83 (1964) RECUEIL 711

547.561.3: 541.223 THE INTERACTION OF CHLORANIL WITH ALIPHATIC AMINES

BY

R. FOSTER Queen’s College, University of St. Andrews, Dundee, Scotland

Although substitution of chlorine usually occurs in the interaction of chloranil with aliphatic arnines, solutions of chloranil in pure triethylarnine yield a product which appears to be a salt, the anion of which is the free radical serniquinone ion derived from chloranil. In acetone solution this salt decomposes to yield 2,3,5- trichloro-6-(2’-diethylaminovinyl)-pbenzoquinone.

The absorption spectra of some aliphatic amine/chloranil mixtures have recently been studied 11 2 . It was reported that the band a t about 285 mp, characteristic of chloranil in solvents such as ethanol or n-heptane was absent, but an absorption was observed in the region 320-380 mp. It has been argued 1* that this band is the perturbed 285 mp band of chloranil, that formation of an n-n intermolecular charge-transfer “outer complex” with the amine increases the electronic charge on the chloranil component of the complex so that the 285 mp electronic transition more closely appro- aches the transition of the semiquinone anion derived from chloranil (I).

. . :O:-

:0:

I

This ion has characteristic maxima in methanol at 426, 452 mp 5 . We observe an absorption spectrum of chloranil in triethylamine similar to that recorded by Slifkin 1. This has a maximum at 314 mp and appears

1 M. A. Slgkiii, Nature (London) 195, 635 (1962). * J . B. Birka and M. A. Slifkin, ibid. 197,42 (1962).

M. A. S/ifkin ibid. 198, 1301 (1963). R. S. 44uIlik.cn J. Am. Chern. Soc. 74, 811 (1952); J. Phys. Chem. 56, 801 (1952).

5 H, Kainer and A. Uberle, Ber. 88, 1147 (1955); R. Foster and T. J. Thomson, Trans. Faraday Soc. 58,800 (1962).

712 R. Foster

to be stable over periods of at least one hour. The spectrum of chloranil in 50% v/v aqueous diethylamine shows a broad band with a maximum at 380 mp. Ethylamine (4M) and chloranil (10-5M) in a 1 :1 v/v mixture of water and ethanol (conditions similar to those used by Slifkin) show two maxima in the range 300-400 mp: a band at 303 mp which decreased with time and a band at 356 mp which increased with time (figure 1).

150 300 3 50 400 Wavelength (m k)

Fig. 1 . Solution of chloranil in 50% v/v aqueous ethanol, 3.3 M with respect to ethyl- amine ( I ) within 3 mins of mixing; (2) after 10 mins; ( 3 ) after 20 mins; (4) after 30 mins. (5) 2.3,5-Trichloro-6-diethylamino-p-benzoquinone in the same solvent. (all at 20").

The Interaction of chloranil wifh aliphatic amines 83 (1964) RECUEIL 713

A solid product was obtained from solutions of chloranil in aqueous diethylamine. This appears to be 2,5-dichloro-3,6-bis-(diethylamino)-p- benzoquinone.

From mixtures of chloranil and excess ethylamine in alcoholic solution a solid product, apparently 2,5-dichloro-3,6-bis-(ethylamino)-p-benzoquinone was obtained. This has an absorption with a maximum at 356 mp in 50% aqueous alcoholic ethylamine solutions, and coincides with the absorption band which is observed to develop in the original mixture (figure 1). The species with absorption at 303 mp which disappears rapidly with time may correspond to an adduct similar to the stable adduct of chloranil with triethylamine described below.

The more complex system involving the tertiary amine was studied in detail. A suspension of chloranil in triethylamine was shaken for three hours at room temperature in a stoppered vessel; after which time the chloranil had apparently been consumed, and there remained a dark green precipitate. A solution of this solid in triethylamine shows a sharp band at 314 mp, coinciding exactly with the absorption previously noted for solutions of chloranil in triethylamine. In appearance the green solid is reminiscent of the product of interaction of sodium (or potassium) iodide with chloran- il 6 , the structure of which is fairly well established 7 as the sodium salt of the chloranil semiquinone ion (I). The resemblance is more than superficial. Both solids show a strong e.s.r. single line absorption. A similar product from triethylamine and 2,5-dichloro-p-benzoquinone shows an e.s.r. with a triplet h.f.s. similar to that previously observed for the corresponding 2,5-dichloro-p-benzo-semiquinone ion 8 . On dissolving in methanol both the sodium salt of I and the green product from triethylamine and chloranil show an absorption (max. 426, 452 mp) which is also shown by a mixture of chloranil and tetrachloroquinol in alkaline solution. This absorption had been previously assigned as due to the ion I 5 . Similar close resemblances of spectra of the sodium salt of I and the green adduct from triethylamine and chloranil are shown in acetone solution (figure 2). The ion I also appears to be generated in acetone solutions of chloranil simply by the addition of strong alkali. This is shown both by optical absorption and by e.s.r. measuements. Furthermore the optical absorption in the visible region by the solid adduct (sh 380, sh 435,770 mp) is similar to that of the sodium salt of I (393, sh 440, 889 mp) 9; their spectra in pure triethylamine are identical, and their i.r. spectra are virtually identical.

This parallelism with the sodium salt of I suggests that the green adduct

H. A. Torrey and W. H. Hunter, J. Am. Chern. SOC. 34, 702 (1912). H. Kainer, D. Sgl and A. C. Rose-Innes, Nature (London) 178, 1462 (1956).

* J . E. IVertz and J. L. Vivo, J. Chem. Phys. 23, 2441 (1955). R. Foster and T. J . Thomson, Trans. Faraday. Soc. 59, 296 (1963).

714 R. Foster

is formed by a single electron transfer from treithylamine to chloranil to yield 11. However this would be a biradical and the e.s.r. due to the cation

I1 I11

should be observed. No absorption which could be assigned to an uncoupled electron on a nitrogen atom or associated with proton in such an ion has been observed. The nature of the cation remains in doubt; tentatively

80 C 0 c, .-

60 2 ru

20

400 500 600 700 800 Wavelength (m k)

Fig. 2. Solution of 8 mg of the green chloranil-triethylamine adduct in 100 ml acetone ( I ) within 4 min of mixing; (2) after 30 min; (3) after 64 min; (4) after 100 min; ( 5 ) after 18 hr. (6) Chloranil in acetone in the presence of sodium hydroxide. (7) Sodium salt of chloranil-semiquinone in acetone. (8) 2,3,5-Trichloro-6-(2'-diethylaminovinyl>pnzo-

quinone in acetone = 8,560). (All at 20").

The Inferacfion of chloranil with aliphafic amines 83 (1964) RECUEIL 715

it is suggested that it may have the structure 111. Acidification of the green adduct with dilute aqueous hydrochloric acid precipitated pure chloranil. This indicates that no replacement of chlorine in the chloranil moiety had occurred.

When free of solvent the adduct appears to be stable, but in solution re- action occurs. This is exemplified in the case of acetone solutions. Figure 2 shows the change in absorption spectrum of a solution of the green adduct with time. The band a t 670 mp is identical with that of 2,3,5-trichloro- 6-(2'-diethylaminovinyl)-p-benzoquinone (IV) prepared by the reaction of chloranil, triethylamine and acetaldehyde in benzene solution lo. Henbest and his co-workers l1 suggested as one possibility of formation of lV, which also occurs in benzene solution in the absence of acetaldehyde 11, a mechanism which involves a charge-transfer complex (V), viz. route a below:

-~ ~~ ~ _ _ _ _ ____ ~~ pp._____.---

Reaction Scheme

route a

Et

N-CHz- \ /

Et (A)

-CH3 +

[AQ ++ A+Q-I v \

\ z \

0

0

( Q )

1

. .

VI route b

f /

/ -NEt3)+

Et \+

N /

Et

Et

N \ /

Et

0-

OH i ~ CH =CHI

I Q J.

Et '\

/ N-CH=CH

Et II 0 IV

~

10 D . Buckley, H . B. Henbest and P . Slade, J . Chem. SOC. 1957,4891. 11 D. Buckley, S. Dunstan and H. B. Hevbest, ibid. p. 4880.

716 R. Foster ~ _ _ ~ _ _,~~~____~

The properties of the green product make it unlikely that this is a charge- transfer complex in the sense of an "outer complex" in Mulliken's descrip- tion. If the adduct has the structure VI, then the only modification of Hen- best's mechanism would be to replace the intermediate V by VI, i.e. route 6.

Experimental Part Spectra in the U.V. and visible were measured using an Optica CF4DR recording

spectrophotometer. The visible spectrum of the solid was measured on a nujol mull contained between microscope slides. Infre-red spectra were measured using a Perkin Elmer infracord spectrometer. E m . measurements were made on a Hilger and Watts Microspin spectrometer.

1. Triethylamine This was purified by Hinsberg's method 12, and finally fractionated, b.p. 89.5"/760 mm.

2. Diethylamine

b.p. 55"/758 mm.

3. Ethylamiw

This was redistilled twice from solid potassium hydroxide and then fractionated,

A commercial (B.D.H.) solution of ethylamine in alcohol was diluted without treatment.

4. Chloranil Twice recrystallised from benzene, m.p. 289".

5. Chloranil-triethylamine adduct A suspension of finely divided chloranil ( lg) in triethylamine (50 ml) in a 100 ml round-

bottomed stoppered flask was shaken for three hours. The green precipitate was then separated by filtration, and washed with a small quantity of triethylamine. It appeared to be homogeneous under a low powered ( x 100) microscope. The infra-red spectrum showed no evidence of contamination by unreacted chloranil. On heating the product began to decompose at 135" although melting did not occur below 234". No means of purification was found. This may account for the poor analytical result.

Found : C 36.57; H 3.23; N 3.87 Calc. for C I ~ H I S C I ~ N O Z (347.08): ,, 41.52; ,, 4.36; ,, 4.03.

6. 2,3,!5-TrichIoro-6-(2'diethylaminovinyl)-p-benzoquinone

zene/40-60" pet. ether, m.p. 131". Solutions in methanol show maxima at 244 mp ( E == 3,760); 322 mp ( E = 17.900) and 680 m p ( E = 5,280).

7. 2,5-Dichloro-3,6bis-(diethylamino)-p-be~~uinone Chloranil (Ig) was added to 30% v/v alcoholic diethylamine (100 ml) and left to stand

for twelve hours with occasional shaking. The solution was evaporated to 50 ml under reduced pressure, the product removed by filtration and recrystallised twice from ethanol m.p. 90". Solutions in methanol show maxima a t 245 mp ( E = 7,630).

Found : C 53.12; H 6.18; N 8.75 Calc. for C14HaoClzNzOa (319.23): ,, 52.67; ,, 6.31; ,, 8.78.

Prepared by the method of Buckley, Henbesf and SladelO, recrystallised from ben-

l2 A. I . Vogel, "Practical Organic Chemistry" 3rd ed. Longmans Green, p. 650.

The Interaction of chloranil with aliphatic amines 83 (1964) RECUEIL 717

8. 2,5-Dichloro-3,6-bis-(ethylamino)-p-~~uinone

insoluble in methanol.

Found : C 45.98; H 4.47; N 10.68 Calc. for C I O H I ~ C I Z N ~ O ~ (263.13): ,, 45.98; ,, 4.60; ,, 10.64.

Prepared in a similar manner to (7) above, recrystallised thrice from benzene, m.p. 250",

The Author wishes to thank Dr. J . C. Orr (Isotopes Laboratory, Queen's College, Dundee) for measurements of e.s.r. absorption.

(Received March 9th, 1964).