1 1 7 . 2 122.2 92!.4 89.6 82.2 The resulting solution was stoppered and permitted to stand 911.8 89.6 84.2 85.2 93.0 107.8 118.2 121.4 at 30' C. for 4 days. Two layers formed and the upper one 9$!.8 90.2 84.6 88.6 95.2 107.5 1 1 8 . 5 122.4 93.0 89.8 83.0 86.0 93.6 108.4 117.6 121.8 was separated off. The lower layer was again saturated with 92.6 90.0 117.8 122.5 hydrogen chloride and again permitted to stand at 30" C. After . . 8 2... 8 8 7... 4 9 4. .. 6 108.6 ... . . . 121.8 two 921.5 additional saturations, 80 per cent of the alcohol had been Av. 9 8 . 5 89.9 83.4 87.0 94.2 108.5 117.9 122.0 converted into the chloride. The crude chloride was washed . with concentrated hydrochloric acid, neutralized, and dried over anhydrous potassium carbonate. This material was rectified a t 150 mm., through an efficient spiral packed column Mixtures containing more than 50 per cent of Z-chloro100 cm. long and 1.5 cm. in diameter. A 300-gram fraction was pentane form anilide mixtures possessing melting points collected which possessed a boiling point range of 96.6' to which represent two different percentage compositions. By 96.8"/760 mm. observing the effect on the melting point of the mixture caused The pentanol-3 from which this chloride ~-CHLOROPENTANE. was prepared was obtained by Grignard synthesis according by addition of known quantities of one of the pure anilides, to the procedure of Lucas and Moyse (5). Three hundred the exact composition was thus definitely established. grams were obtained possessing a boiling point range of 114.8' to 115.5'/760 mm. The procedure used to convert this alcohol Synthesis of Amyl Chlorides into the chloride is described under the preparation of 2-chloropentane. Two hundred and fifty grams of pure 3-chloropentane The amyl chlorides were synthesized from the correspond(b. p. 97.1' to 97.4'/760 mm.) were obtained, a yield of 69 ing alcohols. The boiling points of the two primary isoamyl per cent. alcohols differ by only 4" and the boiling points of pentanol-2 Summary ' . Therefore it is pracand pentanol-3 differ by less than 4 tically impossible to obtain pure samples of these alcohols A new method has been developed for analyzing mixtures by fractionation of commercial products. They were synof two isomeric alkyl chlorides whose boiling points, refractive thesized by methods which could not possibly produce the indices, and densities are too close to each other to serve as other isomer of each pair. the basis for analytical methods. This method is based on the difference in the reaction rate constants of the alkyl l-CHLORO-3-METHYLBUTANE. The 3-methylbutanol-1, from which this chloride was made, was synthesized from isobutyl chlorides with potassium iodide (dissolved in acetone), alcohol according to the following reactions: Two kilograms A method of analysis for mixtures of alkyl bromides, worked of isobutyl alcohol (b. p. 107.5' to 108.0' C.) were converted out by Lauer and Stodola (4) is applicable to mixtures of the into isobutyl bromide by refluxing for 10 hours with 5.6 kg. of corresponding chlorides. 48 per cent hydrobromic acid and 1.7 kg. of concenkrated sulfuric acid, followed by distillation at 200 mm., washing, neutralization, The former method should prove applicable to other similar and rectification. Yield: 49 er cent of material of boiling mixtures of aliphatic chlorides. The latter method, of formpoint 91.0" to 91.5" C. The &rignard reagent made from this ing solid derivatives of the mixtures and then determining bromide was then treated with gaseous formaldehyde (generated the melting points, should prove useful in a large number of by decomposing p-formaldehyde). Hydrolysis of this addition product gave 3-methylbutanol-1. After drying and rectification, analytical problems invdving not only alkyl halide mixtures, a 500-gram fraction was obtained which possessed a boiling but mixtures of other types of organic compounds as well. point range of 130.6' to 131.6'/760 mm. Yield: 47 per cent Especially pure samples of the secondary amyl chlorides based on the bromide. and the primary isoamyl chlorides, used in this work, were The 500-gram sample of this alcohol and 50 ml. of concentrated hydrochloric acid contained in a 1-liter, three-neck flask fitted carefully synthesized and their boiling points accurately with a large reflux condenser and delivery tube for hydrogen determined. chloride, were heated to 90" C. The solution was kept saturated with hydrogen chloride a t this temperature for 6 hours. The Literature Cited mixture was then distilled through a long column until the temperature rose to 100' C. The residual liquid was again (1) Ayres, Eugene, IND. Exu. CHEM.,21, 902 (1929). saturated with hydrogen chloride and the above procedure re(2) Conant, J. B., and Kirner, W. R., J . Am. Chem. Soc., 46, 232 peated. After four additional treatments, about 75 per cent of (1924). the alcohol had been converted into the chloride. The combined (3) Hass, H. B., McBee, E. T., and Weber, unpublished paper, distillatw were washed with concentrated hydrochloric acid, "Syntheses from Natural Gas Hydrocarbons. 11. Identity concentrated sulfuric acid, and neutralized and dried over of Monochlorides Formed on Chlorinationof Simpler Paraffins." anhydrous potassium carbonate. Upon rectification, a 225(4) Lauer, W. M., and Stodola, F. H., J . Am. Chem. Soc., 56, 1215 gram fraction of 1-chloro-3-methylbutane (b. p. 98.7' to 98.9"/ (1934). 760 mm.) was obtained. (5) Lucas, H. J., and Moyse, H. W., Ibid., 47, 1459 (1925). 1-CHLORO-2-METHYLBUTANE. This was prepared from 2(6) Sherrill, M. L., Baldwin, Catherine, and Haas, Dorothea, I b i d . , methylbutanol-1, which was synthesized from butanol-2 ac51, 3034 (1929). cording to the following reactions: Carefully purified butanol-2 (7) Sherrill, M. L., Otto, Belle, and Pickett, L. W., Ibid., 51, 3023 (b. p. 99.5' to 100.0" C.) was converted into 2-bromobutane (1929). using hydrobromic and sulfuric acids as stated above with iso(8) Whitmore, F. C., and Johnston, Franklin, Ibid., 55, 5020 (1933). butyl alcohol. Yield: 53 per cent. Secondary butyl-magnesium RECEIVED February 28, 1935. Taken from a thesis submitted by Paul bromide was prepared and then treated with gaseous formaldeWeber in partial fulfillment of the requirements for the degree of doctor of hyde. Hydrolysis of the addition product gave 2-methylphilosophy. Purdue University. butanol (b. p. 127.5' to 127.8"/760 mm.), yield 40 per cent of plotting these average values against percentage composition is shown in Figure 2. This diagram, which checks very closely the one obtained by Lauer and Stodola on the corresponding bromopentane, was used in analyzing the unknown chloride mixtures.
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