June 20, 1960
3085
EIGHTISOMERS OF BENZENE HEXACHLORIDE
[CONTRIBUTION FROM
THE
DEPARTMENT O F CHEMISTRY, FACCLTY OF
SCIENCE,
KYUSHUUNIVERSITY, FUKUOKA, JAPAN ]
On the Yields of Eight Isomers of Benzene Hexachloride BY YOSHIYA KANDA RECEIVEDSEPTEXBER 28, 1969
A simple statistical calculation of the yields of eight isomers of benzene hexachloride(BHC) on a n assumed chlorination mechanism has been carried out. Processes of chlorination are proposed and several rate controlling factors and coefficients have been estimated from data on the yields of B H C isomers from benzene tetrachloride(BTC) isomers according to a study by Orloff, et el , and the principle for calculating the yields of products has been applied to all the chlorination processes from benzene t o B H C by way of benzene dichloride(BDC) and B T C The calculated values are in fairly good agreement with observed data. A reason for a rich yield of the or-isomer, a possibility of finding still unknown isomers and a possibility of obtaining a good yield of the ?-isomer are also discussed.
Introduction The molecular structures of the benzene hexachlorides (BHC) have been widely studied by means of dipole moments, infrared and Raman spectra, electron and X-ray diffractions, and chemical reactions. The more stable form of cyclohexane is the chair form. If one of the two hydrogen atoms combined to each carbon atom of cyclohexane is substituted by chlorine, many BHC isomers are possible, the form of the chair-like ring remaining unchanged. Thirteen structural isomers would appear to be possible, but only six isomers have been discovered. Identifications of the molecular structures of the six isomers have been completely carried out. The relation between the structures of these six and other unknown isomers and the processes by which they are produced will be discussed. Why the a-isomer is obtained in abundance, and the factors influencing the yield of the 7-isomer (Gammexane) which has an exceptionally strong insecticidal activity, will also be treated. Chlorination Mechanism.-In preparation of BHC gaseous chlorine is introduced into liquid benzene in a flask under sun light. Benzene dichlorides probably are formed first, then tetrachlorides and finally hexachlorides. We should like to trace these processes in detail. According to modern views an additive reaction of halogen to a double bond is interpreted in this way: a t first a C1+ ion combines with the r-electrons of a double bond and a n-complex is formed. The attachment of a C1- ion to the complex completes the halogenation. We extend this idea to the chlorination of benzene and remodel it somewhat so t h a t a simple statistical treatment can be made. Orloff, et a1.,* synthesized five isomers of benzene ) ~ tetrachloride (BTC). Bastiansen, et ~ 1 . studied their molecular structures by means of dipole moment determination and electron diffraction. The hexagons of BTC were all in the chair form. Orloff, et al., also photochlorinated each isomer of BTC and obtained BHC isomers. Their two excellent tables are reproduced here in Tables I (1) See, for example, E . L. Lind, h f . E . Hobbs and P. M. Gross. THIS J O U R N A L , 72, 4474 (1950); Y. Morino, I . hliyagawa and T. Oiwa, Bolyu-Kagaku, 15, 181 (1950). (2) H. C . Orloff, A. J. Kolka, G . Calingaert, hf, E. Griffing and E. R . Kerr, THISJ O U R N A L , 76, 4243 (1953); see also G. Calingaert, M. E. Griffing, R. E. Kerr. A. J. Kolka and H. D. Orloff, ibid.,7.3, 5224 (1951). (3) 0. Bastiansen and J. Markali, Acta Chem. Scand., 6, 442 (1952) 0. Bastiansen, ibid., 6, 875 (1952).
and 11. (The letter “a” signifying “axial” has been used throughout this paper instead of the letter “p” signifying “polar.” Moreover, a misprinted entry in the bottom line of their Table I11 is corrected here.) Tables I and I1 show that there is a considerable amount of cis addition of chlorine to BTC as well as large amounts of trans addition as indicated by Orloff, et al. We now propose a simple reaction mechanism in order to interpret the chlorination of BTC to BHC and also of benzene to BTC. TABLE I THEORETICAL RELATIOSSHIPS BETWEEN B T C RTC configiira. tions
------BHC ee
ASD
BHC
isomers expected--l r a n s Addition cis Addition aa ea or ae
or, eeaa
CY llor“ + a ? I ? aeeaand eaae e and a l / a --L CY and l / e -+ s 7J y,eeea 6 Y e a : 6, feee 0 ff 6 e , eeae 6 ?I e e f , eaea e i / e -+ e ‘ I 1 The symbols indicated as reciprocals ( e . g., l / a ) designate the less favored and hence not isolated high energy conformation.
8,
TABLE I1 PRODCCT DISTRIBUTIOS FROM B T C CHLORIKATIOS BTC isomers
a, eeaa 8 , aeea, -1,eeea
6, eeee e, eeae
eaae
B H C isomers obtained in photochlorination, LX> a 6 7 6 c eeeeaa eeeeee eeeaaa eeeeea eeaeea Other
89 81 39 70
11 40
13
11 17 33
19
1 1
40
27
A chlorine molecule dissociates under irradiation to two chlorine atoms. When a C1 atom ap. proaches an ethylene molecule, the field around the molecule changes and i t is chemically excited. The bond strength of its double bond becomes weakened and n-electrons come to occupy p,orbitals of both C atoms. A C1 atom approaches a p,-orbital in a perpendicular direction to the molecular plane. At approximately the same time the other C1 atom approaches a p,-orbital of the other C atom. The C atoms change their sp2 hybrid orbitals into sp3 orbitals and dichloroethane is produced. These two Cl atoms niay approach the C atoms from the same side or from the opposite side of the molecular plane. Internuclear distances of C=C, C-C and Cl-Cl bonds are 1.33, 1.54 and 2 A.,respectively. The
1.01. h“ BHC
c
8-BT eeee 6 5
B
i/l
e\
’ 9
L
o
a
a
e
F
8
8
7
‘
‘t C-BTC
1;ig. I(h.
t
8
9
/’
C l i l ~ ~ r i ~ ~ ~ptrtoi o c ei i~ ~ cfrom s 6-,
E- a i d
j--BTC t ( J
BHC. processes froin a - , p- aiid 7 -BTC to I:ig. 1(a) -~-Cliloriiiatic~ii BHC.
in the figures. aiblack circle in these means t h a t a C1 atom approaches a p,-orbital of one carbon distance 2 -k. is the closest one between two C1 atom of a double bond in a direction from above atoms. If two chlorine atoms approach the C the double bond down to the carbon, while a white atoms from the same side a t almost the same time, one signifies t h a t a C1 atom comes to a p,-orbital the additive reaction is unfavorable compared of one of the carbon atoms of a double bond in the with trans addition. 1Ve may have a majority opposite direction to t h a t of a black circle. X of the trans addition compound and a minority black circle on the left of a symbol -0-8- means of the czs product. If C1 atoms do not come to the t h a t a C1 atom attacks the left carbon of a double C atoms in the perpendicular direction to the bond of a BTC isomer in the figures from above plane, the addition may be slow since the pT- the ring, while a white circle on the right of the symbol means an attack against the right carbon orbital extends only in t h a t direction. Chlorination of Benzene Tetrachloride.-Orlof, from below the ring. 1Ve introduce three rateet aZ.,discussed in detail three factors controlling controlling factors A, R and C. The l-carbon of the rate of chlorination of B T C : namely, (1) the a-BTC has an axial C1-C . bond in an upward mechanism of addition whether cis or trans, ( 2 ) direction whose C1 atom obstructs a C1 atom a so-called “entry factor” resulting from the represented by a black circle in a symbol -0-5structures of the BTC isomers and involving steric from approaching the 6-carbon from above. and/or electrostatic influences, and, finally, (3) These steric aud,/or electrostatic factors are I letter “e” means that a C1 the steric strains and thermodynamic stability designated “A.” . of the product BHC. Figures l ( a ) and l ( b ) atom comes to an equatorial position when i t illustrate the chlorination mechanism of the six attaches itself to one of carbon atoms of a double isomers of BTC listed in Table I. A C1 atom is bond, while a letter “a” means t h a t a C1 atom represented by a black circle or a white circle 0 . comes to an axial position when i t combines with For 13HC isomers a black circle denotes a C1 atom one of carbon atonis of a double bond. If a or an axial or an equatorial Cl-C bond above the C1 atom comes to the 3-carbon of a-BTC from plane of the hexagon ring which is approximately above, a C1 atom in an equatorial C1-C bond in the paper surface, while a white circle shows a C1 belonging to the 4-carbon obstructs its approach atom of bonds below the plane of the ring. This a little. The blocking factor is represented as in:mner of expression is analogously applied to B T C “B.” When two C1 atonis add cis to a-BTC. i s ~ n i e r s Several symbols, such as -.-e-, are used two I ,::-axial C1 C bonds appear i n -J- and YI-
June 20, 1'360
EIGHTISOMERS
OF
BHC. They repel each other. This repulsion decreases the yields of both isomers. The decreasing factor is expressed as "C." Each factor IS written under a symbolized C1 atom or 0, which the factor concerns, in a symbol -0-e-and others. When a reciprocal form, such as l/a, appears, it is assumed that the form easily goes over to a normal form. No rate factor is introduced in the inversion process. For P-BTC two conformations aeea and eaae exist. The chlorination is traced for both forms. Now we calculate yields of BHC isomers as results of chlorination of each BTC isomer according to the reaction schemes shown in Figs. l ( a ) and l ( b ) . The yields of the products from a-BTC can be calculated as follows: We assume there is 100% of a-BTC a t the beginning of the reaction; of this 807, is subject to trans addition and the remaining 20% is subject to cis addition. The possibility of trans addition is divided in two categories, ie., ee and aa additions. The ee addition is regarded as less favorable than the aa addition. We assign 10% to the ee addition and
3087
BENZEXE HEXACHLORIDE
70% to the aa. The factors A, B and C are assumed to be 0.5, 0.9 and 0.5, respectively. We give a factor BC a value 0.9 X 0.5 X 0.6, which includes an additional coefficient 0.6, necessary since reaction is expected to be much slower when two blocking factors appear a t the same time. For the same reason an additional coefficient 0.2 is introduced in calculation when two C1 atoms in a symbol -0-8- and others have any of the factors simultaneously. However, in the case of the trans aa addition the coefficient 0.2 is omitted since this reaction is presumed to be carried out most smoothly. These values for the factors and coefficients are estimated by a trial-and-error method so that calculated yields of BHC isomers can be adjustedto the observed data as closely as possible. Of course, these estimates are only tentative and a much better set of values could be assigned to them. In this calculation every effort was exercised in order to obtain calculated results nearest to the observed data with the smallest number of parameters and the simplest values. BHC
trans
7 addition CY-BTC 100 L- cis addition
80
lee jaa
10 10 X 0 . 5 ( A ) = 5 168 i 0 70 X 0.9(B) = 63
20
{(eaae
10 10 X 0.5(A) 0.9iB) X 0.5(C).X 0:s X 0.2 = 0.3 10 10 X 0.5(C) = 5
a,92.8%
7, -/,
0.4% 6.8%
In order to calculate the yields of the products from P-BTC it is necessary to know the concentration ratio of the two conformations aeea and eaae. Bastiansen3 calculated the ratio and gave a value 87: 13 from the results of the dipole moment determination. We use this value in the calculation as: \ 10 X 0.87 X 0.5 X 0.5 X 0.2 = 0.4--, BHC 1.3_ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ 710 x 0.13 61.3 a , 86.6%
ee
tians 80
7
a a 70
8-BTC 100 L cis 20
1 I
x
0.87 = 60.9_ _ _ _ _ _ _ J _.__._._____ 18.7 e, 1 2 . 3 % 70 X 0.13 X 0.9 X 0.9 = 7 . 4 - - - 10
\ e a 2o 110 X 0.87 X 0.5 X 0.5 X 0.2 = 0.41 o,8 ( 10 X 0.13 X 0.9 X 0.5 X 0.6 = 0.4
1 ae
7,
1.1%
The calculations of yields of products from other BTC isomers can easily be done as: trans 80
7
*,-BTC loo
cis 20
l e e 10
( a a 70
3
10 X 0.5 = 5 70 X 0.9 X 0.5 X 0.6
6-RTC' loo
E-BTC loo
= =
9 9
\ e e 10 10 ( a a TO 70 X 0.9 X 0.9 X 0.5 X 0.6
( e a 10 i a e 10
10 x 0.9 = 9 10 X 0.9 X 0.5 X 0.6= 2.7
trans 80
\ e e 10 laa i 0
10 X 0.5 = 5 70 X 0.9 X 0.5 X 0.ti
czs 20
8, 0 . 8 ' ; a , 39.S')L
6 , 21.3:;
cis 2o
p-~~e
0.3
5ti.i
i
7
loo
( e a 10 10 X 0.9 ( a e 10 10 X 0.9
i
=
trans 80
7
=
BHC
\ e e 10 10 trens 80 ( a a 70 70 X 0.9 X 0.9
cis 20
18.9
ea 10 10 X 0.5 X 0.9 X0.5 X 0.6 X 0.2 ae 10 10
-
7
=
BHC 6, 1 2 . 4 ' 1 -,!, 47.05;
=
=
BIiC 6 , 25.87;, 7 , 13.9sc
17
e,
BIiC
] 23.9 18.9
ea 10 10 ae 10 10 X 0.9 X 0.5 X 0.5 X 0.6 X 0.2
23.3Cb
e, ;.oc,; e, 69 9:; 7, =
0.3
i,
29 270 0.9yo
31hS
0
BTC
BT C
'/SdS
I,4-BDC t j r
&&
P
tit 4+
6
\a
Y
l f i 4 BENZENE
B
4-f 44% A &&A AC 445 '
l G
c 1;ig. ?(a):
Cliloriiiatiuii processes from beiizeiic to 2,3-BUC and finally to BTC.
The calculated values for the yields of the BHC isomers are seen to be in fairly good agreement with the observed data listed in Table 11. The yields of the products from the unknown {isomer of B T C also may be calculated by the same principle as t h a t applied to the five known B T C isomers. Now, if we knew the relative concentrations of the B T C isomers in the course of the chlorination of benzene, the yield ratio of eight B H C isomers iiiight be calculated very easily. The chlorination iiiechanisin of benzene to B T C will be discussed. Chlorination of Benzene to Benzene Tetrachloride. --Sl'hen benzene is chlorinated, benzene dichloride (HDC) presuniably is obtained first and tlicn benzene tetrachloride (BTC). The reaction 1)rocesses are illustrated i n Figs. !?(a) and 2 ( b ) , in which the conventions used are essentially the same ;is those in Figs. 1 (a) and 1( b ) . Although benzene dichloride has not yet been isolated, we can expect to obtain benzene 2,3dichloride (2,S-BDC) and benzene 1,4-dichloride (1,4-€3DC). For the former the name benzene 1,2tiicliloride ( 1 ,2-HDC) is correct, but we have used the alternate iiaine here so that the numbers will coIiform with those in th.e figures. The cyclohexadiene ring of 2,3-BDC is not coplanar but has a shallow chair-like form. For 2,5-BDC there are two isomers derived through trans aa arid ee additions x i i d one isoiiicr through civ ea or :le a(1cIitioii. . i l l tlirce isoincrs lia\,c optical aritil)odes.
BC B
Fig, 2 ( b j .
Cliloriiiaticm processes fi-om betizeiie t u 1 , I - B U C anti finally to BTC.
IVhen 2,3-BDC is chlorinated to BTC, two C1 atoms attach themselves to the 1- and -&carbon atoms. Since the I-, 6-, 5- and &carbon atoms have sp2-hybridization and one p,-electron with each, it is highly probable t h a t the free valence values of the 1- and 4-carbon atoms are greater than those of the 5- arid G-carbon atoms although the resonance energy due to conjugation of two double bonds may be very small. \Ye obtain, therefore. 1.2,:3,3-BTC as shown in Fig. 2(a). On the other hand, it is also probable t h a t two C1 atoms attack the 1- and 4-carbon atoms in the initial chlorination of benzene. 1l:e obtain one isomer through trans ae and ea additions and two isomers through cis a a and ee additions. The cyclohexadiene ring of I,+BDC has a boat form. The next chlorination occurs a t the 2- arid ;;carbon atonis. IVe obtain 1 .2.3.4-BTC, as shown in Fig. 2(b). Subsequently, we obtain p-, yand 6-BTC from 2,X-RDC which is derived through trans aa and ee additions to benzene, and cy-, tand
