170 CHATTAWBY: THE TRANSFORMATION OF

XIX-The Transformation of Ammor&m Cyanate into Carbamide.

By FREDERICK DANIEL CHATTAWAY.

THE course o€ the reaction which takes place when ammonium cyanate is transformed into carbamide has never been satisfactorily explained. Up to a few years ago it was universally regarded as a peculiar case of isoEeric change, and no consideration was given to the process by which the conversion was effected.

Walker and Hambly (Trans., 1895, 67, 746) then showed that the transformation was reversible, but that in aqueous sdution it was accompanied by a subsidiary irreversible action, in which a portion of the cyanate m i t ~ hydrolysed and converted into ammonium carbonate. From measurements of the velocity of transformation, they showed that the reaction was approximately bimolecular, and not unimolecular, as had up to that time been assumed, and con- cluded that the reacting atomic groups were the ammonium and cyanic ions produced by the. electrolytic dissociation of the ammonium cyanate, and that the transformation should be formulated thus :

NH; + CNO = CO(NHJ2. It has been several times pointed out (compare Goldschmidt

(Zeitsch. Elektrochem., 1905, 11, 5) that this conclusion is not justified, and that the results agree equally well with the assump- tion tha t it is the non-ionised portion of the ammonium cyanate which undergoes transformation. The view tha t the ions take z1o part in the reaction is strengthened by Walker and Eay’s observation (Trans., 1897, 71, 489) that ammonium cyanate, when dissolved in 90 per cent. ethyl alcohol, is converted into carbamide about thirty times as fast under similar conditions as when dissolved in pure water. As the alcohol diminishes the degree of dissociation of the cyanate, and should therefore diminish the rate of carbamide- formation if this is due to the ions, Walker and Kay are forced to assume that the alcohol increases the rate at which the ions interact t o an extent much more than counterbalancing their diminished number, so that on the whole an accelerative effect is produced.

Further evidence that ammonium and cyanic ions are not necw sarily involved in the transformation is found in the circumstance that pure dry solid ammonium cyanate passes over into carbamide when heated; for example, Walker and Kay (Zoc. cit.) found that i t is almost wholly transformed into carbamide when heated a t 61°

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AMMONIUM CYANATE INTO C A ~ B A M ~ D E . 171

for five hours, a small quantity of ammonia and cyamelide being formed a t the same time whilst between 76O and 89O, the exact temperature depending on the rapidity of heating and state of aggregation, it transforms so rapidly that fusion takes place, followed almost immediately by resolidification, the solid mass now only re-melting at 1 2 8 O , three degrees below the melting point of carbamide. I n this rapid transformation, ammonia is given off freely.

We may conclude therefore with reasonable certainty that formation of carhamida is dues to non-ionised ammonium cyanate.

The transformation of ammonium cyanate into carbamide appears to be a simple process until the atomic rearrangement consequent upon it is consluered. It is then seen that if it is to be regarded as intramolecular, it is altogether unusual and exceptional.

The transformation of ammonium cyanate cannot, however, be regarded alone, but must be explained in a, manner that brings i t into line with other reactions in which carbamides are produced, or in which cyanic acid and the isocyanates take part. Any theory whichc attempts to explain the formation of carbamide from ammonium cyanate must, besides the facts established by Walker and his ceworkers, take the following into account:

1. That ammonia is always present while conversion is taking place. 2. That the isocyanic esters ar0 converted into monclsubstituted

carbamides by interaction with ammonia, and into symmetrical di-substituted carbamides by interaction with primary or secondary amines.*

3. That alcohol can react with cyanic acid, cyanates, and isocyanic esters to form urethanes or carbamic &rs.

4. That carbamide, when heated, yields biuret and finally cyanuric acid.

5. That carbamide, when heated with any aromatic primary amine, yields first a monosubstituted and finally a disubstituted carbamide.

6. That carbamic esters are formed when carbamide is heated with alcohols.

7. That cyanic acid and its esters polymerise on keeping. These instances will suffice as examples of the types of reactions

which appear to be closely related and t o admit of a, similar explanation. All these reactions can be simply explained and brougbt into harmony by regarding them as instances of the well- known tendency of the carbonyl group t o add on groups, such as

* The formation of s-disubstituted carbamides from the isocyanic esters by the action of water is due to the hydrolysis of one portion of the ester and reaction of the remaining unchanged ester with the primary amine thus formed.

r N 2

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172 CHATTAWAY : THE TRANSFORMATION OF

R>NH R or R-OH, followed by a subsequent atomic rearrangement involving only the transference of a hydrogen atom from an oxygen atom to a nitrogen atom connected with it through the doubly linked carbon atom, thus":

*N:C:O+H*N: Z *N:C<$? *NH*CO*N:.

The conversion of ammonium cyanate into carbamide should therefore be formulated as follows :

NH,*N:C:O H*N:C:O+NH, 2

HN:C<gg2 Z H,N*CO*NH,.

The isocynnic esters are similarly converted into monoalkyl- and s-dialkyl-carbamides by ammonia -and esters by alcohols thus :

RXCO + NH, -+ R*N:c<O~ NH2

R*N:CO + NE2R -+ R*N:C<NHR OH

amines, and into carbamic

-+ R*HN*CO*NH,;

.+ R*HN*CO*NHR

-+ R*HN*CO*OR'.

Tertiary mines not having a hydrogen atom attached

9

to the . -

nitrogen are not able to react in this manner. The action of alcohols on cyanic acid and the metallic cyanates to form urethanes and the action of water in hydrolysing cyanic acid and the cyanic esters axe similar in type to the foregoing, m is also the behaviour of carbamide when heated alone or with alcohols or amines, and all can be similarly formulated.

When, for example, carbamide is heated alone, the amino-group of one molecule reacts! with the carbonyl group of another; this is followed by the olimination of ammonis, biuret, and finally by a repetition of the process, cyanuric acid being produced, thus:

CO*NH>C,<OH * NH<CO-NH c o * N H > ~ ~ + NH,. N H < ~ ~ * ~ ~ NH2

* Neglecting the precise mode by which thia addition is effected, involving probably the residual aflinity of both oxygen and nitrogen.

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AMMONIUM CYANATE INTO CARBAMIDE. 173

When carbamide is heated with dcohol, the action takes place thus :

The production of substituted carbamides by heating carbamide itself with anilines, which, as the author has shown, is a general reaction for the preparation of monoaryl- and s-diaryl-carbamides, can be similarly formulated, thus : R*NH, + O:C(NH,), ZZ RHN*C(OH)(NH,), Z

Il*NH2 + O:C(NH,)*NHR RHN*C(OH)*NH,*NHR RHN-CO-NH, + NU,.

RHN*CO*NHR + NH,. The polymerisation of cyanic acid into cyanuric acid is the result

of a similar action, cyamelide being formed by an analogous but more complicated process.

It therefore appears that what has hitherto been regarded as the oldest and best-known instance of isomeric change is not a case of isomeric change at all, but a reaction between cyanic acid and ammonia, exactly analogoys to the reactions between isocyanic esters and ammonia or amines, whereby substituted carbamides are formed.

It is pointed out by Mr. D. L. Chapman in the appended note* that this view of the reaction is not inconsistent with the results obtained by Walker and his cclwolrkers.

UNIVERSITY CHEMICAL LABORATORY, OXFORD.

* The view that carbamide is formed from ammonium cyanate by the interaction of the ammonia and the undissociated cyanic acid resulting from the non-electrolytic dissociation of the salt does not appear to be inconsistent with the fact experiment- ally established by Walker and his co-workers that the rate of formation of carb- amide is proportional to the product of the concentration of the two ions provided that we may assume that the ions behave normally.

and &[HNCO] = [NCO’][H’]

For : kI[NHJ = [NHd*][OH’]

whence k~k&“JIHNCO]=[NH~][NCO’][OH’][Ho] = [NH,][NCO’]K, . . . . . . . (i)

From which it follows that the product of the concentrations of the ions is pro- portional t o the product of t4e concentrations of the free ammonia and the un- dissociated cyanic acid in the solution. The fact that the addition of ammonia to a solution of ammonium cyanate does not appreciably affect the rate of conversion of the latter into carbamide does not invalidate the view of the change aavocated by Chattaway, since, by increasing the concentration of the ammonia, the concentration of the undissociated cyanic acid is reduced. A deduction from the above argument ought to be noticed.

(in which 7t is a constant greater than unity and k is d@

From the relation [NH4][NCO’] = k[NH4NCO]n

constant) and the

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