According to Greenish and Bartlett, the addition of sodium fluoride, recommended by Brioux2), acts very favorably, 0,4 p.c. of the fluoride being sufficient. The sodium fluoride does not interfere with the action of the myrosin, but kills micro-organisms that destroy the allyl mustard oil.
Another observation, made by Greenish and Bartlett, is also of interest, viz., that black mustard, harvested in 1905, when tested in 1911 revealed a normal myrosin content, thus showing that storage does not appear to injure the myrosin.
In the course of the chemical reaction which takes place in the formation of mustard oil, the glucoside of the mustard seed, sinigrin (potassium myronate), is hydrolyzed by the protein-like enzyme, in the presence of water, with the formation of mustard oil, (d-glucose and potassium acid sulphate.
C10H18Ns2ko9 + H20 = Csnc8h6 + CeH1206 + KHS04
Sinigrin Water Mustard Oil Grape sugar Potassium bisulphate.
In addition other reactions take place which account for two substances that are never entirely absent in mustard oil, viz., allylcyanide (cyanallyl) and carbon disulphide.
According to Gadamer3) the mustard oil glucosides are in all probability referable to a hypothetical alkyl iminothiocarbonic acid of the general formula R.N: C(SH)OH. In his opinion sinigrin has the following formula:
1) Pharmaceutical Journ. 88 (1912), 203.
2) Annal. Chim. analyt. appl. 17 (1912), 6.
3) Arch, der Pharm. 235 (1897), 47. - Berl. Berichte 30 (1897), 2322, 2328.
Hence sinigrin is an allyliminothiolcarbonic acid, in which the hydroxyl hydrogen is replaced by the potassium acid sulphate radical in ester combination, and the sulfhydryl hydrogen by the glucose residue in ether combination.
W.Schneider1) is occupied with experiments to produce synthetic compounds of the above general formula and thus to supply experimental proof for this hypothetical conception.
When kept in contact with water for any length of time, or when brought into contact with the copper of stills, mustard oil is converted into allylcyanide with separation of sulphur:
In the case of careless distillation the amount of allylcyanide thus resulting may be so large that the entire oil becomes lighter than water2) (Sp.gr. of allyl cyanide 0,835 at 17,5°).
The source of the carbon disulphide found in mustard oil, even in the artificial oil, has not yet been fully ascertained. Experiments3) have shown that when mustard oil is boiled with water in a flask connected with a reflux condenser, no carbon disulphide is formed. However, both carbon disulphide and carbon dioxide are formed in appreciable quantities when mustard oil and water are heated in a sealed tube, hence under pressure, for several hours to a temperature of from 100 to 105°. It may be assumed that mustard oil has a greater capacity for reaction in the nascent state and that under these conditions the presence of water causes a reaction to take place in the direction indicated by the following equation.
1) Berl. Berichte 45 (1912), 2961.
2) H.Will and W. Korner, Liebig's Annalen 125 (1863), 278.
3) Gadamer, Arch. der Pharm. 235 (1897), 53.
Carbon disulphide is also formed by prolonged contact of mustard oil and water. For the detection of carbon disulphide see under "Examination".
The time required to complete the fermentation, so far as pure sinigrin is concerned, is 80 minutes1).
As the potassium acid sulphate resulting from hydrolysis has a destructive effect on the mustard oil in course of being formed, a better yield is obtained (if pure sinigrin be employed) by neutralizing the reaction mixture with alkali. However, an excess of alkali should be carefully avoided, as it reduces the yield very considerably. For this purpose calcium carbonate has been found to be a suitable agent.
With mustard meal, however, the results are very different. Mot only is calcium carbonate of no use, but it is directly harmful. The cause for this peculiar behavior is unknown. Presumably the sinapine, an alkaloid contained in the mustard seed, is responsible. Being liberated by the calcium carbonate, it possibly reacts with the mustard oil, with the formation of a nonvolatile compound similar to thiosinamine1).
Composition. Apart from variable amounts of carbon disul-phide and allyl cyanide present, mustard oil consists almost entirely of allyl mustard oil or allyhsothiocyanate Csnc3h5. Possibly traces of the isomeric thiocyanallyl Cnsc3h5, also of higher boiling (polymeric) substances of unknown composition are present.
C. Pomeranz'2) assumes that natural mustard oil contains the isomeric propenyl mustard oil Csn • CH : CH • Ch3 besides allyl mustard oil CSNCH2CH: CH2 referred to above. For the synthetic oil this has been proved. Upon oxidation it yields not only much formic acid but acetic acid as well, which can result only from the propenyl mustard oil. However, this reaction has not yet been carried out in connection with natural mustard oil, hence the presence of propenyl mustard oil in the latter has not yet been established.
The chemical reactions of mustard oil are essentially those of allylisothiocyanate which can be found in every text-book of chemistry. Here only those are recorded that are necessary for an understanding of the tests.
1) Gadamer, loc. cit.
2) Liebig's Annalen 851 (1907), 354.
For the quantitative determination of allylisothiocyanate in mustard oil, its property to form a solid, non-volatile compound with ammonia is utilized. If an excess of ammonia and alcohol be added to mustard oil, the odor of both mustard oil and ammonia disappears, gradually in the cold, more rapidly when heated. At the same time crystals of thiosinamine are formed.
Thiosinamine or allylthiourea crystallizes in rhombic prisms which melt at 74°. It has a faintly leek-like odor and taste and is readily soluble in water, alcohol, and ether.
If to a small amount of mustard oil twice its volume of concentrated sulphuric acid is added a violent evolution of carbon oxysulphide1) and sulphurous acid2) takes place, resulting in the formation of allylamine sulphate which remains in the test tube as a faintly colored liquid, which, under certain conditions, may congeal.
Artificial Mustard Oil is obtained by the interaction of allyl iodide and potassium thiocyanate in alcoholic solution thiocyanate 175 to 176°, which consists principally of crotonyl mustard oil, had the following properties: d15o0,9941; aD + 0°3'; nD20o1,52398. Upon elementary analysis it yielded results agreeing with the above formula. The thiourea crystallized in long needles which melted at 69 to 70°. The crotonylthiocarbamic acid bornyl ester1) melted at from 55 to 56°. The constitution of the crotonyl group was not ascertained, but the oil probably is not the normal crotonyl mustard oil, the thiourea of which melts at 65 to 66°. The mixture of the normal thiourea with that from the natural oil melted between 45 and 50°2).
C3H5I + Cnsk = Csnc3h5 + KI
Allyl iodide Potassium Mustard oil Potassium iodide.
Allylthiocyanide Cnsc3h5 is first formed, but rearranges itself to mustard oil when heated.
Mustard oil is likewise formed when potassium allyl sulphate and potassium thiocyanate are subjected to dry distillation.
C3H6Ks04 + Cnsk = CSNC3H2 + K2S04
Potassium Potassium Mustard oil Potassium allyl sulphate thiocyanatc sulphate.
The properties of synthetic mustard oil have been described in vol. I, p. 547.
l) A. W. Hofmann, Berl. Berichte 1 (1868), 182. 2) Fluckiger, Arch, der Pharm. 196 (1871), 214.
In addition, the oil probably contained a substance that is identical with allylcyanide (m.p. of the crotonic acid 70° in place of 72°), as well as traces of dimethyl sulphide.
The production of such an abnormal oil from Brassica juncea is contrary to all previous experience. No explanation therefor could be found.
Examination. Inasmuch as there exists no means to distinguish between natural and artificial mustard oils, the latter has been made official in the 5th edition of the German Pharmacopoeia. The requirements are as follows: d15o1,022 to 1,025; soluble in all proportions of 90 p.c. alcohol; it should contain at least 97 p.c. of allyl mustard oil.
Particulars as to the method of analysis will be found on p. 605 of vol. I.
Determination of Carbon Disulphide in Mustard Oil. For the purpose of detecting larger amounts of carbon disulphide added as adulterant, the latter can be converted into copper xanthogenate3) and can thus be determined quantitatively.
For this purpose a current of air is slowly passed through 20 to 25 g. of mustard oil heated on a water bath, the flask being provided with a condenser. The carbon disulphide vapors are carried into a solution of alcoholic potassa and thus converted into potassium xanthogenate. The alkaline solution is then neutralized and 1/10-N-copper sulphate solution added until a drop removed from the reaction liquid is colored reddishl) Comp. M. Roshdestwensky, Chem. Zentralbl. 1910, I. 910.
2) For a different crotonyl mustard oil see Oil of Brassica Napus, p. 527.
3) Macagno, Zeitschr. f. anal. Chem. 21 (1882), 133.
brown by potassium ferrocyanide, in other words until a slight excess of copper sulphate is present and all potassium xantho-genate has been converted into cuprous xanthogenate. From the amount of copper solution consumed the amount of carbon disulphide can be computed (1 cc. corresponds to 0,0152 g. of carbon disulphide).
The precipitate of cuprous xanthogenate may be collected on a filter, washed with water, dried and heated in a crucible to a red heat, thereby converting it into cupric oxide. 1 g. of cupric oxide corresponds to 1,918 g. of carbon disulphide.
For the quantitative determination of the traces of carbon disulphide found in every mustard oil, the method of A.W. Hof-mann1) is resorted to. According to this method the carbon disulphide is converted into a compound with triethyl phosphine, P(C2H5)+Cs2, and weighed.