ICinetics of.Furfural Destruction in Acidic Aqueous Media D. L. WILLIAMS AND -4. P. DUNLOP, The Quaker Oats Company, Chicago, 111. Destruction of furfural in dilute aqueous solutions of mineral acids is a pseudounimolecular reaction, the rate of which is directly proportional to furfural concentration and to hydrogen ion concentration. Increase in temperature accelerates the reaction, and the Arrhenius equation relating reaction velocity and temperature has been derived; this permits calculation oE the extent of destruction in a variety of systems. The results indicate that furfural is relatively stable to dilute aqueous acids at nominal temperatures.
rithm of concentration of-furfural remaining are seen to be a linear function of time; this proves that the destruction of furfural under these conditions is described by the equation for a firstorder reaction: --" = at
kC
where The specific reaction rate constants shown on Figure 1 are in term8 of reciprocal minutes and were derived by the method of least squares. All other values of k were derived similarly. HYDROQEN IONCONCENTRATION. At fixed furfural concentration (1 gram per 100 ml. of solution) the rates of destruction were determined a t several concentrations of hydrochloric and sulfuric acids with the results listed in Table I. In the case of hydrochloric acid the specific reaction rate is proportional to the normality. Under the circumstances the hydrogen ion concentration should be almost directly proportional to normality. The rate constants found in 0.1 N sulfuric acid and in 0.05 N hydrochloric acid are in very close agreement, an indication that the hydrogen ion concentration in each system is practically identical under the experimental conditions. Thus the destruction of furfural is f i s t order with respect to hydrogen ion as well as to furfural concentration. TEMPERATURE. The relative rates of decomposition in aqueous sulfuric acid at several temperatures are reported in Table 11. I n each instance the initial concentration of furfural was 1 gram per 100 ml. of solution. These IC values were applied to the Arrhenius equation, which in its integrated form defines log k as a linear function of the reciprocal of the absolute temperature.
I
N CONNECTION with recent studies a need arose for quanti-
tative data on the stability of furfural under aqueous, acidic conditions. A search of the literature revealed only qualitative observations (rapid darkening of color, formation of resin, etc.), and general information with regard t o ring splitting of such furan compounds as furan itself, 2-methylfuranJ 2,5-dimethylfuran, furfuryl alcohol, etc. In all cases it had been recognized that the rate of destruction could be accelerated by increasing temperature and/or acid concentration, but no attempt was made t o quantitate these factors. The present study was undertaken with the object of deriving a mathematical expression which would permit calculation of the rate of destruction of furfural in aqueous, acidic media a t various temperatures. The furfural used was prepared by vacuum distillation of the ' technical product through a 60-cm. vacuum-jacketed packed column. A mid-fraction (60y0 middle cut) was collected and stored under nitrogen. Acidic aqueous solutions of this furfural were prepared by diluting weighed amounts with acid solutions of the desired normality. REACTION PROCEDURE AND ANALYSES
Aliquots of the acidic, aqueous furfural solutions were charged to Pyrex tubes (25 X 200 mm.) which were then sealed and immersed in an oil bath for the desired time interval. The temperature of the bath was maintained a t hhe indicated temperature b0.5' C. by means of an electrical heating circuit actuated through a control system consisting of a thermocouple and pyrometer. On removal from the bath, each tube was rapidly chilled and then opened for analysis. Quantitative estimation of furfural was made by the Hughes-Acree method (8). [ A t the time this work was carried out, analyses were running consistently low, averaging -1.0 mg. within the limits required by the method (25-100 mg.). All reported results were corrected for this deviation.] Early in the work, however, it was found that the decomposition products interfered with analysis for furfural. This interference was eliminated by neutralizing aliquots and steam distilling prior to analysis, since the troublesome impurities were found to be nonvolatile with steam. This analytical method proved to be entirely satisfactory from the standpoint of speed and accuracy.
log k =
E l -2.303R 7 4-
(3)
-A +c
(4)
logk =
Using the values listed in Table 111, a plot of the data in this fashion gave a straight line, the equation of which was: log k =
- 4365 + 7.145 T
TABLE I. EFFECTO F HYDROGEN I O N CONCENTRATION ON RATE OF FURFURAL DESTRUCTION AT 160' C. Elapsed Time, Min. 0 30 60 90 120 160 180 210 240 270
INFLUENCE OF VARIABLES
FURFURAL CONCENTRATION. The rate of destruction of furfural, a t fixed acid concentration and temperature, was found to be directly proportional to the concentration of furfural. This is shown diagrammatically in Figure 1, wherein plots of loga-
__
Rate constant k =
239
Furfural Remaining, G./100 M1. 0.05 N HC1 0.1 N HC1 0.1 N HzS04 1.00 2.00 1.00 1.00 0.99 1.95 0.94 0.96 1.87 0'.94 0.88 0.94 1.79 0.84 0.79 0.88 1.74 6.88 0.86 1.69 0.73 0.83 1.66 0.84 0.68 0.80 1.57 0.64 0.77 0.78 ... 0.76 ...
...
*..
e . .
1.12 X 10-2 1.14 X 10-9 1.07 X 10-8 2.16 X IO-:
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
240
Vol. 40, No. 2
MECHANISM AND PRODUCTS OF DESTRUCTION C e - 2 QRAMS/IOO MI.. 4 K LL
K
a
Co*lQRAM/100 ML. k=I.lZ xlO-S/MIN.
LL (P
0
A
'
TlUF
0
1 O'
1
0
Figure 1. Effect of Furfural Concentration on Rate of Destruction by 0.1 N Sulfuric Acid at 160' C.
Derivation of the energy of activation, E, gives a value of approximately 20,000 calories per mole. Using Equation 5 one obtains the calculated values for k shown in Table 111. These values are expressed in reciprocal minutes, and they apply only to the destruction of furfural in 0.1 N sulfuric acid. For different acid systems the rate constant may be corrected in accordance with the fact that k is directly proportional to the hydrogen ion concentration. I n Table IV the rate of destruction of furfural in 0.1 A' sulfuric acid has been calculated for several different temperatures. On several occasions the present authors calculated the extent of furfural destruction at other acid concentrations and found good agreement with the experimental values. For example, a sulfuric acid solution (0.047 AT) containing about 7% of furfural was refluxed for 194 hours (under a nitrogen blanket t'o prevent autoxidation), and a t the end of this period only 13.3% of the furfural had been destroyed. The calculated destruction, using Equations 5 and 2, is 12.770, as illustrated:
,
Given: T = 98°C. = 371"A. Acid = 0.047 N HISO, t = 194 hr. = 11,640 min. Co = 7.1 g. furfura1/100 ml. = initial concn. C = final concn., g./100 ml. -4365 log12 = __ 371 7.145 from Equation 5
4
It was noted that the titratable acidity gradually increased during the destruction of furfural by aqueous acids. Part of this formed acid was identified as formic acid by use of the pbromophenacyl ester. I n addition, a resinous tar is formed vhich can be separated into two fractions by virtue of differential solubility in acetone. The acetone-soluble portion is medium broivn in color and possesses high tinctorial poxver. I t 13 also soluble in aqueous sodium hydroxide from which it is reprecipitated on acidification. The acetone-insoluble fraction is very dark brown to black in color, and is insoluble in the common organic solvents and in aqueous alkali. It appears to be a more advanced form of the acetone-soluble resin, as evidenced by transformation of the latter on heating a ith dilute sulfuric acid, into a black resin similar in characteristics t o the acetone-insoluble resin. It is reasonable to assume that the formic acid results from hydrolytic fission of the aldehyde group. The structures of the resinous products are unknown, although consideration of the products of ring scission from 2-methyl-furan ( I ) and 2,5dimethylfuran ( 4 ) suggests that they might be condenration polymers of succindialdehyde as proposed previously by hfarcusson (3).
Ii
11 Iv-CHO 0
HOH
-+
HZC-CHg
I
1
[H+] OHC CHO
+
HCOOH
This postulate, however, requires that one mole of formic acid be formed for each mole of furfural destroyed. Actually, in the present work, the total formed acid never exceeded two thirds of a mole per mole of furfural lost, and it is doubtful if all of this was formic acid in view of the acidic nature of one of the resin fractions. Further investigation of the mechanism is required before any conclusions can be drawn.
+
Sincek
a
.: k H+
=
2.39, X lO-S/min. in 0.1 N HlSOa
DESTRUCTION RATESAT SEVERAL TABLE 11. FURFURAL TEMPERATURES AND CATALYST CONCENTRATIONS Temp.,
k = 1.12 X lO-S/min. in 0.047 N HZSOI
c.
2303 C Now k = -log ' f r o m Equation 2 t C 7.1 :. 1.12 x 10-6 = 2.303 1164010g -C
150 160 170 190
0.1 N 0.1 N
0.1 N 0.05 N
200 210
~
&SO4 Catalyst, Concn. 0.1 N
0.05 N
Specific Reaction Rate Constant k, Min. - 1 0.71 X 10-8
1.12 1.96 5.15 4.43 6.69
or C = 6.2
:.
Destruction = (7'1[:2' ) X 100 = 12.7% KINETICS OF DESTRUCTION REACTION
The destruction of furfural in aqueous, acidic medium must be designated as a pseudounimolecular reaction. As shown previously, the rate is influenced by furfural concentration and by hydrogen ion concentration; hence it is a t least a second-order reaction, Actually it may be third order since, as will be shown later, water is probably a reactant. That is,
-d
[furfural] at
= k
[furfural] [H+] [HzO]
However, since water was present in overwhelming abundance in this work, its concentration was not altered significantly during the course of reaction. Similarly the hydrogen ion concentration was essentially constant (as shown in the next section) for an extended period in all cases studied. Accordingly, when furfural is exposed to dilute aqueous acids, the destruction reaction follows a first order rate equation for a considerable time.
OF TEMPERATURE ON FURFURAL TABLE 111. INFLUENCE DESTRUCTIOS RATEIN 0.1 N H2SOI k x 108 k x 103 T, ' C. 1', A. l/Tabs. Obsvd. log k Calcd.
150 160 190 170 200a
2105
423 433 443 463 473 483
0.00236 0.00231 0.00226 0.00218 0.00211 0,00207
0.71 1.12 1.96 5.15 8.86 13.38
-3,155 -2.951 -2.708 -2.288 -2.053 -1.874
0.70 1.16 1.96 5.21 8.26 12.83
a T h e k values a t ZOOo C. and 210' C. in 0.1 A' sulfuric acid were obtained from those in 0.05 N sulfuric acid (Table 11) b y multip!ication by a factor of 2 t o correct for [ H t ] . Since these two points fall directly on the curve i t appears t h a t at these elevated temperatures, the [H'] in 0.1 N sulfurid acid is almost exactly tnice t h a t in 0.05 A' sulfuric acid.
TABLEIV. RATEOF DESTRUCTION OF FURFURAL IN 0.1 N H2SOa Temp., C. 50 100 150 200 250 300
k Calcd., Min. -1
4.3 x 2.8 x 7.0 x 8.3 x 6.3 x
lo-' 10-6 10-4 10-3 10-3
33.7 x 10-9
R a t e of Deiltruction, %/Min. 0.00004 0,0028 0.07 0.83 6.3 33.7
February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
As a result of the formation of formic acid during the course of destruction of furfural, it might be argued that the over-all reaction would be autocatalytic and thus would proceed a t an ever-increasing rate. This is a t variance with the evidence, presented earlier, which demonstrates that the destruction obeys the equation of a fist-order reaction for an extended period. No doubt the explanation lies in the fact that the amount of formic acid formed was quite small, and, as a consequence of its low degree of dissociation (as evidenced by the value of its ionization constant, 1.76 X compared to that of hydrochloric acid or of sulfuric acid, the formic acid did not significantly alter the initial hydrogen ion concentration in the present work.
241
ACKNOWLEDGMENT
The authors are grateful to S. L. Deutsch for technical assistance, and to I. M. Klotz for his careful review of the work. LITERATURE CITED (1) Harries, C., Ber., 31,43 (1898).
(2) Hughes, E.E., and Aoree, S. F., IND.EKG.CHEM.,ANAL.ED.,6, 292 (1934). (3) Marcusson, J., Ber., 54B,542-5 (1921). (4) Young, D. M., and Allen, C. F, H., “Organic Syntheses,” Vol. XVI, p. 26, note 2 , John Wiley & Sons, Inc., 1936. RECEIVED October 13, 1947. Presented in part before the Physical Chemistry Section a t the second All-Day Technical Conference of the Chicago Section of the A M ~ R I C ACHEMICAL N SOCIETY, January 1947.
Natural Vulcanization Accelerators in Hevea Latex R. F. A. ALTMAW Rubber Research Institute, Buitenzorg, Java
A n extensive study of the literature led to the bdief that latex proteins do not play the predominant role in producing the powerful “natural” vulcanization accelerators, as was suggested by most investigators. It is found that certain nitrogenous bases, either naturally present in fresh latex or formed as the result of putrefaction, heating, and other decomposition processes, are most important in accelerating the rate of cure. With the aid of the systematic method of analysis developed by the author some years ago (6), the nonrubber components of Hevea latex have been separated into various fractions which, one by one, have been examined as to their accelerating properties. The results are given in Figure 1. Apparently the natural vulcanization acceleratorscan be identified on the one hand as the N-bases naturally present in Hevea latex (e), and on the other as choline and probably also colamine, these bases being derived from lecithins and cephalins, respectively. Furthermore, prolamines are also found active.
A
S NUMEROUS investigators have stated, certain nonrubber components of Hevea latex have a decided influence on the process of vulcanization. These components, generally known as natural vulcanization accelerators, have held the attention of both scientists and manufacturers for decades. Reviews of literature on this subject were given by Whitby (86, M A ) , Kindscher (43, and vanRossem (67). However, since a great variety of excellent synthetic vulcanization accelerators has been discovered2, and since it has been considered (15, $0, 67) to be “wrong and beyond hope to believe that the plantation industry as a whole will be able to produce a strictly uniform first latex plantation rubber” (67)-the aim which manufacturers always had in view-this subject now seems to have lost much of the attractiveness which it formerly had for science and industry. Nevertheless an investigation, i n casu, of the isolation and identification of the natural vulcanization accelerators, must be considered motivated for two 1 Present address, Laan van Meerdervoort 52 H , The Hague, Holland. This discovery led t o the statement t h a t the time of vulcanization of even inferior samples of rubber can be reduced to a time of cure which, for practical purposes, Bhows no variation ( 4 7 , 66,68, 9 5 ) .
reasons: First, the problem has not yet been settled in all respects; second, this subject could not be neglected in the author’s study of the influence of the nonrubber constituents on the general properties of rubber. There is a pronounced difference in the accelerating action of substances, whether they are used in ’mixtures containing (a) only rubber and sulfur, ( b ) rubber, sulfur, and an inorganic accelerator, such as litharge, zinc oxide, etc., or (c) rubber, sulfur, and an organic accelerator, such as mercaptobenzothiazole or diphenylguanidine. ACETONE EXTRACT
Mainly as the result of the work of Spence (68, 69, 70, 7f), Beadle and Stevens (IO,7 2 ) , and Weber (85, 84),it was shown that the rate of vulcanization of acetone-extracted pale plantation rubber was remarkably slow. This applies to pure gum mixes as well as to mixes containing zinc oxide. Stevens (76) stated that the properties of acetone-extracted rubber are partly or wholly restored when mixed back with the extracted resin or with foreign resins, such as jelutong resin. Wruck also came to the conclusion that “the resins act as weak positive catalysts of vulcanization” 96). The experiments conducted by van Heurn (40,4 1 ) , however, merely led to the result that the presence of resin reduces the breaking load. I n this connection the investigations of Kratz and Flower (44) should be mentioned; they found, t o the contrary, an increasing vulcanization coefficient as a result of 36hour acetone extraction of raw rubber. Martin and Elliott (.49),investigating the influence of the kind and amount of natural nonrubber constituents on the content of combined sulfur of the vulcanizates, found, as did others, that (a) the amount of combined sulfur a t the standard cure depends on the way in which the rubber is prepared, on the trees from which the rubber is obtained, and also to a less extent on unexplained variations in the finished rubber from time to time; ( b ) the amount of combined sulfur a t the standard cure for rubber of the same type, and from the same trees, varies approximately as the amount of resin constituents of the rubber varies (63, 78, 80); and (c) the resin of crepe rubber has little influence on the rate of combination of rubber with sulfur, whereas the resins of sheets and slabs act as mild and strong accelerators, respectively.
