2306824

Bull. SOC.Chim. Belges, 81 (1972) 65-72
DISCUSSION OF THE MECHANISM OF FREE-RADICAL
PYROLYSES OF CHLOROETHANES
G. HUYBRECHTS, J. KATIHABWA, G. MARTENS (*), M. NEJSZATEN
and J. OLBREGTS
The mechanism usually proposed for the pyrolysis of l,2-CzH4Cl2 is discussed in
terms of existing data on elementary rate constants and using the value
log,,(k,, sec-’) = -(20710 k 630)/4.58 T + (14.33 k0.47)
measured in this work for the rate constant of the reaction
CHzCICHCI
.3
CH2CHC1
+ C1
This discussion is extended to the free-radical pyrolysis of other chlorinated ethanes
using recent literature data. The usual Rice-Herzfeld mechanism is questioned in
view of the discrepancy between experimental results and values predicted from the
knowledge of elementary rate constants of this mechanism.
It is generally accepted [l] that the pyrolysis of 1,2-C2H4C1, at about 700°K
in “seasoned” reactors follows a radical chain mechanism of the type
& CH,ClCH,* + CI
CH2ClCH2CI
Cl
+ CH,ClCH,-
’
CH,CICHCI* + HC1 k,
C1 + CH,CICH,CI
CH,CICHCP
k,
(4)
+ C1
a CHCICHz + HCI
__+
CH,CHCl
(i)
k4
k, ,
The chain termination
C1 + CH,ClCHCI*
(7‘)
b
C2H3C13or CHClCHCl
+ HCI
k;
(ii)
has also been proposed [2-41. This mechanism leads to a first order kinetic law with
an overall rate constant [2, 3, 51
k = ( k , k 2 k,/k,)1/2
(iii)
where k, = k , or k ; . Estimates for the elementary rate constants (k, , k,, k, and k,)
have been proposed [2,3,5] (see table 1, first col.) which led to a satisfactory agreement
(*)
Present address: Solvay Research Center, Neder-over-Heembeek, Brussels.
{
11
I11
47.0
52.5
50.9
10.8
12.8
115.6
-0.3
-3.868
-3.59
75.4
11.9
20.3
20.3'
~
51.0
(,,., {I:$
-2.88
{-3.85h
-4.11''
:::
3.5
16.1 64.8
18.0 {70.0
12.6 49.6
(514.8
(14.9
{
13.7
13.7'
-
1;:q
1.0
I
6.0
{
10.5
72.9
68.8
115.0
46.4
11.88 43.0
12.6 48.3
0.7
< 6.8
-
13.7 20.4
13.7d 20.4'
+0.51
-1.54*
-2.48
-
3.55
9.5
-
3.3
10.8
3.4
9.8
{
12.9
74.3
<15.0
49.1
-0.33
11.88Y:O {-1.54jk
9.62 36 5 -1.71
12.6 48.3 -2.48
5 6.5
0.1
13.7 20.4
13.7' 20.4'
3.3
10.8
{
3.55
9.65
70.1
515.1
44.2
+1.30
13.0 r
.
0{-1.99'
11.6 40.0 -0.89"
12.5 45.1 -1.58
1 7.0 68.7
2.4
12.8 16.8
13.5e 16.1'
3.3
9.8
Activation energies in kcal mole-'; preexponential factors in the units: mole, liter and second.
I Proposed by Howlett (ref. [5]).
I1 Proposed by Goldfinger et al. (ref. 121).
I11 Used in this work (") ref. [6] (the first value for 1,1,2-C2H3C13refers to the pyrolysis to l,l-CzHzC12); (b) this work; c) ref. [2]; (') ref. 171;
r)ref. [8]; (f)see text and table 3 for the A factors and ref. [9] for the activation energies (E _N AH- R T ) .
Experimental values: (g) ref. [S], unpublished data of G. Martens, M. Godfroid and J. Verbeyst lead to a value about 10 times higher; (h) ref. [lo],
(first figures refer to the formation of l,l-C2H2Cl2;it should be noted that the value reported in this ref. as computed from ref. [2] is erroneous,
the correct one is given in the table as k calc 11); (i) mean value taking into account results of ref. [ l l , 121 and unpublished results of G. Martens,
M. Godfroid and J. Verbeyst; ( j ) mean value taking into account results of ref. [l I , 13, 141 and unpublished results of G. Martens and coworkers;
(') ref. [ I l l ; ( I ) ref. [15]; (") ref. [14].
talc
exp.
k
kt
67
78.2
78
I
I1
IIIl
k'
3
1.4
I
6.1
10
22
13.8 23.8
14.3b 20.P
I
I1
I11
k4
3.0
3.1
10.4
10.8
I1
111"
k2
TABLE 1
Preexponential factors (A) and activation energies ( E )for the elementary (k2, k4 and k,/k,) and overaN (k) rate constants of the Rice-Herzfeld mechanism
of the pyrolysis of chloroethanes
z
*
a
Eu
i$
R
2.
n
Discussion of the mechanism of free-radical pyrolysis of chloroethanes
with the experimental data
i) a value
log,,(k,,
67
[q.More accurate values are now available :
1 mole-' sec-')
=
-(3100+300)/4.58T
+ (10.8f0.2)
(iv)
has been measured between 323 and 423 "K ['I.
ii) the rate constant k, has been determined in this work from the competition
between the photochlorination
CH,CICH,CI
+ C1,
+
CHCI,CH,CI
+ HCI
(v)
and the chlorine-photosensitized dehydrochlorination
CH,CICH,CI
(C12)
CHCICHZ
+ HCI
(vi)
of 1,Zdichloroethane. The experimental data are given in the appendix and lead to
log,,(k,,
sec-
') = -(20710f
(vii)
630)/4.58 T + (14.33 f0.47)
in the temperature range 433410°K.
iii) an upper limit for the ratio kl/k, in the rate law (iii) can be estimated from the
equilibrium constant K = k,/k- of the reaction.
CH,CICH,Cl
(1)
CHZCICHZe
+ C1
kl
k-
(-1)
(viii)
1
Assuming no activation energy for the radical recombination (- l)(*), one calculates
= (79.4 f 3) kcal molefrom the enthalpy change AH,",, N
[9] of reaction
(viii) an activation energy
El
= (78 f3) kcal
mole-
'
(ix)
Table 2 shows that the entropies of the chloroethyl radicals (R-)are about
(4.6f 3) cal mole- deg- higher than those of their corresponding hydrogenated
molecules (RH). Using S"(CH,CICH,) = 65.9 cal mole- deg- [17], this leads to
S"(CH,CICH,.) = (70.5f3) cal mole-' deg-'. Use of this value with So(Cl)= 39.5
[18] and S"(CHzCICH,CI) = 73.7 cal mole-' deg-' [17] yields an entropy change
AS&, 2:
= (36.3 3) cal moledeg- for reaction (viii). From this, one
calculates the ratio of preexponential factors
'
'
'
,
log, (A , / A -
'
mole 1-
I)
= 6.1 f0.7
(4
This value is an upper limit for log,,(A,/A,, mole 1 - I ) since A, must be greater
than A - according to the proposed mechanism (i). Taking these results into account
one obtains for the overall rate constant k (see eq. (iii))
,
log,,(k, sec-') = -(50900f2000)/4.58 T + ( < 15.6f0.7)
(xi)
which is to be compared to the experimental value [q
log,,(k, sec-') = -47000/4.58 T + 10.8
(*)
This assumption will also be used for reactions (7) and (7') (see eqs. (i) and (ii)).
(xii)
G. Huybrechts, J. Katihabwa, G. Martens, M. Nejszaten and J. Olbregts
68
TABLE 2
Standard entropies s"for chloroethyl radicals ( R ) and chloroethanes (RH). The standard state
is the ideal gas at 1 atmosphere and 25 "C. The units are cal mole-' deg-'
R.
cclzcclz~
CHClZCC12'
CH,CICHCI.
CHZCICHZ.
CHSCH2.
(")
So(R.)
So(RH)
P(R')-S"(RH)
96.4f2.5 [8]
91.8jz3.0 [7]
76.7 f 3.0 (")
91.0 [17]
86.7 [I71
73.7 [I71
65.9 [I71
54.9 [I 71
5.4 f 2.5
5.1 f 3 . 0
3.0f3.0
59.8 [20]
Measurements
CHzCHCl
of the preexponential factors A'z
+ C1
(2')
gz?
4.9
and A4 for the reactions
CHZCICHCl- led to !oglo(Az', 1 mole-' sec-') = 10.5 [16]
(4)
and log,,(A4, sec-') = (14.33 f0.47)(thiswork).FromthesevaluesandS0(CHzCHCI)=
63.1 [17] and S"(C1) = 39.5 [I81 cal mole - I deg -'onecalculatesS"(CH2C1CHCI.) =
(76.7f3) cal mole-'deg-'.
It should be pointed out that CI atoms are presumed to
add to the less chlorinated C atom of CHzCHCl (reaction (2')) as was observed in the
case of CHCICClz [19].
TABLE 3
Calculation of the ratio of preexponential factors log(A1/A-l, mole 1 - l ) of the rate constant of the
initiation step and its reverse in the Rice-Herzfeld mechanism
R
CHzClCHz-
CHzClCHCI.
CHzCICCIz~
CHCIzCHCI.
CHCIzCC12.
S'(R.)
S'(RC1)
70.5 (")
73.7 [17]
76.7 ( b )
80.6 [17]
85.2 (")
85.1 [I71
85.2 (")
86.7 [I71
91.8 [7]
91.0 [I71
AS",
log Al1A-l
28.0
6.1
27.3
6.0
31.3
6.8
29.7
6.5
32.0
1.0
S"(R.) = S o ( R H ) f 4 . 6 see text;
See table 2; entropies correspond to a standard state of ideal gas at 1 atm and 25°C and are
given in cal mole-' deg-'.
AS, = AS,-RAn-AnR In ( R T )
with An = I and T = 298°K
AS,, = So(Cl)+So(R)-S"(RCI)
with F(C1) = 39.5 cal mole-' deg-' [I81
The maximum value calculated from equation (xi) for the overall rate constant k
at the mean temperature of pyrolysis (700°K)
log,O(k700, sec-')= (-0.3f1.3)
(xiii)
is more than 3 orders of magnitude higher than the value calculated from equation (xii)
10g10(k700, sec-') = -3.86
(xiv)
Therefore, the pyrolysis of 1,Zdichloroethane can be explained by the mechanism (i) only if the rate constant for reaction (7) (disproportionation) or (7') (total
Discussion of the mechanism of free-radical pyrolysis of chloroethanes
69
recombination) is about
times faster than that for reaction (- 1) (eq. (viii)),
Since this is not reasonable it must be concluded that this pyrolysis is strongly inhibited in all the experimental conditions under which it was studied. In any case, the
mechanism (i) does not fit the value reported I5-j for the rate constant of this
reaction.
Similar considerations hold also in the case of the pyrolysis of 1,1,24richloroethane, the tetrachloroethanes and pentachloroethane as shown in table 1 (col. 2-5).
In each case, the experimental data lie at least a factor of 20 below the calculated
values. This questions the use of a Rice-Herzfeld mechanism for the interpretation of
the experimental data.
APPENDIX
Thermal decomposition of the CH,ClCHCP radical.
The Competition between the photochlorination
CH,ClCH,CI
+ C1,
+
CH2ClCHCl,
+ HCl
and the chlorine-photosensitized dehydrochlorination
CH,CICH,CI
(a,)
t
CH,CHCI
+ HCI
(xvi)
of 1,2-C2H4C1, has been studied in the gas-phase between 433 and 510°K.The
kinetic apparatus has already been described [21]. Mixtures of purified tank chlorine
(Solvay) [22] and 99.9% pure 1,2-C,H4CI, (Sohay*) were irradiated with 436 nm
light in a cylindrical Pyrex reaction cell (diameter = 3.8 cm, length = 12 cm). The
incident light intensity, determined from the rate of photochlorination of C,HCI, [23]
was about 2.5x lo-'' einstein cm-' sec-'. The total pressure was measured by
means of a Pyrex bourdon gauge while the chlorine partial pressure was determined
by means of a logarithmic photometer [24]. Partial pressures ranged between 150 and
450 torr for l,2-C,H4Cl, and between 8 and 36 torr for C1,. The changes in total
pressure and in chlorine partial pressure lead to the rates of dehydrochlorination
( u d ) (see eq. (xvi)) and of photochlorination (ucl) (see eq. (xv)) respectively. The ratio
V C l / V d is proportional to the chlorine pressure and depends neither on the 1,2-C2H4C1,
pressure nor on the light intensity, i.e.
(xvii)
over the temperature range studied. Typical results obtained at 454.2"K are shown in
figure 1. Figure 2 shows an Arrhenius plot of the constant k obtained for each experiment: the straight line corresponds to
log,,(k, mole-' 1)
= (19790*580)/4.58
T-(5.58&0.27)
(xviii)
Note added in proof
While this paper was in the course of publication, a paper by Holbrook, Walker and
Watson ( J . Chem. SOC.(B), 577 (1971)) on the pyrolysis of 1,2-dichloroethane was
published which leads to similar conclusions.
(*) This unstabilized product was kindly supplied by the Solvay Research Center, Nederover-Heembeek, Brussels.
G. Huybrechts, J. Katihabwa, G. Martens, M. Nejszaten and J. Olbregts
I0
I
I
I
10
20
30
10
>=
G,
\
5
C
p(CI,), torr
Fig. 1 - Influence of the pressures (in torr) of C12(p(Clz)) and l,2-CzH4C1,(p(D)) and.of
the incident light intensity lo on the ratio of the chlorination and dehydrochlorination
at 454.2"K.
rates of 1,2-CZH4C1z(ucl/ud)
0 p ( D ) = 150, 0 p ( D ) = 450, lo = 2.5 x lo-'' einstein cm-2 sec-I.
einstein cm-z sec
0 q ( D ) = 150, I . = 3 x
Straight line calculated from equation (xviii).
and was obtained by least squares. The quantum yields are always higher than 2 500.
The results can be explained by the simultaneous chain propagating steps
CHZCICHCL. + C12
(3)
CH2CICHC12
+ C1
k,
(xix)
and
The radical CH2CICHCI* is formed by attack of a C1 atom on 1,2-C2H4CI2(see
reaction (2) of scheme (i)). Because of the long chains, the ratio of the rates of chlorination and dehydrochlorination is simply given by
v C J V ~= (k3/kJ
C12
(xxi)
Comparison of equations (xvii), (xviii) and (xx) leads to
log1,(k3/k4, mole-' 1) = (19790&580)/4.58T-(5.58f0.27)
(xxii)
Substituting the known (*) value of k,
log,,(k,,
(*)
mole-' 1 sec-') = -(920f50)/4.58 T+(8.75f0.20)
(xxiii)
This value has been obtained [25] from the photochlorination of CzH3CI in intermittent
light between 298 and 328°K and it is supposed that CI atoms add to the less chlorinated
C atom of C2H3CI as was observed in the case of C2HCI3 [19].
Discussion of the mechanism of free-radical pyrolysis of chloroethanes
71
Fig. 2 - Arrhenius plot for the rate constant k (in mole-' 1) of eq. (xvii). Straight line
calculated from equation (xviii).
one obtains
log,,(k4, sec-') =
- (20710f630)/4.58 T+(14.33f0.47)
(xxiv)
'
The value of the activation energy E4 = (20.7h0.6) kcal mole- is very close
to that (22f 3 kcal mole- ') calculated from the estimated dissociation energy
D(.CHClCH,-Cl) = (22.6f3) kcal mole- [9], assuming no activation energy for the
reverse of reaction (xx). The value of the preexponential factor log,,(A4, sec-')
=(14.33f0.47)isinlinewith thoseobservedforthelossofaC1atomfrom CHCl,CCl,*
(13.7) [7] and CCI,CCl,* (13.5) [8] radicals.
The authors thank Dr. G. R. De Mar6 for valuable discussions.
LABORATOIRE DE CHIMIE PHYSIQUE I
Facultk des Sciences
Universitk Libre de Bruxelles
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72
G. Huybrechts, J. Katihabwa, G. Martens, M. Nejszaten and J. Olbregts
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[3]
[4]
[5]
[6]