G U A S I D I N E ASD KITROUS ACID.
11*
BY WILDER D. BAh-CROFT .49D S. LOUISA RIDGWAY
Although considerable work has been done on guanidine, its structure is far from being a settled question-as is shown by the different formulae adopted by various authors. A study of the reaction between guanidine and nitrous acid was undertaken in order that some interesting points in regard to the reaction itself might be cleared up, and that some light might’ be shed on the structure of guanidine through the speed of the reaction. Work on the decomposition products at various stages is being done by Mr. B. C . Belden’ of this laboratory. The reaction of nitrous acid and primary aliphatic amines has long been known to give the corresponding alcohol, nitrogen, and water. Conversely the evolution of nitrogen with nitrous acid has been used in the detection and estimation of primary aliphatic amines. This method was brought into prominence by Van Slyke’s? scheme for the analysis of proteins and protein residues; and more especially by his very convenient apparatus. Van Slyke found experimentally how amino groups in natural amino acids, in which he was particularly interested, reacted. The large majority gave up their nitrogen quantitatively in five minutes, but he found some that reacted more slowly. H e noticed that other compounds containing KHz groups reacted a t different speeds. His method has been extended by other workers t o a large number of compounds. The time needed for complete evolution of nitrogen varies considerably even in simple compounds according to the position of the amino group. Dunn and Schmidt3 showed this admirably for various amino acids having the amino group alpha, beta, gamma, delta, and epsilon to the carboxyl group. The reaction velocity becomes slower for each increase in the number of carbon atoms separating the two groups. Taylor,l in a recent series of papers has studied the reaction carefully in order to determine the order and the velocity in different cases. His method of carrying out the reaction was necessarily considerably different from that of Van Slyke. I n the latter an unknown excess of nitrous acid is present in such high concentration as to be decidedly unstable, forming S O and HKOs. Taylor found that pure nitrous acid of not greater than 0.2 N strength could be stabilized quite well if it were not shaken, no gas bubbled through it, and if it were not in contact with air. He used 0.05 ?j solutions, and determined the * This work is done under the programme now being carried out a t Cornell University and supported in part by a grant from the Heckscher Foundation for the Advancement of Research established by August Heckscher at Cornell University. J. Phys. Chem., 35, 2684 (1931). J. Biol. Chem., 9, 185 (1911);12, 275 (1912). J. Biol. Chem., 53, 401 (1922). J. Chem. SOC., 1927, 1923;1928, 1099,1897;1929, 2052; 1930, 2741.
G U A S I D I S E .4ND S I T R O U S ACID
2951
amount of unchanged nitrogen compound rather than the volume of nitrogen evolved. He found that the velocities varied greatly according to the type of compound and the place of the amino group in it. Velocity varies according to the product, [compound H'] [SO2-] [HSOZ], and the reaction is of the third order. The compounds investigated were ammonia, methyl amine, n-propyl amine, diethyl amine, glycine ethyl ester, and some aliphatic amino acids. I n these latter, velocity varies as [?;Hz' RCOO-] [HSOZ]?, but a complex is probably formed between S H i RCOO- and "02, making the first expression given perfectly general. Other factors than the relation of the amino group to the compound have been found to influence the velocity of the reaction. Dunn and Schmidt found temperature to be important. Taylor found that salts and mineral acids have a depressing effect due to lowering of the nitrite ion concentration, while the ion of the compound, nitrite ion, or nitrous acid in excess have an accelerating one. Seither Dunn and Schmidt or Taylor have given any results on the action of nitrous acid on the S H z group of amides. From their fundamentally different structure, they would be expected to act differently. I t has been found that mineral acid rnust be added before they react. I n the case of more complex compounds there is a great deal of confusing data and disagreement as to what they signify. It is either definitely or tacitly assumed by most authors that, if a compound reacts with puke nitrous acid to give off one or more atoms of nitrogen, the nitrogen evolved represents one or more -KHn groups present in the original compound. If acid must be added before nitrogen is evolved, it is assumed that the original compound did not contain an - S H Z group, but that under the influence of the acid, it rearranged to form one. (Acetic acid is present in the Van Slyke apparatus, so in cases where there is a slow reaction which might be due to acetic acid, the reaction with pitre nitrous acid must be carried on in another way.) This explanation is simple and plausible as long as we are dealing with compounds not haling more than one nitrogen atom on a carbon, as for example acetamide. It may still be the best one for every case. A review of the work done on guanidine in this connection brings up some interesting questions. Pellizzari' reported a small amount of cyanamide formed by the reaction of nitrous acid on guanidine (not in the Van Slyke apparatus) : he also reported nitrosoguanidine as an intermediate compound. Hale and 1-ibrans' made a small number of runs on guanidine in a modified \-an Slyke apparatus to use for comparison in other work. With glacial acetic acid and mixtures of glacial acetic and higher fatty acids nearly at the boiling temperature, two nitrogens of guanidine were liberated in 12-1 j minutes. With hot joCc and 8 j"c acetic acid, s.4ycand 9.6Vc respectively were evolved. Plimmer,3using the Van Slyke apparatus, found very little reaction in the presence of acetic acid, gradually increasing reaction up to 1/3 of the total nitrogen in the presence of increasing amounts of HCI up to 1.6 Tu'
~
-
_
_
_
* Gazz., 51 I, 224 (1921). * J. Am. Chem. SOC.,40, 10j9 (1918). J. Chem. SOC., 127, 2651 (192j).
2952
WILDER D. B.4KCROFT AND S. LOUISA RIDGWAT
(approx.), liberation of about 2 3 of the nitrogrn in the presence of 2 S acid, and nearly complete reaction with considerably more wid. .A11 the runs lasted approxirnatcly 2 1 hours. The following figures are part of a tablc given in his article: Temp 7.3
I1 I1
I? ID
13
10 cj 10
I1 11
I? 1.4
J1
Thc extraordinary thing ahout I'liinmer's data is thar there is no figure between 31.6'; nitrogen evolvtd anti 71.8'; nitrogen evolved. It looks as though thc second nitrogtm cnnic off instantaneously \\-hen it came off at all, while the third nitrogen ciinic off sloivly :iiid incompletcly. ThiA is so unlike anything t h a t is IiIio\vn 11i:it the re:ictlon h:is been studied with some care. A;: might have bcrn r~spectrti,t h c results trirrictl out t o tie another c : m of the misleading espeririicnt . Plininier v a s not misled tieeause he apparently nevw noticed the rtwi:irk:ihle fcntures of hi; data. Plimmcr's results are in disagreement lvith tho si:iieriitrnt of 1ir:illl \rho says that guanidine reacts nitrogen :itonis even in the presencc of cxccss mineral acid. lrred :I r:oiiccntration of less than I . j S :is an excess rensoni and l1wI,':dane2 on the other hxid agree with Plinimer. tigations w r c going on independently a t the same time, islied f i r G t , they do not give full data. They disagree in that Ihey get complcatc evolution of nitrogen Jvith 6' C and 1 2 ~ ' ; sulphuric acid (npp'ox. 1 . 2 and 2 . 1 SJ,11-hile P l i ~ i ~ mne\-er rr gets :in entirely complete reuctioii even in the presence of tlie larger amount of acid. It may he that there is scinie specific difference between hydrochloric and sulphuric acids. In gencrnl our results confirm those of Plimnier. Through all this work, various formulae have k e n assigned to guanidine to account for its behaviour, not only ton-ard nitrous acid? but in its other J. Chem. Soc., 107, 1,396 '1915 Biochern. J., 20, 1264 (1926 .
2953
G C A S I D I S E AKD NITROUS ACID
reactions, syntheses, and decompositions. H S : C.(SHz)2 has few supporters now as the formula for free guanidine. &all1 in order to explain his results postulates : XH3 alkali hydroxides XH3 __--
I H 1 : C/’
\I
I
--i
II
H2S.C’
\\
acids
MI
4-
I1
S
Plimmer? adopts I which he considers changes to H S : C(SI-I2)2in the presence of strong acid and accounts for the liberation of 2/13 of the nitrogen. This is analogous t o the structures Kernery assumes for urea:
O
OH
S H 3
SHn
Hynd and RlacFarlane4 consider that since guanidine is a very strong monacid base, the formula should be
XH3
\I KH
\
/’
OH
H
dissociating to give a guanidine ion
In the presence of strong acid, they consider that this then changes to
/s C1 H.Hs l . C
’
\ I XH3
J. Chem. Soc., 107, 1396 I 191j).
* J. Chem. SOC.,127, 2651 (1925).
“The Chemistry of Urea” (1923). Biochem. J., 2 0 , 1264 (1926~.
,
WILDER D. BANCROFT AXD S . LOUISA RIDGWAY
2954
the free amino group reacts with nitrous acid, and urea is formed, immediately reacting with nitrous acid. All of these formulae have been discussed by Bancroft and Barnett.’ They have criticized that of Hynd and MacFarlane on the basis that it is unnecessary and improbable to have two pentavalent nitrogens on the same carbon. In any case guanidine can only be a monacid base, for all the nitrogens are on the same carbon. According to Sidgwick? all of these ring formulae for guanidine and the corresponding ones for urea are impossible on the best modern theory of atomic linkage. From the covalency rule, nitrogen can have only four covalent linkages, three of which are normal and one coordinate. Its external shell can not contain more than eight electrons. I t has five to start with and therefore can not take on more than three more. A nitrogen with three normal covalent linkages may join coordinately with a positive ion such as hydrogen and thus form a positive ion with four covalent linkages. This is capable of forming an ionic linkage with a negative ion such as chlorine. XH4‘ in KH4C1 is an example of such a case. All so-called pentavalent nitrogens have one ionized link. He gives as the only possible formulae: SH,
H x : (.?, a n d considerable :mounts cd both nitrous and the added acid arc present. If the acid is strong, ;is tlic mineral acids, we have HY02 S a ' + so,11' -t -1--+ 1 3 ' and larger ;i:nounts of nitroii5 arid are forrnctl. T h t this is actu:rlly the C W E is shown by the niuck, dtcjicr blue .solution Tvheii mineral ncici is ndtled. Theoretically t h e larecr thi, i'\;c+s of nitrous acid thc faster the rc:iction rractsslorvly nithacctic ncid takes place, and this might esplairi ~Ii~-eu:inidine and quickly with hydrochloric :inti sulphuric :icids. Prscticnllyl :i very hrgc excess acts almmt tlic s:irne as R different very large exces:'. This is ~ r i l illustrated by the fact r h : t r the nxiction in the 5 ' m Slykc, :tpp;iratus (nhcrc. there is it large csctss of nitrous ;icitl r ' w n in the ;tbsencc of mineral acidi has been reported as prnrt ically nionrimo!ecul:ir, while 'h.y!nrjl working more carefully and without ii large cscess, finds thttt rhe reaction is triniolecular. The difference in amount of nitrous acid liberxted by w : i k and strong acids can riot be sufficient to :tccoiint for the difference in the r c x t i o n velocity in the presence of :wtk :ind strong :lei& and in tlie prcsitnce of different amounts of the mnc The second p~>Psiijl(~ eii'cc~.is :!):it ( i f t h e liydrogtw ions:. Since e:ich mid has its own ionization constillit \he Iiydrogcri ion coiieenirat ion ;hoiild hc different in each case :ind t h e spcrd of the rwciion dift'c.rent. B u t if t h c hydrogen ion concentrarion wrre :ibniit the s:tnie in er]i:i;.:dc'nt amounts of two acids, the diffcrrncr. might ~ c l !lx! within 1 hc iiiilit. of cuperinitvt:ii rrror. The eflect of ~ h c iiyclrogrn . ion ccj 111 is iuflicient I O ;iccount for the difierence in re:ictioii in ilie prewnc.c~( i t ' :?trrliii' :inti xc:ik acid'. ?'hnt the hydrogen ion c o n c c ~ i i ~ ~ : i t i on~uis i ~ et:!ken into :iccoiixri V Y S ihown ir! nnotherway. Even n.ith :in cs of nitroiis acid conip:ir,iilile t o that i n the 1-an Slyke sppttr:i tiis in the prcsencr of ininrra! :icitl. iornic(1 by dissolving nitrogen trioxide in iv:iti.r, t h e reaction vrlocity w : i ~grcntly cut down in a low acid concentration. I t ~vvouldseem thn! the hydrogen ions acted in zonie way on guanidine (and o t h t r compounds vhich (lo nt react with pure nitrous acid) t o change it hito a form atiackcd by tile nitrous acid. Thrre must further be some effect due t n the acid itself. Sulphuric acid is less highly ionized than hydrochloric acid, but a t two of the concentrations tried, it x i s the more rffrctive in promoting the re:iction. Besides the dif-
--.
+-
+
+
+
~
J. Chem. SOC., 1928, logy, 1b9;; 1929, 2 0 . j ~ .
+
2961
G U A S I D I S E A N D SITROUS ACID
ference in reaction rates in the presence of different quantities of the same acid, cf Fig. j, are far too great t o be explained in any way on the basis of change in hydrogen ion concentration alone. The work of Hale and %brans,’ t o which reference has already been made, gives us another illustration of a change in reaction velocity with different amounts of acid for which explanation on the basis of hydrogen ion concentration is impossible. For the same time and conditions, they get approximately 2 1 3 of the nitrogen evolved with glacial acetic and 1/10 with 8 ~ ~ : . The facts of the reaction of guanidine and nitrous acid may be summarized to date in this way. With pure nitrous acid, guanidine does not react. K i t h acid as a catalyst, it reacts slowly until a certain concentration of acid (which is specific for the acid) is reached: it then reacts fairly fast to give off the first two atoms of nitrogen. The third atom of nitrogen comes off more sloivly than the first two. At present we can give no fuller explanation of the bearing of these facts on the decomposition of guanidine than is given in the following discussion. B. C. Eelden? of this laboratory has made experiments t o ascertain the products formed by guanidine in its decomposition. This must be done largely outside the Tan Slyke apparatus for there the concentration of inorganic salts is so high a s t o interfere with the tests for the small amounts of nitrogenous material present. Our first hypothesis was that urea is formed. The equation for the reaction of an amino group with nitrous acid as it is usually written gives urea from guanidine, the two substances arc known to be closely related, Hynd and 31acFarlane3 favor that view, and we obtained qualitative tests for urea in the guanidine reaction mixture after reaction in several cases. These reactions, taken in part from Kerncr,4 may he considered to represent this reaction: CS*HZ?;H? H O S O + C’OStHd Sz HzO . COS?H, HOSO+-IISCO S2 2 H20 HSC’O HOSO-S? 1-190 CO, and H S C O f HzO H-1 +C’Oz SHJ SHdA HOSO?Sz f HA 2 1120. But urea obviously cannot be formed, for in the 1-an Slyke apparatus it would be acted on immediately in the presence of the strong mineral acid and the reaction would proceed to an end a t once. Actually the last forty percent of the nitrogen is very slow t o come off. The next suggestion was that cyanamide is formed, possibly through nitroso guanidine as an intermediate, as Pellizzarij reported in his experiments. These equations would represent the reaction: C‘S3Ha H O S O CS8H4.S0 H20 CSaH4.?;O+C?;SH? S ? Hz0 C‘S-SHI HOXO ---* H S C O ?;z HzO,
+ + +
+
+- + + + - + +
+
---f
+
‘
J. Am. Chem. Soc., 40, 10j9 f1918) J. Phys. Chem., 35, 2684 i1g31). Biochem. J., 20, 1264 (1926). “The Chemistry of Urea” (1923). Gazz., 5 1 I, 224 (1921).
+ + + + +
-+ +
2962
WILDER D. B A N C R O m A S D S. LOUISA RIDGWAT
and the HXCO reacts as before. By experiments with pure nitrous acid, guanidine carbonate, and sulphuric acid, all in aqueous solution, Belden’ showed the formation of 80% of the theoretical amount of cyanamide, assuming that the reaction was stopped before any of i t decomposed. In experiments which Werner! did with cyanamide and nitrous acid (in much more concentrated solution and under different conditions from those in the Van Slyke apparatus) he found that forty percent of the nitrogen of cyanamide reacted rapidly and that the last sixty percent reacted slowlp, due, he assumed, to the fact that forty percent of the cyanamide is in the CS.SH2 form at the beginning and that, the HY:C:SH form only changes slowly to the first as it is used up. This last agrees very well with our data, better than assuming that the first nitrogen or fifty percent of the cyanamide nitrogen comes off rapidly. However, pure cyanamide put into the Van Slyke apparatus under the conditions, as nearly as possible, in which it would be formed there from guanidine, reacted slowly (with j cc. of “21). Some of it should have reacted rapidly i.e. the part corresponding to the first forty percent of the nitrogen of cyanamide or approximately the second thirty percent of guanidine.
Summary I. The more important factors affecting the speed of the reaction,
+
RCH2 NHZ HOXO, are discussed. 2 . Evolution of nitrogen by the action of nitrous acid is generally considered to mean that the compound originally contained, or rearranged to contain, an -NH2 group. This may not be true if more than one nitrogen is attached to a carbon. 3. The work on nitrous acid and guanidine to date is summarized. 4. The various formulae given to guanidine are discussed. KO conclusions are drawn. j. Experimental results on the reaction of guanidine and nitrous acid in the presence of hydrochloric and sulphuric acids of 1.j to 1.9normality are given. This work agrees generally with previous work, especially that of Plimmer. I t is more detailed for the acids and the concentrations of those acids which it covers. 6. Guanidine and pure nitrous acid do not react. In the presence of acid up to a certain concentration, they react slowly. I n the presence of acid above that concentration, they react quickly to liberate two atoms of nitrogen. The third atom of nitrogen is evolved more slowly. 7. Of the possible effects of acid in promoting the reaction between guanidine and nitrous acid-greater concentration of nitrous acid, hydrogen ion concentration, and a specific effect due to the acid itself-the last two are most active. J. Phys. Chem., 35, 2684 ( 1 9 3 1 ) . “The Chemistry of Urea” (1923).
GUANIDINE AND NITROUS ACID
2963
8. Some preliminary experiments on the decomposition products of guanidine by nitrous acid are discussed.
This paper has raised several important questions which it has left unanswered. It was written to summarize the work so far because of the intrinsic interest and importance of the subject, and in an effort to formulate the issues clearly. These may be considered to be: ( I ) the structure of guanidine; ( 2 ) the probable rearrangement in the presence of acid by which it is made open to attack by nitrous acid; (3) the specific effects of different acids; (4)the nature of the decomposition products of the reaction between guanidine and nitrous acid in the intermediate and final stages, and the consequent explanation of the time-reaction curve. Cornell Cniversily.
