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Cyanogera Bromide and Cyanogen. by AUGUSTUSEDWARD DIXON and JOHNTAYLOR

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Cyanogera Bromide and Cyanogen. by AUGUSTUSEDWARD DIXON and JOHNTAYLOR View Article Online / Journal Homepage / Table of Contents for this issue 974 DIXON AND TAYLOR: GI.- Cyanogera Bromide and Cyanogen. By AUGUSTUSEDWARD DIXON and JOHNTAYLOR. CYANOGENbromide, in cold aqueous solution, or in the presence of such dilute acids as do not of themselves chemically decompose it, shows no evidence of suffering ionic dissociation. The dilute aqueous solution has the same odour as the solid compound; even after long keeping it yields with silver nitrate no turbidity; it is neutral to litmus, and the pungent vapour fails to give the guaiacum and copper sulphate reaction for hydrogen cyanide ; moreover, the solution is a very feeble conductor of electricity. Although the mixture produced by treating cyanogen bromide with alkali hydroxide contains but alkali bromide and alkali cyanate, Chattaway and Wadmore are of opinion (T., 1902, 81, 199) that hypobromite must first be formed, and then reduced. That cyanate is not directly formed in the reaction with alkali hydroxide is proved from the following facts: (1) Alkali cyanate is not reduced to cyanide by hydriodic acid, ferrous sulphate and alkali, sulphurous acid, alkali sulphite, or even by treatment with aluminium and alkali hydroxide. Further, it has no action on carbamide, either alone or in presence of alkali, (2) If cyanogen bromide is treated with alkali iodide, followed -by alkali, the mixture contains cyanide, but no cyanate, and, when acidified, yields free iodine. (3) If it is treated with ferrous sulphate, and subsequently with alkali and ferric salt, the mixture on acidification gives Prussian- blue, but contains no cyanate. Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. (4) The bromide, if mixed first with sodium sulphite and then with alkali, contains sulphate and cyanide; no cyanate is present. (5) A moderately concentrated solution of cyanogen bromide, mixed with carbamide, effervesces on the addition of alkali hydr- oxide, nitrogen being evolved ; hypobromite therefore appears to be present. The mixture in this case contains not only cyanide but also some cyanate. In cases (2) to (5) mentioned above, if the addition of the alkali hydroxide precedes that of the various reducing agents named, no reduction occurs. A direct experiment showed that hypobromite instantly converts potassium cyanide into the cyanate. With very dilute solutions of carbamide no effervescence occurs, cyanate, but no cyanide, being formed; even with highly concen- trated solutions, however, some cyanate is detectable. These pheno- mena are explained by the fact that carbamide tends to give deriv- atives in which the haloid element is joined to nitrogen (Chattaway, View Article Online CYANOGEN BROMIDE AND CYANOGEN. 97 5 I’roc. Roy. Soc., 1908, A, 81, 381; T., 1901, 79, 274); and since these behave as oxidising agents, the carbamide in such dilute solutions plays merely the part of a bromine-carrier; thus, when potassium cyanide (1 gram-molecule per litre) is oxidised by alkaline hypobromite (0.2 gram-molecule per litre), the volume-relations are unaffected by the previous addition of 1 molecule of carbamide for each molecule of cyanide present; but with hypobromite at five times the above concentration nitrogen escapes, and the proportion of hypobromite required to complete the oxidation is considerably greater than in the absence of the carbamide. In all the above reactions the positive ion, joining the cyanogen, is eliminated as cyanide, the bromine being absorbed by the remainder of the molecule. The fact that water is non-ionised suffices to explain why it has no action on cyanogen bromide. According to Chattaway and Wadmore (Zoc. cit.) hydrogen sulphide reacts quantitatively with cyanogen bromide, as shown by the equation: CN*Br+ H,S =Hm+HBr +- S; ‘‘ a little ” thiocyanic acid, however, being produced, the formation of which is attributed to the action of the sulphur on the hydrogen cyanide. Our experimental results were as follows: A 2 per cent. solution of hydrogen cyanide, when shaken or gently warmed with flowers of sulphur, yielded with ferric chloride no red coloration; in pres- ence of hydrogen bromide the same negative result was obtained. Various methods were tried of precipitating sulphur in solutions containing hydrogen cyanide or potassium cyanide acidified with hydrochloric acid; in no case (provided that the cyanide solution Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. was kept acid) could any trace of thiocyanic acid be detected. Moreover, cyanogen bromide, when treated with potassium xanthate, yielded cyanide and a precipitate of sulphur, but no thiocyanic acid was found in the resultant mixture. In order to determine the relative amount of thiocyanic acid produced in the reaction between cyanogen bromide and hydrogen sulphide, a dilute aqueous solution of the former was exactly saturated with a dilute solution of the latter; after removal of the sulphur by filtration, the thiocyanic &id contained in the clear liquor was determined by Barnes and Liddle’s method; it was thus found that almost exactly one-half (0.505) of the cyanogen engaged had gone to form thiocyanic acid. Since cyanogen bromide is not ionised by water or by dilute acids, it is natural to suppose that its chemical changes are brought about through union with the ionised fractions of the material presented, this union being followed by decomposition of the 3s2 View Article Online 976 DIXON AND TAYLOR: resultant additive compound. In many cases that material cannot develop the higher valence of the nitrogen atom, whilst the carbon atom has available two valencies, through which this combination can take place; for example, the reaction with alkali hydroxide is represented thus : K,O + :C:NBr+OR*CK:NBr+KBrO + KCN+KBr + KCNO (no regard is here paid to the molecular structures of the products). The reaction with sodium sulphite is similarly explained : NaO-SO*O*CNa:NBr--+Na@N+ SO(ONa)*OBr--+ NaCN + NaBr + SO,. In the case of hydriodic acid: C"HI:NBr-+HCN + IBr and IBr + HI = HBr + I, ; with potassium xanthate : ELooCS*S*~*K--+ KCN + EtO*CS-SBr and Bl-N EtO-CS=SBr+ H,O =EtO-CS*OH+ HSBr-+HBr + S. More difficult to explain is the reaction between cyanogen bromide and hydrogen sulphide, for HS=C'H:NBr would yield HCN + HSBr instead of HSCN + HBr ; moreover, Gutmann has shown (Rer., 1909, 42, 3628) that when alkali sulphide is used the reaction proceeds in accordance with the equation : K,S + CNBr =KBr + KSCrN. From analogy to the alkali oxide reaction the primary change in the case of alkali suiphide is: K,S + CN*Br= CK(:NBr)*SK +KCN + KSBr ; Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. then : CNK + BrSK= KBr + CNSK, just as CNK + BrOK = KBr + CNOK. TEat potassium thiohypobromite is really produced and then desulphurised, ae- shown in these equations, receives support from the following facts. (a) Alkaline solution of potassium sulphide, when mixed with potassium cyanide followed by acid, gives no reaction for thiocyanic acid. (b) Potassium cyanide, mixed first with excess of alkaline hypo- bromite, next with alkaline sulphide, and then acidified, yields a liquid containing no thiocyanic acid (in other words, cyanate is not changed by alkali sulphide into thiocyanate). (c) The mixture of cyanide and sulphide, if treated with alkaline hypobromite and then acidified, reacts copiously for thiocyanic acid. View Article Online CYANOGEX BROMIDE AND CYANOQEN. 977 These facts may be interpreted as follows : alkali sulphide acting on the hypobromite thus : KBrO + KSH = KOH + EBrS ; the resultant thiohypohromite now transforms the cyanide into thio- cyanate. In the case of hydrogen sulphide it is possible that free thiohypo- bromous acid is first liberated, a portion of which decomposes forth- with, for sulphur is quickly (although not instantaneously) precipitated. Here the mechanism of the change is represented it9 follows : CH(:NBr)*SH--+HG'N + HSBr, the resultant thiohypobromous acid being desulphurised by the hydrogen cyanide. Since thO former, however, if produced at all, very soon decomposes, the hydrogen cyanide, unless able to seize at once the whole of the available sulphur, must undergo more or less incomplete transformation into thiocyanic acid. Consistent with this view is the fact that the presence of hydro- chloric acid serves to inhibit the production of thiocyanic acid. If thiohypobromous acid is analogous to hypobromous acid, its decom- position must be accelerated by concentrated hydrochloric acid, with consequent diminution in the amount of available sulphur. Conversely, whatever delays the decomposition of the thiohypo- bromous a2id or accelerakes the rate at which the cyanide can desulphurise it, favours the production of thiocyanic acid ; alkali may act in either or both of these ways. When aqueous hydrogen sulphide is used, >lie large bulk required tends to produce ultimately solutions of almost equal concentration ; at the commencement of mixing, however, this is not the case; we Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. have found that the variation in the amount of thiocyanic acid formed at different concentr,ations of the cyanogen bromide is quite r eadi 1y appreciable. Since the products of the change are hydrobromic acid, thio- cyanic acid, hydrogen cyanide, and sulphur, alkalimetric determin% tion of the total number of equivalents of acid formed from a known quantity of cyanogen bromide measures the amount of thio- cyanic acid present in the mixture. This was checked by indepen- dent measurements of the total acidity from 2 molecules of bromide (with N/10-alkali and methyl-orange), and of the thiocyanic acid (by Barnes and Liddle's method), the result, by calculation from the total acidity, and by direct determination, showing a difference of 0.05 equivalent in the amount of thiocyanic acid. In the experiment previously mentioned (p.
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