Cyanogera Bromide and Cyanogen. by AUGUSTUSEDWARD DIXON and JOHNTAYLOR

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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 hydroxide, 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 concentrated solutions, however, some cyanate is detectable. These pheno- mena are explained by the fact that carbamide tends to give derivatives 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 presence 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 hypobromite, 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 thiocyanate. In the case of hydrogen sulphide it is possible that free thiohypobromous 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 hydrochloric acid serves to inhibit the production of thiocyanic acid. If thiohypobromous acid is analogous to hypobromous acid, its decomposition must be accelerated by concentrated hydrochloric acid, with consequent diminution in the amount of available sulphur. Conversely, whatever delays the decomposition of the thiohypobromous 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, thiocyanic 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 thiocyanic 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. 975), the cyanogen bromide being very dilute, for 2 molecules taken, 1-01 equivalents of thiocyanic acid were found present; in the latter ones, for the View Article Online

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same amount of bromide (used at concentrations from one-tenth to fivetenths of a gram-molecule per litre), the corresponding figures for the thiocyanic acid varied from 0.85 to 0.59 equivalent. To observe further the effect of concentration, excess of gaseous hydrogen sulphide was brought into contact wit.h a solution of cyanogen bromide, containing 1.2 decigram-molecules per litre ; reckoned as before, the amount of thiocyanic acid was now 0.43 equivalent. From the resiilts obtained it appears that the proportion of thiocyanic acid is highly variable.

Sodium Thiosu7phate and Cyanogen Bromide. When cold neutral aqueous solutions were mixed, alkalinity was developed, lasting for one or two minutes; the final products were hydrogeri cyanide, alkali sulphate, bromide, tetrathionate, thio- cyaEate, and sulphite. The alkalinity is due to the production of alkali cyanide: NaO*S02*SNa+ CN*~r~NaO.SO2*S=CNa:NBr--+ NaCN + KaO*SO,-SBr ; then : NaO*SO,-SBr + NaS=SO2=ONa=NaBr + N+S,O,. Next, the alkali cyanide reacts with the tetrathionate (see Marshall, T., 1908, 93, 1729), producing hydrogen cyanide, together with alkali sulphate, sulphite, and thiocyanate, the complete interaction being expressed by the equation: 4CN*Br+ 8Na,S20, + H,O = Na,S04 + Na,SO, -+ 2NaSCK + 2IICN -t3Na,S40, + 4NaBr. This was verified by the observations (i) that about one-half of Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. the cyanogen in a known amount of the bromide went to form thiocyanate, and (ii) that when cyanogen bromide was treated with N / 10-sodium thiosulpha.te, followed by A'/ 10-iodine, the apparent molecular ratio of cyanogen bromide to thiosulphate consumed was 2:3; thus, four molecules of bromide absorb eight molecules of thiosulphate; but as the resultant mixture contains 1 molecule of sulphite, equivalent, so far as iodine-absorption is concerned, to two of thiosulphate, the apparent ratio of 4 :6 is explained.

Thiocar bainide in A queous Solution, and Cyanogen Bromide. If these are mixed in presence of sodium hydrogen carbonate (which has no action on the bromide) carbon dioxide escapes; the resultant solution contains sodium thiocyanate, sodium bromide, and cyanamide, but no cyanide: G'SN,H, + CN*Br=HBr + HSCN + NH,-CN. View Article Online

CYANOGEN BROMIDE AND CYANOGEN. 979

If, instead of the bicarbonate, a strong acid be present, the products consist solely of hydrocyanic acid, hydrobromic acid, and a salt of formamidim disulphide, together with a trace of sulphuric acid. IVhen the components are mixed in simple aqueous solution all the products named above are formed. According to a recent hypothesis (Dixon and Taylor, T., 1912, 101, 2502) it is supposed that in certain reactions of thiocarbamide the production of isothiocarbamide derivatives is brought about by the primary formation of additive compounds, resulting through the development of quadrivalence on the part of the sulphur atom, and the subsequent decomposition of the substances thus produced ; for example : SC(NH2)z+ AcCl= AcSCI:C(NH&=HCl+ AcS*C(:NH)*NH2. A strongly ionised acid, HX, could, in like manner, become united with the sulphur atom to yield a compound, C(N%),:SHX, readily transformable into the basic form, NH,*C(NH)*SH, unstable by itself, but capable of retaining this configuration so long as it remains associated' with a strong acid; and whilst the molecule is in this condition an oxidising agent, incapable of attacking the comparatively stable amino-group, could attack the SH-hydrogen. On the withdrawal of the combined acid the hydrogen at the *SH-group, migrating, like an acyl radicle, to the imidic nitrogen, thiocarbamide would be regenerated. There is reason to believe that such reversion occurs; for, in the reaction with cyanogen bromide, if the acidified solution of thiocarbamide is neutralised with sodium hydrogen carbonate, and then treated with the bromide, thO products arq the same as when the Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. preliminary treatment with acid is omitted. The mechanism of these reactions is explained as foIlows : where no acid is present, or allowed to become free, the thiocarbamide unites primarily with the whole molecule of cyanogen bromide ; the elimination of the elements of hydrogen bromide from (NH2),C:S:CN*Br produces NH,-C(XH)*S.CN, the residue decomposing (like certain alkylisothiocarbamides) into cyanamide and thiocyanic acid (thiocyanic acid or cyanogen hydrogen sulphide being the analogue of alkyl hydrogen sulphide). When the interaction takes place in presence of a strong acid the primary union between the isothiocarbamide salt and the cyanogen bromide (as in the case of hydrogen sulphide) occurs at the *SH-group, with formation of NH2*C(NH)*C-S*CH:NBr. This decomposes into hydrogen cyanide and NE,*C(NH)*S*Br,the latter of which, attacking another molecule of isothiocarbamide salt, yields hydrogen bromide and formamidine disuIphi.de : View Article Online

980 DLXON AND TAYLOR:

NH,*C(:NH)*SH + BrS*C(:NH)*NH, = NH,=C(:NH)-S*S*C(:NH)*NH2+ HBr ; thus, the non-production of thiocyanic acid is accounted for, as well as the production of all the substances detected in the reaction, save the trace of sulphuric acid. This last, no doubt, proceeds from the formamidine disulphide, which, as Werner has shown (T., 1912, 101, 2182), is very easily oxidised. The reaction with thiocarbamide in presence of acid is essentially parallel to that with cyanogen bromide and thiosulphate already mentioned. At all events, the explanation is similar for both; in each case an additive compound is formed; both decompose, giving cyanide and an unstable residue containing *SBr; and in each case that residue combines with another moIecule of the original thio- compound, eliminating hydrogen (or sodium) bromide. The results when tlhe interaction takes place in simple aqueous solution are now easily interpreted. At first, cyanamide is formed, together with hydrobromic and thiocyanic acids ; these, combining with t%e yet unchanged thiocarbamide, and so producing isothiocarbamide salt, render possible the formation of hydrogen cyanide and formamidine disulphide.

The Constitution of Cyanogen. To cyanogen the constitution usually assigned is NiC*CiN, the chief grounds for this formula being that the substance is producible from oxamide or from glyoxime, and that, when treated with hydrochloric acid, it yields oxamide. On the other haEd, when passed into cold aqueous alkali hydr- Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. oxide it decomposes, forming alkali cyanide and alkali cyanate; in ordinary circumstances such behaviour would be used as an argu- ment to show that the two carbon atomb are not singly linked. Moreover, when kept in aqueous solution, cyanogen yields not merely oxalic acid, but alsu hydrocyanic acid and ammonium carbonate; it may be recalled, too, that hydrocyanic acid, under the same conditions, yields some oxalic acid. Further, although the methods of formation named above are consistent with the view that cyanogen is a normal, rather than an isn-compound, other methods of preparation are known, which do not suggest a normal constitution for the product; for example, silver cyanide or mercuric cyanide when heated yields cyanogen ; there is considerable reason to believe that these are isocyanides, and hence (unless transformation is effected by the heating) the constitution of the resultant molecule is quite as likely to be :C:N*N:C: as to be NiC=C'iN. Besides, the compound is producible View Article Online

CYAKOGEN BROMIDE AND CYANOGEN. 981

without strong heating, namely, through the decomposition of cupric isocyanide. It may, perhaps, be objected that it is not certain that the cupric salt is an isocyanide; but cyanogen bromide (which certainly is not Br-CiN), when treated with cold aqueous potassium cyanide (probably K*N:C:), is said to yield ordinary cyanogen. Further, the reaction with alkali hydroxide or with alkali sulphide so closely resembles that of cyanogen bromide with the same materials as to suggest that, in the case of cyanogen itself, one cyanogen radicle plays towards the other the same part as the halogen radicle in the bromide; in which case the changes may be explained on precisely similar lines, thus : K*1;7*OK +K,O -+ -+ K(CN) + K( CNO). N*(CN)5 N-(CN) K*E*SK 1;' +K2S -+ K(CN) + K(CNS). K.(CN) -+ N*(CN) There is no special difficulty in accounting for the slow production of oxalic acid (or oxamide) from aqueous cyanogen, supposing it to be an iso-compound:

or for that of hydrocyanic acid and ammonium carbonate, for here water (of course, very slowly) may behave like alkali hydroxide (see above), the resultant cyanic acid decomposing into ammonia and carbon dioxide. Another point seems worth notice. By the dehydration of an

Published on 01 January 1913. Downloaded by Gazi Universitesi 23/03/2016 14:32:46. amide of the type R*CO*NH,, the syste-n R*CqX. presumably occurs; here the free valencies, if they are to link up, must neces- sarily do so between the carbon and the nitrogen; the product, therefore (apart from molecular rearrangement), must be R-CiN. In the dehydration of oxamide, however, this condition no longer holds; if two molecules of water are eliminated, the residue for the :Y*N: . moment is so that-unless it be assumed that the carbon :CON: ' valencies are incapable of uniting with one another-the system fi;? would represent that proceeding from the most equable distri- C.N bution of the four pairs of temporarily free valencies. Such a compound, breaking, according to circumstances, either at the doubly-linked carbon or at the singly-linked nitrogen pair of atoms, could yield all thO products directly obtainable from cyanogen ; thus, alkali hydroxide, by rupturing the double bond, could yield View Article Online

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those compounds the genesis of which is not easily intelligible on the assumption that the carbon atoms are but singly linked; whilst hydrochloric acid, attacking preferably the nitrogen atoms, might readily cause a hydrolysis which would leave the carbon atoms in single linking. This formula, too, meets to some extent at least the difficulty as to the production of ordinary cyanogen from isocyanogen radicles; for, by the union of the two pairs of unsaturated carbon valencies, a, cyclic compound could originate : 8: :fl --+ fl=g N-N N-N The constitution here suggested as best according with the proper- ties of cyanogen is so far similar to that of Gutmann for cyanogen bromide, namely, g>Br, that, in the former, a tervdent group N uN (CN) replaces bhe tervalent halogen of the latter, thus : I I>(CN). N Chattaway and Wadmore’s formula for cyanogen bromide does not really conflict with that of Gutmann (Zoc. cit.), for the feeble extra valencies of the tervalent haloid element would very readily be broken, ths carbon valencies thus becoming “ free,” and hence able to manifest the unsaturated character peculiar to the molecule, including its poisonousness. In like manner the cyanogen molecule, assisted, perhaps, by its own internal strains, could easily open out into a compound, :C:N*N:C:, the poisonousness of which might well be anticipated.

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