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Ultraviolet absorption spectra ot derivatives of symmetric triazine—II
SpectrochimicaAeta, 1958, Vol. 12, pp. 127 to 138.
Pergamon
Press
Ltd., London
Ultraviolet absorption spectra of derivatives of symmetric triazine-II 0x0~triazines and their acyclic analogs R. C. HIRT and R. G. SCHMITT Research Division, American Cyanamid Company, Stamford, Connecticut, U.S.A. (Received 16 Decena6er1957) Abstract-The ultraviolet absorption spectra of the oxygen-containing symmetric triazines, their ions, and their acyclic analogs are correlated with their molecular structures in terms of conjugated double bonds between carbon and nitrogen atoms of the heterocyclic ring and exocyclic nitrogen and oxygen atoms. The probable structure of the positive ion of melamine (2:4:6-triamino-s-triazine) is deduced from these correlations. Spectrophotometrically determined ionization constants are reported.
Introduction ultraviolet absorption spectra of a number of mono-, di- and triamino derivatives of symmetric triazine were presented and discussed in Part I of this series [l]. This paper presents the spectra, structures and ionization constants of the oxygen-containing triazine derivatives ammeline, ammelide and cyanuric acid, and their acyclic analogs biguanide, guanylurea and biuret. Correlations between the observed spectra and the structures of the neutral and ionic forms The preferred structures of these of these molecules are discussed in detail. molecules are here presented :
THE
Preferred structures of neutral molecules:
NH2
0
C N’ 4
A H\N/ \ /” I/ I I I J T o// \N/c\o ,,,/:yc\&,, gHc\NNc\NH2 V
B
\N
?
6
HN/\N/\
H\N/c\N/H
H\V/c\N
NH2
2
iI Ammeline
Melamine
H 2 N’
NH2
NH2
I:
d ‘N’
NH2 d H N/
‘NH
Ammelide
NN/
Cyanuric
riJH2 /
NH2
NH2
C
C
\N/
2 I&
Biymide
Guanylures
127
Bimcet
NO
acid
R. C. HIRTand R. G. SCHMITT
Experimental procedure The data presented were obtained on either a Cary Model 11, or a Beckman Fused quartz cells of I, 5, 10, 20, 50 and 100 mm Model DU spectrophotometer. light path length were used, as well as micrometer 3aly cells which were adjustable buffered systems, 0.1 N HCl from 500 to O-01 mm in length [2]. Non-absorbing Dissociation constants were obtained and 0.1 N NaOH, were used as solvents. spectroscopically by means of the “Spectra-titrimeter” [3]. The data are presented as plots of the log of the molar absorptivity (molar Beer’s Iaw was used in extinction} versus wave-number (kaysers, K or cm-l). the form A = (E/M) . b . c, where A is the absorbance (optical density), E is the molar absorptivity (molar extinction), dip is the formula weight, b is the cell length in millimeters and c is the concentration in grams per 100 ml. All of the compounds mentioned were prepared by various members of these laboratories and recrystallized or resublimed to obtain a high degree of purity. In most cases, samples were identical to those in previous publioations 11, 41. Rlesults The observed bands of the compounds studied are summarized in Table 1. The pH range over which these positions and intensities apply and their dissociation constants are also indicated. The spectra of these compounds are shown jn Figs. l-7.
Discussion It has been established that melamine (A), in the solid state and in neutral solution, exists in the symmetrical, triamino form [l, 5, 61. A complete discussion of the ultraviolet spectrum of melamine in the un-ionized stat.e has been given by HIRT and SALLEY [1]. These authors assigned the weak transition occurring at 42,500 K or 2350 A to a symmetry-forbidden p-electron transition. Although the spectrum of the melamine ion (Fig. 1) has been reported in connection with determination of its ionization constant [7, 8, 91 and with analytical applications [lo], no attempt was made to assign a structure to the ion. Melamine in acidic solution may be hydrolyzed progressively by the loss of one, two and three These amino groups to ammeline, ammelide and cyanuric acid respectively. related compounds do show similarities and progressions in their ultraviolet spectra [II] (see Figs. 2, 3, 4) which may be utilized in determining the structure of the melamine ion. NH2
A,
x $7 H 2 N’
LN’clNH
2
Ultraviolet absorption spectra of derivatives of symmetric triazine-II
Table 1. Spectral data and ionization constants of compounds examined
Compound
I
Form
PK,
iIaximum
daximum
Intensity
(A)
(IQ
k)
___-
below -1
2360
42,400
20,650*
1st positive ion
l-4
2350
42,550
10,200
neutral molecule
6-13
2350-S
42,550-S
1st positiie ion
l-3
2300
43,480
20,300
neutral molecule
6-8
2300
43,480
7,650
1st negative ion
11-13
2300-S
43,480-S
3,580
1st positive ion
below 0
no band
no band
neutral molecule
3.5-5.5
2220
45,050
14,080
1st negative ion
8.5-13
2260
44,250
9,730
no data
no data
no data
no band
no band
2140 ^
46,730
10,170
2200
45,050
6,360
2nd positive ion
Melamine
r_
Range of pH values
-0 5.1
Ammeline
2,820
4.5 9.4
Ammelide 1.8 6.9 -13.5
2nd negative ion Cyanuric acid
above
14
neutral molecule
l-4.5
1st negative ion
8-9
2nd negative ion
12-14
neutral molecule
l-11
1st negative ion
above 14.5
1st positive ion
l-6
neutral molecule
lo-13
2nd positive ion
below 1.5
6.5 10.6
Biuret
no band
no band
13.2
Guanylurea
2160 no band
46,300
5,670t
no band
8.0
Biguanide 3.2
1st positive ion 13.3 neutral molecule
i S = shoulder, E = estimated * Value in cont. HCl t Value at pH 13
-
2180 no band
45,870 no band
5-10
2300
43,480
above 14.5
2300
43,480
129
19,380
12,360 8,900E
R. C.
1!-
HIRT
and R. G. SCHMITT
3:
I,,,,,
I
3G00
,
i
,
2500
2200
.p b”
A Fig. 1. Ultraviolet
spectra of rflelamine, m12NHCl in water .---.-.-.. (pH > 6) . . . . . ., in 0.1 N HCl -, 5
4
..’
.-.
....
..
‘.’
::’ , //
3 c 3 2 ,:’
/ 1
1 4c
35 000
lo
45000
cd I1,,,
3G00
I
I
I
2500
2200
,
A Fig. 2. Ultraviolot
spectra of ammeline, in 0.1 N NaOH - - - -, in water (pH = 7) in 0.1 N HCl -.
130
. . . .,
Ultraviolet absorption spectra of derivatives of symmetric triazine-II
cni’ ,(I
3000
II
I
I
I
2500
I
2200
A
Fig. 3. Ultraviolet spectra of amm~~,NinH~~l
N NaOH - - - -, in buffer (pH = 4.7)
-.
. . . .,
5
4
... c , :
...
;‘_ \
/( /; 3
.f.:;’
lu
1,’ /;
g 2
,I; /!’ /
1
/ 35000
40000
4: 50 00
Cl-6
Fig. 4. Ultraviolet spectra of oyan~Oa~$~C~OH
(pH = 12) - - - -, in buffer (pH = 9)
-. 131
. . . .,
R. C. HIRT and R. C. Scam 5
4
--...-.^.-i----
3
.-_ .--
tir 2 2~
.--.-___I
1
35000
4(
cni’ A Fig. 6. Ultr&wioIet spectra of biumt, in 0.1 N N&H
~_.
~
- - - -, in buf%r (pH = 8)
-
I
/I
35000
‘ crri’
Jr1
3000
If
11
2500
2200
,
A
Fig. 6. Ultraviolet spectra of panylurea,
in 0.1 M NaOH - - - -, in 0.1 N HCI -.
132
. . . ..
Ultraviolet absorption spectra. of derivatives of symmetric triazine-II
JII
II
3000
I
I
2500
I
I
2200
A
Fig. 7. Ultraviolet spectra of biguanide, in 1 N NaOH - - - -,
in buffer (pH = 9)
. . . .
inO.lNHClp.
There are several possible forms in which cyanuric acid can exist in the solid or un-ionized form in solution; the trihydroxy form (B), the tricarbonyl form (C), and combinations of these two (D, E, F). PH N
2\
A HO’
N 4
‘N’
I/
7
H\N/c\*7
H\N/c\N
A O/ \N/
‘OH
I
A \*
II
HO/c\NAH
I
I3 (B)
(C)
03
0
b
N/c\N/H
N'
'N
1 HO’
‘N’
I
ltl
H (W
03) 133
e ?H
R, C. HIRT a.ndR. G. SCEMITI~ HUGHES [5] presented evidence, based largely on the calculated and observed heats of formation, for the tricarbonyl form of cyanuric acid. KLOTZ [S] described the spectra of oyanuric acid at three pH values, and attributed the large change in intensity of absorption with pH to an ionized. form having a double bond within the ring conjugated with the carbonyl group o&side the ring. Thus the trioarbonyl form, which has no conjugation of the double bonds, displays weak absorption in the ultraviolet, while the anion with two conjugated double bonds has moderately strong absorption near 45,500 K (2200 A). The ultraviolet absorption spectrum of cyanuric acid and its pH dependence have been studied in these laboratories with samples of very high purity. Three distinct forms for oyanurio acid were observed in the pH range of from 1 to 14, the spectra of which are shown in Fig. 4. Values for pLf, of 6.5 and 10.6 were determined. The spectrum at pH 1 (0.1 N HCl) shows very weak absorption, which is in agreement with KLOTZ, and must be attributed to the tricarbonyl form of oyanuric acid (C) as it is the only possible nonconjugated structure. It follows then that the spectrum at pH 9 is due to the singly-ionized form and has the structure (G) and the curve at pH 12 is the doubly-ionized form, structure (H).
(Q)
(m
(1)
It is interesting to note that the doubly-ionized form of cyanuric acid absorbs at somewhat longer wave length (max. at 45,500 K) than the singly-ionized form (max. at 46,700 ,K or 2140 8). The longer conjugated system in structure (H) serves as a satisfactory explanation to these observations and lends support to the conclusion that cyanuric acid exists as the tricarbonyl form in the neutral state., A third ionic fofm (I) may exist in the solid salt “trisodium cyanurate”; it is not observable spectroscopically in,solut,ion. It shouId be noted that the infrared spectrum of solid cyanuric acid shows the presence of carbonyl groups {band at 1715 K), although no statement may be made concerning the number of such groups present. This gives further confirmation of the carbonyl structure of cyanuric acid. It seems reasonable to assume that ammehne and ammelide should have molecular structures which are in~rmediate between those of melamine and cyanuric acid; that is, with one and two carbonyl groups respectively [4]. The infrared spectra of solid ammeline and ammelide showed the presence of carbonyl groups. It is also interesting to note that the calculation of the heat of formation from PAULINQ’Sdata [IZ] shows that the carbonyl form has 10 kcal/mole more than the hydroxy form. This fact was noted by KLOTZin his discussion of cyanuric acid [S], snd appears implicitly in the reasoning on eyanuric acid carried out by HUGHES[5]. 134
Gltraviolet &xq&ion
sp&sa
of dssivstives of qmmetric i&x&m-If
To illustrate the structure-spectra correlations that can be made for the neutral and ionic forms of melamine, ammeline, ammelide and cyanuric acid, Fig. 8 was prepared in which the various structures were divided into groups co~esponding to the shape of.their ultraviolet absorption curve. The structures for ammeline, ammelide and the ionic form of melamine are deduced from the now proven structures of cyanuric acid and its ions and the neutral form of mefamine by comparing their observed speotra. There are four distinct types of spectra observed which we oall “shoulder”, “resolved”, “peak” and “weak”, corresponding to three, two, one and no double bonds in the ring. Ammeline
/r”
Type of spectra observed
“Shoulder”
I
“Resolved”
“Peak”
“Very
weak”
MELAMINE
basic
ond neutral
I
basic
very
basic
acidic
very
acidic
neutral
basic
neutral
acidic
CYANURIC ACID
solid
so1t
very
basic
basic
neutral
and acidic
Fig. 8, Correlation between the u&r&violet absorption epectr& and the molecular structures of melamine, smmefine, csmmeiideand cyanuric acid.
135
mdR.G.
R.C.HIRT
displays three forms, a neutral molecule
SCHMITT
and both a positive
and a negative
ion:
0-
N
(4
/
h
\NP\N
6
‘N’
H 2N/
H
\N
C
‘NH
I:
HN+N’\hTH 2
2
(J)
0
0
II
H \
2
(IO
/H
7
!I
H
P\
‘N’
ST
‘N
1:
+ /c\NP\cH HZN
H+N 2 j
2
I!
‘N’
‘NH
2
I% CL)
Of)
There are two possible structures for the positive ion of ammeline, (L) and (M). By comparing the wavelength maxima of ammeline at pH 1 with that of its acyclic analogs, guanylurea and biguanide (see Table l), it becomes apparent that structure (L) with its C=N-C=N conjugated system is the more likely bhoice. Both the positive ion of ammeline and the positive ion of biguanide have their maximum absorption at identical energies (43,500 K or 2300 A), whereas the neutral form of guanylurea (Y) with its C=N-C=O conjugated system absorbs at 2180 A. The effect of the amino substituents on these three molecules should be about equal. Ammelide was reported [ll] to have a neutral form (I?) and a singly-ionized negative form (0); recent work with strong NaOH and HCl solutions have shown the existence of a positive ion ( Q) and have indicated the presence of a doubleionized negative form (N): ?/\ 7
‘N’ y
H N/c\N/c\o_
R C
H
H ‘N
!I C H N/ LNi io-
0
2
2
\N/
fi C \N/
I
I
H
H \N/
R
C
\N/
I
H
I
2\ NP\ NH, I3
W)
(0)
(P)
(8)
The spectra of melamine cation (pH l), of un-ionized ammeline (pH i’), of ammelide anion (pH 11) and the oyanuric acid double-negative ion (pH 12) are These data suggest that the positive ion quite similar in shape and intensity. 136
Ultraviolet absorption spectra of derivatives of symmetric triazine-II
of melamine should be represented by an isomelamine not as the normal form of melamine (S).
4 H 2N’
‘N’
type of structure
h
‘NH
H 2 N’
2
CR)
(R) and
II
‘N”,H
2
(8
The existence of the nonconjugated forms of ammelide (Q) and of cyanuric acid (C), which did not display bands in the ultraviolet region above 2000 A, suggested the examination of compounds having analogous structures. These were guanylurea to correspond to ammelide, and biuret to correspond to cyanuric acid in the arrangement of C, N and 0 atoms and their connecting bonds. Biuret, though commonly written in a linear form with right-angle branches, Y P as: H,N-C-NH-C-NH,,
may more realistically
as:
Hz?
be written with 120” angles,
S”H2
The resemblance to cyanuric acid is apparent and no bands are found. Upon ionization in basic solution, a proton is lost, and an absorption band appears at 46,300 K or 2160 A as in Fig. 5. The structure should then be written, in a form analogous to singly ionized cyanuric acid, as:
I
I
op\N/c\o m Guanylurea, which shows no bands in acid solution, must be written in a non-conjugated form, and be analogous to ammelide. If the conventional structure (V) is written for guanylurea, the ion, showing absorption at 45,870 K or 2180 A, as in Fig. 6, would have to be written as (W): HzfT
7H2
Her
THz
R. C. HIRT and R. G. SCHMITT
However, the presence of the strongly basic guanyl group would favor the acceptance of a proton from the water rather than the delivery of one; the structures would more likely be given as: H2r
+ 2\ H2N
H,N
YH2
N
/A
(4 lNi
H N/
0
NH,
2
I!I (x)
A 10
(Y)
The existence in the neutral molecule of the conjugated C=N-C=O with a band at 45,800K or 2180 A closely corresponds to the same system in ammelide with a band at 45,000 K or 2220 A (structure (0)); a C-N-C=N conjugated system would absorb at lower energy as in ammeline positive ion (L) with its band at 43,500 K or 2300 A. To check this, biguanide, which can have only a C-NC-N conjugation, was examined and found to absorb at 43,500 K, as in Fig. 7. Neutral biguanide and its positive ions are written as:
H2N
H2N
NH2
A H N’ ‘N’ 2
(Z)
b ‘NH
H N/ 2
NH2
H2N
A 1: NN/ NNij[
2
(a)
H+N/ 2
NH2
c! b \N/ NNh B (8)
2
It is interesting to note that the addition of a single proton to biguanide does not shift its absorption band position, but only decreases its intensity; the addition of a second proton makes a conjugated resonating system impossible, and the absorption band disappears into the vacuum ultraviolet region.
References 111 HIRT R. C. and SALLEY D. J., J. Chem. Phye. 1953 21 1181. PI HIRT R. C. and KING F. T., Analyt. Chem. 1952 24 1545. [31 KING F. T. ctnd HIRT R. C., Ap@. Syoectrosc. 1953 7 164. [41 HIRT R. C., Ap$. Spectroec. 1952 6 15. 151 HUGHES E. H., J. Amer. Chem. Sot. 1941 63 1737. [61 DEWAR M. J. S. and PAOLONI L., Trans. Faraday Sot. 1957 53 261. [71 DIXON J. K., WOODBERRY N. H. and COSTA G. W., J. Amer. Chem. Sot. 1947 60 699. PI KLOTZ I. M. and ASEOUNIS T., J. Amer. Chem. Sot. 1947 60 801. 191 COSTA G. W., HIRT R. C. and SALLEY D. J., J. Chem. Phys. 1950 18 434. [lOI HIRT R. C., KING F. T. and SCHMITT R. G., Analyt. Chem. 1954 26 1273. [ill HIRT R. C., Talk before Symposium on Molecular Structure and Spectroscopy. The Ohio State University, Columbus, Ohio, June 1950. [121 PAULING L., Nature of the Chemical Bw~d pp. 53, 131. Columbia Univ. Press 1940.
138
Pergamon
Press
Ltd., London
Ultraviolet absorption spectra of derivatives of symmetric triazine-II 0x0~triazines and their acyclic analogs R. C. HIRT and R. G. SCHMITT Research Division, American Cyanamid Company, Stamford, Connecticut, U.S.A. (Received 16 Decena6er1957) Abstract-The ultraviolet absorption spectra of the oxygen-containing symmetric triazines, their ions, and their acyclic analogs are correlated with their molecular structures in terms of conjugated double bonds between carbon and nitrogen atoms of the heterocyclic ring and exocyclic nitrogen and oxygen atoms. The probable structure of the positive ion of melamine (2:4:6-triamino-s-triazine) is deduced from these correlations. Spectrophotometrically determined ionization constants are reported.
Introduction ultraviolet absorption spectra of a number of mono-, di- and triamino derivatives of symmetric triazine were presented and discussed in Part I of this series [l]. This paper presents the spectra, structures and ionization constants of the oxygen-containing triazine derivatives ammeline, ammelide and cyanuric acid, and their acyclic analogs biguanide, guanylurea and biuret. Correlations between the observed spectra and the structures of the neutral and ionic forms The preferred structures of these of these molecules are discussed in detail. molecules are here presented :
THE
Preferred structures of neutral molecules:
NH2
0
C N’ 4
A H\N/ \ /” I/ I I I J T o// \N/c\o ,,,/:yc\&,, gHc\NNc\NH2 V
B
\N
?
6
HN/\N/\
H\N/c\N/H
H\V/c\N
NH2
2
iI Ammeline
Melamine
H 2 N’
NH2
NH2
I:
d ‘N’
NH2 d H N/
‘NH
Ammelide
NN/
Cyanuric
riJH2 /
NH2
NH2
C
C
\N/
2 I&
Biymide
Guanylures
127
Bimcet
NO
acid
R. C. HIRTand R. G. SCHMITT
Experimental procedure The data presented were obtained on either a Cary Model 11, or a Beckman Fused quartz cells of I, 5, 10, 20, 50 and 100 mm Model DU spectrophotometer. light path length were used, as well as micrometer 3aly cells which were adjustable buffered systems, 0.1 N HCl from 500 to O-01 mm in length [2]. Non-absorbing Dissociation constants were obtained and 0.1 N NaOH, were used as solvents. spectroscopically by means of the “Spectra-titrimeter” [3]. The data are presented as plots of the log of the molar absorptivity (molar Beer’s Iaw was used in extinction} versus wave-number (kaysers, K or cm-l). the form A = (E/M) . b . c, where A is the absorbance (optical density), E is the molar absorptivity (molar extinction), dip is the formula weight, b is the cell length in millimeters and c is the concentration in grams per 100 ml. All of the compounds mentioned were prepared by various members of these laboratories and recrystallized or resublimed to obtain a high degree of purity. In most cases, samples were identical to those in previous publioations 11, 41. Rlesults The observed bands of the compounds studied are summarized in Table 1. The pH range over which these positions and intensities apply and their dissociation constants are also indicated. The spectra of these compounds are shown jn Figs. l-7.
Discussion It has been established that melamine (A), in the solid state and in neutral solution, exists in the symmetrical, triamino form [l, 5, 61. A complete discussion of the ultraviolet spectrum of melamine in the un-ionized stat.e has been given by HIRT and SALLEY [1]. These authors assigned the weak transition occurring at 42,500 K or 2350 A to a symmetry-forbidden p-electron transition. Although the spectrum of the melamine ion (Fig. 1) has been reported in connection with determination of its ionization constant [7, 8, 91 and with analytical applications [lo], no attempt was made to assign a structure to the ion. Melamine in acidic solution may be hydrolyzed progressively by the loss of one, two and three These amino groups to ammeline, ammelide and cyanuric acid respectively. related compounds do show similarities and progressions in their ultraviolet spectra [II] (see Figs. 2, 3, 4) which may be utilized in determining the structure of the melamine ion. NH2
A,
x $7 H 2 N’
LN’clNH
2
Ultraviolet absorption spectra of derivatives of symmetric triazine-II
Table 1. Spectral data and ionization constants of compounds examined
Compound
I
Form
PK,
iIaximum
daximum
Intensity
(A)
(IQ
k)
___-
below -1
2360
42,400
20,650*
1st positive ion
l-4
2350
42,550
10,200
neutral molecule
6-13
2350-S
42,550-S
1st positiie ion
l-3
2300
43,480
20,300
neutral molecule
6-8
2300
43,480
7,650
1st negative ion
11-13
2300-S
43,480-S
3,580
1st positive ion
below 0
no band
no band
neutral molecule
3.5-5.5
2220
45,050
14,080
1st negative ion
8.5-13
2260
44,250
9,730
no data
no data
no data
no band
no band
2140 ^
46,730
10,170
2200
45,050
6,360
2nd positive ion
Melamine
r_
Range of pH values
-0 5.1
Ammeline
2,820
4.5 9.4
Ammelide 1.8 6.9 -13.5
2nd negative ion Cyanuric acid
above
14
neutral molecule
l-4.5
1st negative ion
8-9
2nd negative ion
12-14
neutral molecule
l-11
1st negative ion
above 14.5
1st positive ion
l-6
neutral molecule
lo-13
2nd positive ion
below 1.5
6.5 10.6
Biuret
no band
no band
13.2
Guanylurea
2160 no band
46,300
5,670t
no band
8.0
Biguanide 3.2
1st positive ion 13.3 neutral molecule
i S = shoulder, E = estimated * Value in cont. HCl t Value at pH 13
-
2180 no band
45,870 no band
5-10
2300
43,480
above 14.5
2300
43,480
129
19,380
12,360 8,900E
R. C.
1!-
HIRT
and R. G. SCHMITT
3:
I,,,,,
I
3G00
,
i
,
2500
2200
.p b”
A Fig. 1. Ultraviolet
spectra of rflelamine, m12NHCl in water .---.-.-.. (pH > 6) . . . . . ., in 0.1 N HCl -, 5
4
..’
.-.
....
..
‘.’
::’ , //
3 c 3 2 ,:’
/ 1
1 4c
35 000
lo
45000
cd I1,,,
3G00
I
I
I
2500
2200
,
A Fig. 2. Ultraviolot
spectra of ammeline, in 0.1 N NaOH - - - -, in water (pH = 7) in 0.1 N HCl -.
130
. . . .,
Ultraviolet absorption spectra of derivatives of symmetric triazine-II
cni’ ,(I
3000
II
I
I
I
2500
I
2200
A
Fig. 3. Ultraviolet spectra of amm~~,NinH~~l
N NaOH - - - -, in buffer (pH = 4.7)
-.
. . . .,
5
4
... c , :
...
;‘_ \
/( /; 3
.f.:;’
lu
1,’ /;
g 2
,I; /!’ /
1
/ 35000
40000
4: 50 00
Cl-6
Fig. 4. Ultraviolet spectra of oyan~Oa~$~C~OH
(pH = 12) - - - -, in buffer (pH = 9)
-. 131
. . . .,
R. C. HIRT and R. C. Scam 5
4
--...-.^.-i----
3
.-_ .--
tir 2 2~
.--.-___I
1
35000
4(
cni’ A Fig. 6. Ultr&wioIet spectra of biumt, in 0.1 N N&H
~_.
~
- - - -, in buf%r (pH = 8)
-
I
/I
35000
‘ crri’
Jr1
3000
If
11
2500
2200
,
A
Fig. 6. Ultraviolet spectra of panylurea,
in 0.1 M NaOH - - - -, in 0.1 N HCI -.
132
. . . ..
Ultraviolet absorption spectra. of derivatives of symmetric triazine-II
JII
II
3000
I
I
2500
I
I
2200
A
Fig. 7. Ultraviolet spectra of biguanide, in 1 N NaOH - - - -,
in buffer (pH = 9)
. . . .
inO.lNHClp.
There are several possible forms in which cyanuric acid can exist in the solid or un-ionized form in solution; the trihydroxy form (B), the tricarbonyl form (C), and combinations of these two (D, E, F). PH N
2\
A HO’
N 4
‘N’
I/
7
H\N/c\*7
H\N/c\N
A O/ \N/
‘OH
I
A \*
II
HO/c\NAH
I
I3 (B)
(C)
03
0
b
N/c\N/H
N'
'N
1 HO’
‘N’
I
ltl
H (W
03) 133
e ?H
R, C. HIRT a.ndR. G. SCEMITI~ HUGHES [5] presented evidence, based largely on the calculated and observed heats of formation, for the tricarbonyl form of cyanuric acid. KLOTZ [S] described the spectra of oyanuric acid at three pH values, and attributed the large change in intensity of absorption with pH to an ionized. form having a double bond within the ring conjugated with the carbonyl group o&side the ring. Thus the trioarbonyl form, which has no conjugation of the double bonds, displays weak absorption in the ultraviolet, while the anion with two conjugated double bonds has moderately strong absorption near 45,500 K (2200 A). The ultraviolet absorption spectrum of cyanuric acid and its pH dependence have been studied in these laboratories with samples of very high purity. Three distinct forms for oyanurio acid were observed in the pH range of from 1 to 14, the spectra of which are shown in Fig. 4. Values for pLf, of 6.5 and 10.6 were determined. The spectrum at pH 1 (0.1 N HCl) shows very weak absorption, which is in agreement with KLOTZ, and must be attributed to the tricarbonyl form of oyanuric acid (C) as it is the only possible nonconjugated structure. It follows then that the spectrum at pH 9 is due to the singly-ionized form and has the structure (G) and the curve at pH 12 is the doubly-ionized form, structure (H).
(Q)
(m
(1)
It is interesting to note that the doubly-ionized form of cyanuric acid absorbs at somewhat longer wave length (max. at 45,500 K) than the singly-ionized form (max. at 46,700 ,K or 2140 8). The longer conjugated system in structure (H) serves as a satisfactory explanation to these observations and lends support to the conclusion that cyanuric acid exists as the tricarbonyl form in the neutral state., A third ionic fofm (I) may exist in the solid salt “trisodium cyanurate”; it is not observable spectroscopically in,solut,ion. It shouId be noted that the infrared spectrum of solid cyanuric acid shows the presence of carbonyl groups {band at 1715 K), although no statement may be made concerning the number of such groups present. This gives further confirmation of the carbonyl structure of cyanuric acid. It seems reasonable to assume that ammehne and ammelide should have molecular structures which are in~rmediate between those of melamine and cyanuric acid; that is, with one and two carbonyl groups respectively [4]. The infrared spectra of solid ammeline and ammelide showed the presence of carbonyl groups. It is also interesting to note that the calculation of the heat of formation from PAULINQ’Sdata [IZ] shows that the carbonyl form has 10 kcal/mole more than the hydroxy form. This fact was noted by KLOTZin his discussion of cyanuric acid [S], snd appears implicitly in the reasoning on eyanuric acid carried out by HUGHES[5]. 134
Gltraviolet &xq&ion
sp&sa
of dssivstives of qmmetric i&x&m-If
To illustrate the structure-spectra correlations that can be made for the neutral and ionic forms of melamine, ammeline, ammelide and cyanuric acid, Fig. 8 was prepared in which the various structures were divided into groups co~esponding to the shape of.their ultraviolet absorption curve. The structures for ammeline, ammelide and the ionic form of melamine are deduced from the now proven structures of cyanuric acid and its ions and the neutral form of mefamine by comparing their observed speotra. There are four distinct types of spectra observed which we oall “shoulder”, “resolved”, “peak” and “weak”, corresponding to three, two, one and no double bonds in the ring. Ammeline
/r”
Type of spectra observed
“Shoulder”
I
“Resolved”
“Peak”
“Very
weak”
MELAMINE
basic
ond neutral
I
basic
very
basic
acidic
very
acidic
neutral
basic
neutral
acidic
CYANURIC ACID
solid
so1t
very
basic
basic
neutral
and acidic
Fig. 8, Correlation between the u&r&violet absorption epectr& and the molecular structures of melamine, smmefine, csmmeiideand cyanuric acid.
135
mdR.G.
R.C.HIRT
displays three forms, a neutral molecule
SCHMITT
and both a positive
and a negative
ion:
0-
N
(4
/
h
\NP\N
6
‘N’
H 2N/
H
\N
C
‘NH
I:
HN+N’\hTH 2
2
(J)
0
0
II
H \
2
(IO
/H
7
!I
H
P\
‘N’
ST
‘N
1:
+ /c\NP\cH HZN
H+N 2 j
2
I!
‘N’
‘NH
2
I% CL)
Of)
There are two possible structures for the positive ion of ammeline, (L) and (M). By comparing the wavelength maxima of ammeline at pH 1 with that of its acyclic analogs, guanylurea and biguanide (see Table l), it becomes apparent that structure (L) with its C=N-C=N conjugated system is the more likely bhoice. Both the positive ion of ammeline and the positive ion of biguanide have their maximum absorption at identical energies (43,500 K or 2300 A), whereas the neutral form of guanylurea (Y) with its C=N-C=O conjugated system absorbs at 2180 A. The effect of the amino substituents on these three molecules should be about equal. Ammelide was reported [ll] to have a neutral form (I?) and a singly-ionized negative form (0); recent work with strong NaOH and HCl solutions have shown the existence of a positive ion ( Q) and have indicated the presence of a doubleionized negative form (N): ?/\ 7
‘N’ y
H N/c\N/c\o_
R C
H
H ‘N
!I C H N/ LNi io-
0
2
2
\N/
fi C \N/
I
I
H
H \N/
R
C
\N/
I
H
I
2\ NP\ NH, I3
W)
(0)
(P)
(8)
The spectra of melamine cation (pH l), of un-ionized ammeline (pH i’), of ammelide anion (pH 11) and the oyanuric acid double-negative ion (pH 12) are These data suggest that the positive ion quite similar in shape and intensity. 136
Ultraviolet absorption spectra of derivatives of symmetric triazine-II
of melamine should be represented by an isomelamine not as the normal form of melamine (S).
4 H 2N’
‘N’
type of structure
h
‘NH
H 2 N’
2
CR)
(R) and
II
‘N”,H
2
(8
The existence of the nonconjugated forms of ammelide (Q) and of cyanuric acid (C), which did not display bands in the ultraviolet region above 2000 A, suggested the examination of compounds having analogous structures. These were guanylurea to correspond to ammelide, and biuret to correspond to cyanuric acid in the arrangement of C, N and 0 atoms and their connecting bonds. Biuret, though commonly written in a linear form with right-angle branches, Y P as: H,N-C-NH-C-NH,,
may more realistically
as:
Hz?
be written with 120” angles,
S”H2
The resemblance to cyanuric acid is apparent and no bands are found. Upon ionization in basic solution, a proton is lost, and an absorption band appears at 46,300 K or 2160 A as in Fig. 5. The structure should then be written, in a form analogous to singly ionized cyanuric acid, as:
I
I
op\N/c\o m Guanylurea, which shows no bands in acid solution, must be written in a non-conjugated form, and be analogous to ammelide. If the conventional structure (V) is written for guanylurea, the ion, showing absorption at 45,870 K or 2180 A, as in Fig. 6, would have to be written as (W): HzfT
7H2
Her
THz
R. C. HIRT and R. G. SCHMITT
However, the presence of the strongly basic guanyl group would favor the acceptance of a proton from the water rather than the delivery of one; the structures would more likely be given as: H2r
+ 2\ H2N
H,N
YH2
N
/A
(4 lNi
H N/
0
NH,
2
I!I (x)
A 10
(Y)
The existence in the neutral molecule of the conjugated C=N-C=O with a band at 45,800K or 2180 A closely corresponds to the same system in ammelide with a band at 45,000 K or 2220 A (structure (0)); a C-N-C=N conjugated system would absorb at lower energy as in ammeline positive ion (L) with its band at 43,500 K or 2300 A. To check this, biguanide, which can have only a C-NC-N conjugation, was examined and found to absorb at 43,500 K, as in Fig. 7. Neutral biguanide and its positive ions are written as:
H2N
H2N
NH2
A H N’ ‘N’ 2
(Z)
b ‘NH
H N/ 2
NH2
H2N
A 1: NN/ NNij[
2
(a)
H+N/ 2
NH2
c! b \N/ NNh B (8)
2
It is interesting to note that the addition of a single proton to biguanide does not shift its absorption band position, but only decreases its intensity; the addition of a second proton makes a conjugated resonating system impossible, and the absorption band disappears into the vacuum ultraviolet region.
References 111 HIRT R. C. and SALLEY D. J., J. Chem. Phye. 1953 21 1181. PI HIRT R. C. and KING F. T., Analyt. Chem. 1952 24 1545. [31 KING F. T. ctnd HIRT R. C., Ap@. Syoectrosc. 1953 7 164. [41 HIRT R. C., Ap$. Spectroec. 1952 6 15. 151 HUGHES E. H., J. Amer. Chem. Sot. 1941 63 1737. [61 DEWAR M. J. S. and PAOLONI L., Trans. Faraday Sot. 1957 53 261. [71 DIXON J. K., WOODBERRY N. H. and COSTA G. W., J. Amer. Chem. Sot. 1947 60 699. PI KLOTZ I. M. and ASEOUNIS T., J. Amer. Chem. Sot. 1947 60 801. 191 COSTA G. W., HIRT R. C. and SALLEY D. J., J. Chem. Phys. 1950 18 434. [lOI HIRT R. C., KING F. T. and SCHMITT R. G., Analyt. Chem. 1954 26 1273. [ill HIRT R. C., Talk before Symposium on Molecular Structure and Spectroscopy. The Ohio State University, Columbus, Ohio, June 1950. [121 PAULING L., Nature of the Chemical Bw~d pp. 53, 131. Columbia Univ. Press 1940.
138