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The interactions of chloranil in aqueous solvents—I The absorption spectrum of chloranil in 50 per cent aqueous ethanol
Spectrochimica
Acta, 1967, Vol. 28),, pp. 1751 to 1756. PergamonPress Ltd. Printed in Northern Ireland
The interactions of chloranfl in aqueous solvents--I The absorption spectrum Ol chloranil in 50 per cent aqueous ethanol M. A. SLIFKIN D e p a r t m e n t of Pure and Applied Physics, Ro y al College of Advanced Technology, Salford and
1%. A. SUMNER and J. G. HEAT~COT~ Department of Chemistry, Royal College of Advanced Technology, Salford
(Received 13 August 1966)
Abstract--A study has been made of the visible and ultra violet absorption spectra of a solution of ehloranil in 50 per cent aqueous ethanol when buffered at different pH values. The changes in the spectra with time have also been studied. It is shown that the characteristic purple colour of an aqueous solution of chloranil is not due to chloranilic acid. Spectral details are also given for triehlorohydroxyquinone in 50 per cent aqueous ethanol. INTRODUCTIOI~
IN a s t u d y of the interaction of some amino acids and 50 per cent aqueous ethanol [1] it was not e d t h a t the unstable an d developed a purple colour assumed to spectrum of ehloranil at different p H values, and as a studied.
proteins with ehloranil in spectrum of chloranil was be chloranilic acid. The function of time, has been
:EXPERIMENTAL
The solvents were made up b y mixing equal volumes of ethanol and requisite buffer solution. Spectral measurements were made using a Unicam SP700 recording s p e c t r o p h o t o m e t e r and t h e stoppered cuvettes (10 m s ) were made of fused silica. All spectra were measured at a t e m p e r a t u r e of 22 -4- ½°C. Buffer solutions were supplied b y Messrs. British Drug Houses Ltd. RESULTS AND DISCUSSION
The spectrum of freshly made up chloranil in 50 per cent aqueous ethanol at p H 9 exhibits a maj or absorption band at 33.2 k K with a width of 6.7 kK. (The width of th e b a n d is measured between the points where t he optical density is one half of the m a x i m u m optical density.) A minor band is observed at ~-~18.6 kK, the absorption ratio between t he two bands being a p p r o x i m a t e l y 20: 1. After three hours, the ba nd a t 33.2 k K in unchanged b u t t h a t at 18-5 k K increases slightly to give an absorption ratio between t he two bands of 13: 1. After four days, there are v e r y m a r k e d changes in the spectrum. The b a n d of m a x i m u m i n t e n s ity shifts to 31 k K with an increase in absorption of about 25 per cent over t h a t of th e original 33.2 k K band. The absorption at 18.5 k K decreases to a b o u t one t h i r t i e t h of the absorption a t 31 kK . The spectrum at p H 9 after four days is [1] J. B. BIRKS and M. A. S n i p ,
1Vature 197, 42 (1963).
1751
1752
M . A . SLIFXn¢, R. A. S ~ R
and J. G. HEATHCOTE
i d e n t i c a l w i t h t h e s p e c t r u m of chloranilic acid as r e p o r t e d b y BERTOLACINI a n d BARNEY [2]. T h e s p e c t r u m of a f r e s h l y p r e p a r e d solution of chloranil in 50 p e r cent a q u e o u s e t h a n o l a t p H 7, e x h i b i t s a m a j o r a b s o r p t i o n b a n d a t 33-5 k K w i t h a w i d t h o f 5 k K . T h e r e are m i n o r b a n d s a t 22.5 k K a n d , ~ 1 8 k K . A f t e r a b o u t 6 hr, t h e b a n d a t 33.5 k K is u n c h a n g e d b u t t h e one a t 22.5 k K d i m i n i s h e s slightly a n d t h a t a t ~ 1 8 k K increases slightly. T h e r a t i o of t h e a b s o r p t i o n of t h e b a n d a t 18 k K t o t h a t a t 33.5 k K is 1 : 13. O v e r a p e r i o d of f o u r d a y s o n l y slight c h a n g e s in o p t i c a l d e n s i t y are o b s e r v e d . T h e m a j o r a b s o r p t i o n b a n d shifts f r o m 33.5 t o 33.2 k K . T h e r a t i o of t h e a b s o r p t i o n of t h e b a n d a t 18 k K t o t h a t a t 33.2 k K is 1 : 13. A f r e s h l y p r e p a r e d solution of chloranil in 50 p e r cent a q u e o u s e t h a n o l a t p i t 4, e x h i b i t s a m a j o r a b s o r p t i o n b a n d a t 32.6 k K w i t h a w i d t h of o n l y 1.4 k K , b u t t h e r e is a slight a b s o r p t i o n in t h e region of 27 k K to 17.5 k K . A f t e r f o u r d a y s , little c h a n g e is o b s e r v e d in t h e b a n d a t 32.6 k K , b u t t h e r e is a slight decrease in a b s o r p t i o n in t h e region of 24 k K a n d a slight increase in a b s o r p t i o n in t h e region of 18 k K . T h e r a t i o of t h e a b s o r p t i o n a t 18 k K t o t h a t a t 32.6 k K is a b o u t 1 : 13. T h e results are s u m m a r i z e d in T a b l e s 1 a n d 2. F i g u r e s 1, 2 a n d 3 show s p e c t r a of chloranil a t different p H values. Table 1. Position of peaks in the absorption spectra of solutions of chloranil pH
Age of solution
4
30 rain 4 days 30 rain 6 hr 4 days 30 mln 3 hr 4 days
7 9
Frequency of major peak (kK)
Frequency of minor peak (kK)
32.6 32.6 32.5 33.5 33.2 33.2 33.2 31
Width of major peak (kK)
-18 -18 18 18.5 18.5 18.5
1.4 5 6"7
Table 2. Ratio of absorption of major and minor peaks of solutions of ehloranil twenty four hours old Solution No.
pH
Optical density of major peak
Optical density of minor peak
Ratio minor: major
1 2 3
4 4 7
0.39 0.63 0.60
0.029 0-047 0.43
1 : 13.4 1 : 13.4 1 : 13-9
4
7
0.36
0.30
1 : 12
5 6
9 9
0.65 0.26
0.049 0.019
1 : 13.3 1 : 13.2
I t is seen t h a t s h o r t l y a f t e r p r e p a r i n g solutions o f chloranil, t h e s p e c t r a c h a n g e slightly t o give a p u r p l e colour due t o t h e n e w b a n d a t 18 k K . O v e r t h e r a n g e of p H v a l u e s covered, t h e r a t i o of t h e m a x i m u m a b s o r p t i o n o f t h i s b a n d t o t h e old
[2] R. J. BERTOLACINIand J. B. BARNEY, Anal. Chem. 30, 302 (1958).
The interactions of chloranil in aqueous s o l v e n t s - - I
1753
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Frequency
~ ~x~X,,~\
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50 ,
x. 20
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Fig. 1. Absorption spectra of ebloranil (2.56 × 10-t~/) in 50 per cent aqueous ethanol at p H 9. (a) 3 hr after preparation ( - - - - - - ) . (b) 4 days after preparation ( - - × - - ) .
band at ~ 3 3 k K is about 1 : 13. This must represent some new species--obviously not chloranilic acid as previously suggested [1]. It is only in alkaline solution that chloranil is eventually converted to chloranilic acid. The rather marked changes in the width and positions of the chloranil peak due to alteration in pH must be due to solvation. HAUSS~,R and MULLIKEN [3] have shown that the spectral bands of ehloranil in several different solvents have quite marked differences in position and width. Thus the peak absorption of chloranil in heptane occurs at ~ 3 4 kK and in perfluorheptane at ~ 3 6 kK. The widest band occurs in ethanol. They attribute these effects to strong solvent interaction. Chloranil (I) is a well-known electron aceeptor [4]. Ethanol has been shown to be a weak electron-donor in water [5] and therefore it is probable that there is some charge transfer between ethanol and chloranil in water. The amount of charge transfer will be a function of pH [1]. The new species formed shortly after the preparation of the chloranil solutions has obvious affinities with chloranilic acid (III), both possessing the broad featureless band with m a x i m u m absorption at about 18 kK. The new species is probably some intermediate stage in the conversion of ehloranil to chloranilic acid. [3] K. H. I~USSER and R. S. MvIz_z~¢, J. Phys. Chem. 64, 367 (1960). [4] C. BRIEGLEB Elelctronen-Donator-Acceptor.Komplexe. Springer (1961). [5] M. A. S L ~ , Spectrochim. Acta 21, 1391 (1965).
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The interactions of chloranil in aqueous solvents--I
1755
One i n t e r m e d i a t e is a k n o w n s t a b l e c o m p o u n d t r i c h l o r o h y d r o x y q u i n o n e ( I I ) . S o m e of t h i s c o m p o u n d h a s b e e n s y n t h e s i s e d following t h e t e c h n i q u e o f HA~COCK et al. [6]. S p e c t r a of t r i c h l o r o h y d r o x y q u i n o n e h a v e b e e n m e a s u r e d in t h e s a m e s o l v e n t s as chloranil. T h e s p e c t r a s h o w similar f e a t u r e s t o t h a t o f one d a y - o l d chloranil. H o w e v e r t h e r a t i o s o f a b s o r p t i o n of t h e m a j o r p e a k a t ~ 3 4 k K t o t h e m i n o r p e a k a t 18 k K are less t h a n t h o s e o f chloranil. T r i c h l o r o h y d r o x y q u i n o n e in p H 4 a n d p H 7 solution is r e l a t i v e l y stable. A t p H 9 t h e s p e c t r u m c h a n g e s t o t h e c h a r a c t e r i s t i c s p e c t r u m o f chloranilic acid w i t h a s t r o n g b a n d a t 31.4 k K a n d a v e r y w e a k b a n d a t 18 k K , t h e r a t i o of a b s o r p t i o n s b e i n g a b o u t 24: 1. T h i s is s u m m a r i z e d in T a b l e 3. O t h e r similarities b e t w e e n t h e s p e c t r u m o f t r i c h l o r o h y d r o x y q u i n o n e a n d chloranil are t h e b a n d w i d t h s of t h e m a j o r p e a k s a n d t h e p o s i t i o n s o f t h e m a j o r p e a k s a t different p H values. Table 3. Spectral details of trichlorohydroxyqulnone in 50 per cent aqueous ethanol
pH
Major peak Optical v(kK) density
Immediately on mixing 4 32.7 7 34 9 34 After 24 hr 4 32.7 7 34 9 31.4~
Minor peak Optical ~,(kK) density
Ratio of major to minor absorption
Width of major peak (kK)
0.620 0.854 0-700
18 18 18
0-132 0.108 0"061
4.8:1 7-9:1 11-6:1
1.4 5 5*
0.658 0.734 0.524
18 18 18
0.114 0.076 0.022
5.6 : 1 9.7 : 1 24 : 1
1.4 5.2 8.7
Concentration = 4.53 × 10-SM. * Increases rapidly with time: after 60 min reaches 6-9 kK. t Peak at 34 k K still visible. o
o
0
CL~CL Cl~Cl--'--'~~ l l ~ c l 0 [
0-
OH CL 0-Cl-~J---C l oH
0-
o
o
o ]II
[6] J. W. HANCOCK,C. E. MORRE~ and D. RHU~, Tetrahedron Lettera 22, 98 (1962).
1756
M.A. SLIF~,
R. A. S U ~ E R and J. G. HEATHCOTE
I t would a p p e a r t h a t despite some quite m a r k e d resemblances b e t w e e n t h e spectra o f old chloranil a n d t r i c h l o r o h y d r o x y q u i n o n e , t h e t w o solutions are different. I t is possible t h a t t h e old solutions o f chloranil c o n t a i n a n equilibrium m i x t u r e of chloranil, t r i c h l o r o h y d r o x y q u i n o n e a n d chloranilic acid or o t h e r i n t e r m e d i a t e species w i t h t r i c h l o r o h y d r o x y q u i n o n e as t h e d o m i n a n t species. Possible p a t h w a y s w h i c h h a v e b e e n p u t f o r w a r d for t h e successive stages o f the conversion of chloranil t o chloranilic acid are [6, 7] as shown a b o v e . T h u s a n y of these species could be p r e s e n t in old solutions of chloranil. Acknowledgements--The spectrophotometer was obtained with a Medical Research Council grant to M. A. S. Triehlorohydroxyquinone was synthesised by B. K~OGH. We would like to thank Dr. T. MELIA for helpful criticisms.
[7] C. A. BISHOP and L. K. J. T o n g Tetrahedron Letters 4.l, 3043 (1964).
Acta, 1967, Vol. 28),, pp. 1751 to 1756. PergamonPress Ltd. Printed in Northern Ireland
The interactions of chloranfl in aqueous solvents--I The absorption spectrum Ol chloranil in 50 per cent aqueous ethanol M. A. SLIFKIN D e p a r t m e n t of Pure and Applied Physics, Ro y al College of Advanced Technology, Salford and
1%. A. SUMNER and J. G. HEAT~COT~ Department of Chemistry, Royal College of Advanced Technology, Salford
(Received 13 August 1966)
Abstract--A study has been made of the visible and ultra violet absorption spectra of a solution of ehloranil in 50 per cent aqueous ethanol when buffered at different pH values. The changes in the spectra with time have also been studied. It is shown that the characteristic purple colour of an aqueous solution of chloranil is not due to chloranilic acid. Spectral details are also given for triehlorohydroxyquinone in 50 per cent aqueous ethanol. INTRODUCTIOI~
IN a s t u d y of the interaction of some amino acids and 50 per cent aqueous ethanol [1] it was not e d t h a t the unstable an d developed a purple colour assumed to spectrum of ehloranil at different p H values, and as a studied.
proteins with ehloranil in spectrum of chloranil was be chloranilic acid. The function of time, has been
:EXPERIMENTAL
The solvents were made up b y mixing equal volumes of ethanol and requisite buffer solution. Spectral measurements were made using a Unicam SP700 recording s p e c t r o p h o t o m e t e r and t h e stoppered cuvettes (10 m s ) were made of fused silica. All spectra were measured at a t e m p e r a t u r e of 22 -4- ½°C. Buffer solutions were supplied b y Messrs. British Drug Houses Ltd. RESULTS AND DISCUSSION
The spectrum of freshly made up chloranil in 50 per cent aqueous ethanol at p H 9 exhibits a maj or absorption band at 33.2 k K with a width of 6.7 kK. (The width of th e b a n d is measured between the points where t he optical density is one half of the m a x i m u m optical density.) A minor band is observed at ~-~18.6 kK, the absorption ratio between t he two bands being a p p r o x i m a t e l y 20: 1. After three hours, the ba nd a t 33.2 k K in unchanged b u t t h a t at 18-5 k K increases slightly to give an absorption ratio between t he two bands of 13: 1. After four days, there are v e r y m a r k e d changes in the spectrum. The b a n d of m a x i m u m i n t e n s ity shifts to 31 k K with an increase in absorption of about 25 per cent over t h a t of th e original 33.2 k K band. The absorption at 18.5 k K decreases to a b o u t one t h i r t i e t h of the absorption a t 31 kK . The spectrum at p H 9 after four days is [1] J. B. BIRKS and M. A. S n i p ,
1Vature 197, 42 (1963).
1751
1752
M . A . SLIFXn¢, R. A. S ~ R
and J. G. HEATHCOTE
i d e n t i c a l w i t h t h e s p e c t r u m of chloranilic acid as r e p o r t e d b y BERTOLACINI a n d BARNEY [2]. T h e s p e c t r u m of a f r e s h l y p r e p a r e d solution of chloranil in 50 p e r cent a q u e o u s e t h a n o l a t p H 7, e x h i b i t s a m a j o r a b s o r p t i o n b a n d a t 33-5 k K w i t h a w i d t h o f 5 k K . T h e r e are m i n o r b a n d s a t 22.5 k K a n d , ~ 1 8 k K . A f t e r a b o u t 6 hr, t h e b a n d a t 33.5 k K is u n c h a n g e d b u t t h e one a t 22.5 k K d i m i n i s h e s slightly a n d t h a t a t ~ 1 8 k K increases slightly. T h e r a t i o of t h e a b s o r p t i o n of t h e b a n d a t 18 k K t o t h a t a t 33.5 k K is 1 : 13. O v e r a p e r i o d of f o u r d a y s o n l y slight c h a n g e s in o p t i c a l d e n s i t y are o b s e r v e d . T h e m a j o r a b s o r p t i o n b a n d shifts f r o m 33.5 t o 33.2 k K . T h e r a t i o of t h e a b s o r p t i o n of t h e b a n d a t 18 k K t o t h a t a t 33.2 k K is 1 : 13. A f r e s h l y p r e p a r e d solution of chloranil in 50 p e r cent a q u e o u s e t h a n o l a t p i t 4, e x h i b i t s a m a j o r a b s o r p t i o n b a n d a t 32.6 k K w i t h a w i d t h of o n l y 1.4 k K , b u t t h e r e is a slight a b s o r p t i o n in t h e region of 27 k K to 17.5 k K . A f t e r f o u r d a y s , little c h a n g e is o b s e r v e d in t h e b a n d a t 32.6 k K , b u t t h e r e is a slight decrease in a b s o r p t i o n in t h e region of 24 k K a n d a slight increase in a b s o r p t i o n in t h e region of 18 k K . T h e r a t i o of t h e a b s o r p t i o n a t 18 k K t o t h a t a t 32.6 k K is a b o u t 1 : 13. T h e results are s u m m a r i z e d in T a b l e s 1 a n d 2. F i g u r e s 1, 2 a n d 3 show s p e c t r a of chloranil a t different p H values. Table 1. Position of peaks in the absorption spectra of solutions of chloranil pH
Age of solution
4
30 rain 4 days 30 rain 6 hr 4 days 30 mln 3 hr 4 days
7 9
Frequency of major peak (kK)
Frequency of minor peak (kK)
32.6 32.6 32.5 33.5 33.2 33.2 33.2 31
Width of major peak (kK)
-18 -18 18 18.5 18.5 18.5
1.4 5 6"7
Table 2. Ratio of absorption of major and minor peaks of solutions of ehloranil twenty four hours old Solution No.
pH
Optical density of major peak
Optical density of minor peak
Ratio minor: major
1 2 3
4 4 7
0.39 0.63 0.60
0.029 0-047 0.43
1 : 13.4 1 : 13.4 1 : 13-9
4
7
0.36
0.30
1 : 12
5 6
9 9
0.65 0.26
0.049 0.019
1 : 13.3 1 : 13.2
I t is seen t h a t s h o r t l y a f t e r p r e p a r i n g solutions o f chloranil, t h e s p e c t r a c h a n g e slightly t o give a p u r p l e colour due t o t h e n e w b a n d a t 18 k K . O v e r t h e r a n g e of p H v a l u e s covered, t h e r a t i o of t h e m a x i m u m a b s o r p t i o n o f t h i s b a n d t o t h e old
[2] R. J. BERTOLACINIand J. B. BARNEY, Anal. Chem. 30, 302 (1958).
The interactions of chloranil in aqueous s o l v e n t s - - I
1753
--05
40
--
0-4
--
05
--
02
6O
d o
i
I
,,
E v~ c
o
u
X
o
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I-
,\
80--
--
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~ x \
×
~X tO0
I
40
35
I
Frequency
~ ~x~X,,~\
T - - ~x"r' 25
50 ,
x. 20
15 0
kK
Fig. 1. Absorption spectra of ebloranil (2.56 × 10-t~/) in 50 per cent aqueous ethanol at p H 9. (a) 3 hr after preparation ( - - - - - - ) . (b) 4 days after preparation ( - - × - - ) .
band at ~ 3 3 k K is about 1 : 13. This must represent some new species--obviously not chloranilic acid as previously suggested [1]. It is only in alkaline solution that chloranil is eventually converted to chloranilic acid. The rather marked changes in the width and positions of the chloranil peak due to alteration in pH must be due to solvation. HAUSS~,R and MULLIKEN [3] have shown that the spectral bands of ehloranil in several different solvents have quite marked differences in position and width. Thus the peak absorption of chloranil in heptane occurs at ~ 3 4 kK and in perfluorheptane at ~ 3 6 kK. The widest band occurs in ethanol. They attribute these effects to strong solvent interaction. Chloranil (I) is a well-known electron aceeptor [4]. Ethanol has been shown to be a weak electron-donor in water [5] and therefore it is probable that there is some charge transfer between ethanol and chloranil in water. The amount of charge transfer will be a function of pH [1]. The new species formed shortly after the preparation of the chloranil solutions has obvious affinities with chloranilic acid (III), both possessing the broad featureless band with m a x i m u m absorption at about 18 kK. The new species is probably some intermediate stage in the conversion of ehloranil to chloranilic acid. [3] K. H. I~USSER and R. S. MvIz_z~¢, J. Phys. Chem. 64, 367 (1960). [4] C. BRIEGLEB Elelctronen-Donator-Acceptor.Komplexe. Springer (1961). [5] M. A. S L ~ , Spectrochim. Acta 21, 1391 (1965).
175t
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The interactions of chloranil in aqueous solvents--I
1755
One i n t e r m e d i a t e is a k n o w n s t a b l e c o m p o u n d t r i c h l o r o h y d r o x y q u i n o n e ( I I ) . S o m e of t h i s c o m p o u n d h a s b e e n s y n t h e s i s e d following t h e t e c h n i q u e o f HA~COCK et al. [6]. S p e c t r a of t r i c h l o r o h y d r o x y q u i n o n e h a v e b e e n m e a s u r e d in t h e s a m e s o l v e n t s as chloranil. T h e s p e c t r a s h o w similar f e a t u r e s t o t h a t o f one d a y - o l d chloranil. H o w e v e r t h e r a t i o s o f a b s o r p t i o n of t h e m a j o r p e a k a t ~ 3 4 k K t o t h e m i n o r p e a k a t 18 k K are less t h a n t h o s e o f chloranil. T r i c h l o r o h y d r o x y q u i n o n e in p H 4 a n d p H 7 solution is r e l a t i v e l y stable. A t p H 9 t h e s p e c t r u m c h a n g e s t o t h e c h a r a c t e r i s t i c s p e c t r u m o f chloranilic acid w i t h a s t r o n g b a n d a t 31.4 k K a n d a v e r y w e a k b a n d a t 18 k K , t h e r a t i o of a b s o r p t i o n s b e i n g a b o u t 24: 1. T h i s is s u m m a r i z e d in T a b l e 3. O t h e r similarities b e t w e e n t h e s p e c t r u m o f t r i c h l o r o h y d r o x y q u i n o n e a n d chloranil are t h e b a n d w i d t h s of t h e m a j o r p e a k s a n d t h e p o s i t i o n s o f t h e m a j o r p e a k s a t different p H values. Table 3. Spectral details of trichlorohydroxyqulnone in 50 per cent aqueous ethanol
pH
Major peak Optical v(kK) density
Immediately on mixing 4 32.7 7 34 9 34 After 24 hr 4 32.7 7 34 9 31.4~
Minor peak Optical ~,(kK) density
Ratio of major to minor absorption
Width of major peak (kK)
0.620 0.854 0-700
18 18 18
0-132 0.108 0"061
4.8:1 7-9:1 11-6:1
1.4 5 5*
0.658 0.734 0.524
18 18 18
0.114 0.076 0.022
5.6 : 1 9.7 : 1 24 : 1
1.4 5.2 8.7
Concentration = 4.53 × 10-SM. * Increases rapidly with time: after 60 min reaches 6-9 kK. t Peak at 34 k K still visible. o
o
0
CL~CL Cl~Cl--'--'~~ l l ~ c l 0 [
0-
OH CL 0-Cl-~J---C l oH
0-
o
o
o ]II
[6] J. W. HANCOCK,C. E. MORRE~ and D. RHU~, Tetrahedron Lettera 22, 98 (1962).
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M.A. SLIF~,
R. A. S U ~ E R and J. G. HEATHCOTE
I t would a p p e a r t h a t despite some quite m a r k e d resemblances b e t w e e n t h e spectra o f old chloranil a n d t r i c h l o r o h y d r o x y q u i n o n e , t h e t w o solutions are different. I t is possible t h a t t h e old solutions o f chloranil c o n t a i n a n equilibrium m i x t u r e of chloranil, t r i c h l o r o h y d r o x y q u i n o n e a n d chloranilic acid or o t h e r i n t e r m e d i a t e species w i t h t r i c h l o r o h y d r o x y q u i n o n e as t h e d o m i n a n t species. Possible p a t h w a y s w h i c h h a v e b e e n p u t f o r w a r d for t h e successive stages o f the conversion of chloranil t o chloranilic acid are [6, 7] as shown a b o v e . T h u s a n y of these species could be p r e s e n t in old solutions of chloranil. Acknowledgements--The spectrophotometer was obtained with a Medical Research Council grant to M. A. S. Triehlorohydroxyquinone was synthesised by B. K~OGH. We would like to thank Dr. T. MELIA for helpful criticisms.
[7] C. A. BISHOP and L. K. J. T o n g Tetrahedron Letters 4.l, 3043 (1964).