pectrophsto metric
ethsds for Olefins
Cosorimetric Deterrni nu tion of Conius uted Diohfi ns A.
P. ALTSHULLER
and I. R. COHEN
Air Pollution Engineering Research, Robert A. Tuff Sanitary Engineering Cenfer, Public Healfh Service, U. S. Department of Health, Education, and Welfare, Cincinnati 26, Ohio
b In a new colorimetric method conjugated diolefins are coupled with p-nibrobenzenediazonium fluoborate in a 2-methoxyefhanol-phosphorlc acid solvent medium. Isoprene-type diolefins couple to form products with strong absorption near 4 9 0 mp, while butadiene couples to form a product with a maximum near 4 0 5 mp. The intensities of these maxima are linearly related to concentration between a t least 0.3 and 30 pg. per ml. for isoprene-type diolefins and 20 and 200 pg. per ml. for 1,3-butadiene. A 2- to 4-hour reaction period is necessary to obtain optimum intensities. No appreciable interference occurs from paraffinic, acetylenic, simple aromatic, and most other types of olefinic hydrocarbons. Some aldehydes, ketones, and phenols interfere moderately. Isoprene has been efficiently collected and determined from dilute isoprene-air mixtures, and in several liquid mixtures containing various other hydrocarbon components, including 1,3-pentadiene, unconjugated diolefins, and various types of mono-olefins.
T
conjugated diolefins, 1,3-butadiene, 2-methyl-1,3-butadieneJ and trans-1,3-pentadieneJ have been identified in various combustion gases, including automobile exhaust (3, 1I ) and cigarette smoke (4).The acyclic diterpene, phytadiene, also a conjugated diolefin, has been reported in cigarette smoke (4).Upon occasion, butadiene or isoprene exists as an atmospheric pollutant near petrochemical or synthetic rubber plants. Eye irritation and plant damage similar to those experienced in the Los Angeles area have been produced by the photolysis of nitrogen oxides in the presence of olefins (2, 10). Conjugated diolefins have been shown to be among the most potent of olefins in causing these eye irritation effects (10, 11). Spectrophotometric analysis of the whole class of conjugated diolefins or portions of this class should be most useful. Among the limited number of possible colorimetric reactions available, the reactions of conjugated diolefins with strong coupling agents such HE
as p-nitrobenzenediazonium salts appeared applicable to the development of a colorimetric method. Meyer and his coworkers have shown that strong diazonium salts will couple with hydrocarbons to form colored crystalline azo compounds (6). Subsequent work by Neyer proved that strong diazonium salts such as diazotized p-nitroaniline and 2,4-dinitroaniline would couple with lJ3-butadiene, 1,3-pentadiene , 2-methyl-1 ,3-butadieneJ and 2,3-dimethyl-lJ3-butadiene to form yellow to orange azo compounds (6). A coupIing product also was obtained with mesitylene using diazotized 2,4,6trinitroaniline (7). The study of the coupling reactions of hydrocarbons was continued some years later by Terent’ev and his coworkers in a series of investigations ( 1 2 4 1 ) . This group found that the unconjugated diolefin, 1,5-hexadieneJ showed no apparent activity with either diazotized p-nitroaniline or 2,4-dinitroaniIine. The conjugated diolefins, 2,4hexadiene. 2-methyl-lJ3-pentadiene,and 2,5-dimethy1-2,4-hexadieneJ did enter into reaction with these diazonium salts (14). Slow reactions were also noted with the mono-olefins, isobutylene and 2-methyl-2-buteneJ particularly with the diazonium salt prepared from 2 , 4 dinitroaniline (14). No reactions were noted with acetylene, hydrogen, carbon dioxide, or carbon monoxide (16). It was also shown that styrene and dimethylstyrene do not react, but that indene does react with diazotized 2,4dinitroaniline (16). The compound 01 - phenyl 1,3 - butadiene also is sufficiently reactive to couple with the diazonium salt of p-nitroaniline
-
($11*
It was shown that 100 ml. of gas containing 0.2% of butadiene would cause a yellowing of a diazotized solution of p-phenylenediamine (16). -4 solution containing diazotized Z,$-dinitroaniline was later (13) recommended as a qualitative color reagent for diolefins. Terent’ev found that 2,3-&methyl1,3-butadieneJ 1,3-cyclohexadieneJ and cyclopentadiene can be determined quantitatively by reaction with a known amount of’ diazotized p-nitroaniline and by titrating the unreacted
diazonium compound with %naphthol (17). This method was applied to determining the content of these conjugated diolefins in complex liquid mixtures containing other hydrocarbons, including hexane, heptane, cyclohexane, cyclohexene, and benzene (12, 17). This method mas shown to be useful in determining diolefins in cracked and pyrolyzed products of organic compounds (I7 , 19). and used to determine monomer concentrations in a study of the polymerization of oi-phenyl-l,3butadiene ($1). Despite the considerable number of investigations of the coupling of active methylene compounds (9), relatively little use has been made of these reactions for analytical purposes. The main exceptions for the hydrocarbons are the papers of Terent’ev and his coworkers on the analytical uses of cow pling reactions of diolefine (10, 18, Ii5,19). Consequently, an inyestigation of the spectrophotometric aspects of these coupling reactions appeared of interest. EXPERIMENTAL DETAILS
The diazonium salt used in most of the present work was commercial grade p-nitrobenzenediazonium Auoborate (Eastman Kodak) recrystallized from water. The purified material consisted of yellow or light yellow needles of the diazonium salt. The diolefins used included 1,3butadiene, 2-methyl-1,3-butadienej 2,3dimethyl- 1,3-bu tadiene , 1,3-pentadiene, 1,s-hexadiene, 2,5-dimethyl-lJ5-hexadiene, and 2,5-dimethyl-2,4-hexadiene. The 1,a-butadiene used is Phillips Petroleum Special Purity Grade of 99.3% purity. The 2-methyl-IJ3-butadiene used is also Phillips Petroleum 9970 purity. Gas chromatographic analysis gave 99.7 weight % purity. Gas chromatographic analysis of the other diolefins produced the following results (weight per cent purity) : IC & K Laboratories, 2,3-din~ethyi-l,3-butadiene, 92.6; Chemical Procurement Co., lJ3-pentadiene, 91.3; Aldrich Chemical Co., lJ5-hexadiene, 99.9; Aldrich Chemical Co., 2,B-dimethyl1,5-hexadieneJ 85.5; and K & K Laboratories, 2,5-dimethyl-2,4-hexadiene, 95.6. Dimethylfulvene also was obtained from K &. X Laboratories. Most of the mono-olefins and aromatic hydrocarbons were Phillips Petroleum 99+ VOL. 32, NO. 13, DECEMBER 1960
m
1848
mole 70 or reagent grade materials. The terpenes were provided by the Hercules Powder Co. The allo-ocimene and myrcene were of better than 98% purity, according to the supplier. The aldehydes, ketones, phenols, and aniline were either reagent grade or the best commercial grade available. The approximate solubility of p nitrobenzenediazonium Auoborate was determined in methanol, acetone, sulfuric acid, 2-methoxyethanol, and 2-ethoxyethanol. 2-Methoxyethanol proved to be the best solvent. About 2 grams of the diazonium salt can be dissolved in 100 ml. of this solvent. Most of the determinations mere made using 2-methoxyethanol nearly or just saturated with the diazonium salt. The salt was dissolved either by gentle heating to 40 O to 50' C., or by 5 'gorow shaking a t room temperatures. Temperatures above 50' should be avoided to minimize the decomposition of the p-nitrobenzenediazonium fluoborate in solution. The diazonium salt reagent should be prepared fresh for each day's use. One, 5, and 8 parts by volume of sample in 2-methoxyethanol or 2methoxyethanol-phosphoric acid were diluted to 10 parts by volume with 2% by weight of p-nitrobenzenediazonium ffuoborate in 2-methoxyethanol-phosphoric acid or 2-niethoxyethanol. These dilutions correspond to 1.8, 1.0, and 0.4% by weight of diazonium salt in the final solutions. The phosphoric acid was included in the diazonium reagent in making up the 1.0 and 1.8% solutions. I n making up the final solutions containing 0.4% diazonium salt the phosphoric acid was included in the sample solution rather than in the diazonium salt solution. The isoprene-air mixtures were generated from a diffusion cell having a diffusion tube 3.40 mm, in diameter and thermostated at 15' (I). The isoprene was collected in a series of three fritted-glass bubblers. Absorbance vias usually read with a Gary Model 11 spectrophotometer. However, a small fraction of the readings were made using a Beckman NIodel B spectrophotometer.
Table I. Absorptivities of Coupling Reactions of p-Nitrobenzenediazonium Fluoborate with 2-Methyl-l,3-butadiene and 2,3-Dimethyl-lt3-butadiene in Various Solvent Systems Diolefin 2-Methyl-l,3-butadiene
Solvent" 2-Methoxyethanol 2-iilethoxyethanol-25 % phosphoric acid 2,3-Dimethyl-l,3-butadiene 2-Methoxyethanol 2-Methoxyethanol-9 yo acetic acid 2-hlethoxyethanol-15y0 phosphoric acid
24
4
0.008 0.025
24 24
0.011 0,021
4
0.029
a -411 solutions prepared by diluting 1 ml. of solution containing diolefin t o 10 ml. with 2 q -nitrobenzenediazonium fluoborate in 2-methoxyethanol or 2-methoxyethanol-acid. 8alculated from observed absorbances and concentration of diolefin, pg. per ml., in Eolution before dilution t o 10 ml.
71
RESULTS
I n the preliniinary measurements 2methoxyethanol alone was used as the solvent with p-nitrobenzenediazonium fluoborate as the coupling reagent. Absorption bands around 490 mp were observed for isoprene-type diolefins and in the vicinity of 400 nip for 1,3-butadiene and lJ3-pentadiene. A number of mono-olefins and unconjugated diolefins were tested, ineluding 2-methgl-lbutene, 2-methyl-2-butene, 1-hexene, a-hexene, irans-4 - methyl - 2 pentene, cycloliexene, 1,5 - hexadiene, 2,4,4trimethyl - 1 - pentene, and 2,5-diNegligible methyl 2,5 - hexadiene. reactivity was shown for the acyclic terminal olefins, including 2-methyl-lbutene, 1-hexene, and 2,4,4-trimethyl-l-
-
1844
0
ANALYTICAL CHEMISTRY
pentene. Furthermore, the optical absorbances of the coupling products of 2-hexeneJ trans-4-methyl-2-pentene, cyclohexene, and 1.5-hexadiene 'i? ere less than 0,5% of that given by equal amounts of isoprene. The two branched chain, internally double-bonded compounds 2-methyl-2-butene and 2,5diniethyl-2,5-hexadiene reacted sufficiently with the diazonium salt to give optical absorbances IThich are about 2% of that given by isoprene. The coupling products of 2-hexene, cyclohexene, and 2-methyl-2-butene had weak absorption maxima in the same region as isoprene. The absorption spectra of a number of aromatic hydrocarbons, aldehydes, ketones, and phenols also were measured under the same conditions. Hou-ever, a number of serious disadvantages were encountered in using 2-methoxyethanol as the solvent. The reaction times necessary to obtain the desired intensities were in the range of 18 t o 24 hours, and allowed appreciable decomposition of the diazonium salt in solution. Even more serious was the lack of a good linear relationship between concentration of diolefin and optical absorbance of the coupling products for all of the conjugated diolefins used, including 1,3-butadiene1 1,3pentadiene, 2-methyl-1 ,3-butadieneJ 2,3dimethyl-1,3-butadiene, and 2,5-dimethyl-2,4-hexadiene. It was felt that a solvent system of higher acidity might eliminate at least some of the above problems by increasing the rate of reaction of the diazonium salt with the diolefins and by stabilizing the diazonium salt in solution, This assumption proved to be correct. A 2-methoxyethanol-acetic acid solvent system was briefly studied first and a twofold increase in intensity for the isoprene coupling product was observed during the 24-hour reaction period. A 2 - methoxyethanolphosphoric acid solvent system was then investigated. I n this system, the
reaction rates were so greatly increased that the absorptivities observed after 4 hours !+-ere two to three times those found after 24 hours using only 2methoxyethanol as the soh-ent. This improvement in seneitivity is shown by the results given in Table I. The shorter reaction times and the high acidity of the solvent medium also eliminated the problem of serious decomposition of the diazonium salt in solution during the reaction period. Finally, good linear relationships between optical absorbance and concentration of diolefin could now be obtained for some of the conjugated diolefins tested. Consequently, the 2-met hoxyet hanol-phosphoric acid soivent system was used for thc rest of the present investigation. RESULTS FOR 2-METHYL- A N D 2,3DIMETHYL- 1,3-BUTADIENE
Rates of Reaction. The rates of formation of the coupling product have been follo&ed spectrophotometrically for the reaction of several of the diolefins with p-nitrobenzenediazonium fluoborate for reaction periods up to 6 hours. I n particular, the reaction rates of 2-methy1-1,3butadiene and 2,3-dimethyl-1,3-butadiene have been studied in detail at various phosphoric acid concentrations, temperatures, and reagent concentrations in the final solutions (Tables I1 and 111). -4reaction time of 4 hours appears to be about the best compromise between optimum intensity and analysis time. The specific absorptivities a t 2 hours range from 0.5 to 0.75 of those a t 4 hours, and average 0.6 to 0.7 of those after 3 hours. The absorptivities after 6 hours are 10 to 30% higher than those at 4 hours for the coupling reaction of isoprene with the diazonium salt. The absorptivities after 4 and 6 hours for the 2,3 - dimethyl - 1,3butadiene - p-nitrobenzenediazonium
fluoborate reaction are essentially the same, The absorptivities reached after 2 or 3 hours may be sufficient for some applications An exception t o these results occurs for the allo-ocimene-2,7-dimethyl-2,4,6octatriene - p-nitrobenzenediazonium reaction, which proceeds much more rapidly. After less than 30 minutes, the intensity of the absorption maxima has reached a maximum, and then a gradual decrease in intensity occurs. Reagent Concentration. T h e intensities at t h e absorption maxima of the products of several diolefins were investigated a t various reagent concentrations in the final solution (Tables I1 and 111). When collecting a trace component from an essentially unlimited gas saniple it is advantageous to use the 1.0 or 0.4 weight reagent concentrations. The absorptivities a t 25", computed using the concentration of the isoprene after dilution, of the 1.8% reagent solutions are almost twice those of the l.OyG reagent solutions and almost three times those of the 0.4% reagent solutions. However, the smaller dilution ratios of the latter two solutions more than compensate for their lower intensities. The absorptivities given in Tables I1 and I11 are computed on the basis of initial concentrations before dilution to take differences in dilution into consideration. For example, using these values, a t 25' and a t optimum acid concentrations, 10 pg. per ml. of isoprene in 1 nil. of sample solution when diluted up to 10 mL will give an absorbance of only 0.25. However, 10 pg. per ml. of isoprene in 8 ml. of sample solution when diluted up to 10 mi. will give a n absorbance of 0.68. Phosphoric Acid Concentration. The solutions were prepared with 10, 15, 20, 25, and 307, of concentrated phosphoric acid (85%) in t h e final mixtures. A few determinations also were made with t8he concentrated phosphoric acid used below 10% and above 30%. The absorptivities for 2-methyl-1,3butadiene are maximized in solutions containing about 25% concentrated phosphoric acid. However, particularly at a 25" reaction temperature, there is little difference in the intensities in 20 or 25% phosphoric acid solutions, Absorptivities for 2,3-dimethyl-l, 3butadiene are niaximiaed in solutions containing about 15% concentrated phosphoric acid. Again, a t the 25" reaction temperature there is little difference among the intensities at 15, 20, and even 257, phosphoric for the 0.4 and 1.0% reagent concentrations. Reaction Temperature. T h e effect of temperature was investigated under a number of experimental conditions, b u t t h e most complete data obtained I
Absorptivities for Coupling Product ob Isoprene with p-NitrobeKzenediazonium Fluoborate" Reagent Content, a, fig.-~~Pvfl, Cm.-1 Time, T:~P., 1.8% 1% 0,470 C. Hr. 0,008 . . . R.t.b 10 2 0.0116 ... 0 .i i i 0 10 25 4 0.061 10 4 35 oIoi4 0 :635 2 R.Lb 15 0 ,'o&c 0.019 0.054 R.t.6 4 15 ... 0,090 4 15 35 0.104 15 35 6 0.02202 0,0019 0.068 &'0,005 20 3 25 0.0245f 0.0015 0.088zk 0.005 0.088%"0.005 4 20 25 0.0255zk 0.0013 0.125 f 0.010 20 25 6 0.0297 f 0.0025 0.097 f 0.010 0,090 20 3 35 0.0294 f 0.0024 0.120 f 0.010 0.122k 0.006 20 4 35 0.0260f 0.0028 0.10 f 0.02 0.154 20 6 35 0.0213f 0.0029 0.078 f 0.006 25 3 25 0.0243i 0.0029 0.100 zk 0.008 0.0672 ' 0 . 0 3 3 25 4 25 0.0255i 0.0029 0.128f 0.003 25 25 6 0.0312rrt 0.0023 0.143 =!= 0.006 0 ,io3 25 3 35 0.0315f 0.0022 0.166& 0.006 0.133 =k 0.016 25 4 35 0.0302f 0.0032 0.15 i 0.02 0.219 25 6 35 0.012 ... 30 2 R.t. 0.023* ... 0.044 &'O.OOS 30 4 25 0.102 & 0.011 *.. ... 30 4 35 ... ... 0.13 30 25 6
Table II.
. . 1
a
6 c
hbsorptivities calculated using isoprene concentration, pg./ml., before d-iiution. Roomotemperatwe, 25" f 2". At 25 .
were for isoprene a t 25' and 3 5 O , at 20 and 2 5 7 , phosphoric acid, and at 0.4, 1.0, and 1.87, reagent in the final solutions. The data clearly indicate a n increasing dependence on temperature as the reagent concentration decreases. For example, for isoprene a t 25y0 phosphoric acid concentration and 4-hour reartion time, the ratioe of the specific absorptivities a t 35" to those a t 25" for 1.8, 1.0, and 0.4% reagent are, respectively, 1.30, 1.66, and 2.00. These appreciable temperature coefficients clearly indicate the necessity for adequate temperature control during the reaction period. RESULTS FOR OTHER DIOLEFINS
A number of other acyclic diolefins, inclu6ing 1,3-butadiene, 1,3-pentadieneJ
2-methyl-l .3-pentadieneJ 2,S-dimeth~12,4 - hexadiene, 2,5 - dimethyl - 1,5hexadiene, l,g-hexadiene, and the acyclic terpene. niyrcene, were investigated in the coupling reaction. Two terpenes having cyclohexadiene structures, aterpinene and a-phellandrene, also were used in a limited. number of memurements. Finally, the acyclic terpene, dlo-ocimene (2,6 - d.imethy1 2,4,6octatriene), and dimethylfulveae wew studied to find out the type of spectra resulting from the cotipiing of trieneb with p nitrGbenaenediaaonium fluoborate. Because of hhe importance of htadiene, a considerable effort was mhd9 to find satisfactory reaction condition3 for this compound. At the 1.8 and 1.6% reagent concentrations, tlir r e l a t w ~ ~ ~ h i p I
I
Absorptivities for Coupling Product of 2,3-Dimethyl-'i ,%butadiene with p-Nitrobenzenediazonium Fluoboratea Time, T;mp., HJ'OI, Reagent Content, a, p g . - l M1. Cm.-1 Hr. C. 7% 18% 1.070 0 47-; 0 151 f 0 008 0.14 R.t.b 10 0.0265 i 0 007 4 0 230 rt. 0 016 0.13 10 35 ... 4 0 022 R.t. 10 6 0.0281 i 0 0004 ... 2 R.t. 15 0 168 f 0 OOlc 0.0294 0 0013 0.165 4 Ret. 15 0 242 f 0 002 0 , 1.55 4 35 15 ... 0.12 R.t. 20 0.024 2 0 112 f 0 008? 0.15 R.t. 20 0.0268i 0.0019 4 0 200 0 013 35 20 ... 4 0.035 0.13 2 0.0153 I 0.0003 R.t. 25 0.057 % 0 . 0 0 2 ~ 0.0262 0.0ll 0.16 4 R.t. 25 0.13 4 35 25 R.t. 30 0.0100 $'O.OCyI 2 R.t. 30 0.0172i 0.007 4 ... 4 .. 35 30 a Absorptivities calculated using 2,3-dimethyl-?,2-butadiene concetitrations, pg. /d, Se-
Table 111.
...
*
I . .
b
fore dilution. Room temperature 25" i 2'.
c
At 25".
VOL. 32, NO. 13, DECEMBER I960
e
1845
Table
IV. Absorptivities for Product of 1,3-Butadiene with p-Nitrobenzenediazonium Fluoborate"
Time, Hr. 4 4 4 3 4 6 3 4
Ttmp., C. R.t. 35 R.t. 35 35 35 35 35
HYPO^,
%
10 10 20 20 20 20 25 25
Reagent Content, a, pg.-1 Ml. Cm.-1 1.8% 1.0% 0.4% 0.001b
, . .
.
I
.
0 .O l b
0.003b
... ... ... ... ...
o.oiiib
0.022b 0 .036b
...
0.009'Z 0.001 0.012 f 0.001
0.014f 0.002 0.014 f 0.002 0.023& 0.002
Absorptivities calculated using 1,a-butadiene concentrations, pg./ml., before dilution. n'onlinear relationship between absorbance and lJ3-butadiene concentration, and wave length of absorption maxima varies with concentration of diolefin. a
between absorbance and diolefin concentration was not a linear one. Furthermore, the peak maxima shifted with concent
