Kinetics of Cyclodimerization of Hexafluoropropene and of Its

Cycloaddition to Chlorotrifluoroethene

BERNARD ATKINSON and CHRISTOS TSIAMIS Dcpartrnent ( i f Chemistry, Imperial College of Science and Technology, London S W7

2A Y , England

Abstract

The thermal cyclodimerization of hexafluoropropene and its cycloaddition to chlorotri- fluoroethene have been studied a t 570-700 K and 8-62 kPa. Both reactions give 1,2-substi- tuted hexafluorocyclobutanes. For the intercombination product 1-chloro-2-trifluoro- methyl-hexafluorocyclobutane infrared, nuclear magnetic resonance and mass spectra as well as vapor pressure are reported. The rates of reaction can be expressed by the following equations:

2CFz = CFCF:$ - C ~ F ~ C F R ) ~ , k = 8.2 X 103 exp(-143,700 J/mol RT)mR/mol-sec

CFz = CFCFR + CF2 = CFCl - C4F&ICF:j, k = 7.25 X 10:l exp(-118,400 J/mol RT)m3/mol-sec

The reactions probably proceed by a biradical mechanism.

Introduction

1,1,2-trifluoroethenes undergo thermal cyclization to stable cyclobutane derivatives [ 1,2] in which the dominant isomers correspond to head-to-head addition. The rate of reaction is markedly dependent on the nature of the fourth substituent [3-51. For hexafluoropropene the reactivity is known to be exceptionally low [6], but no quantitative measurements of rate constants have been reported. The aim of the present work was to deter- mine the activation energies and other kinetic parameters for cyclodi- merization of perfluoropropene and for its addition to chlorotrifluoroethene and so to provide a quantitative basis for discussing the reactivity of per- fluoropropene.

International Journal of Chemical Kinetics, Vol. XI, 1029-1043 (1979) c’ 1979 John Wiley bi Sons, Inc. 0538-8066/79/0011- 1029$01.00

1030 ATKINSON A N D TSIAMIS

Experimental

Materials

Chlorotrifluoroethene was obtained from the Matheson Company Inc., and hexafluoropropene was purchased from Peninsular Chemical Reagents. Both compounds were purified by gas-solid chromatography using Poropak R. The purity as determined by infrared spectroscopy and gas chroma- tography was better than 99.8%.

1,2-Dichlorohexafluorocyclobutane (approximately 50% cis) was pre- pared by pyrolyzing chlorotrifluoroethene a t 625 K and about 80 kPa for 1 hr. It was separated and purified by trap-to-trap distillation and gas chromatography.

An equimolecular mixture of cis-trans isomers of 1,2-di( trifluo- romethy1)-hexafluorocyclobutane was obtained by the pyrolysis of hexa- fluoropropene a t 650 K and pressure of about 100 kPa for about 8 hours. Purification was by fractional distillation and gas chromatography.

1-Chloro-2-trifluoromethyl-hexafluorocyclobutane (approximately 40% cis) was prepared by pyrolyzing mixtures consisting of 90% vlv hexafluo- ropropene and 10% v/v chlorotrifluoroethene at about 690 K and about 100 kPa for 3-4 h. Reactants and products were separated by trap-to-trap distillation using melting toluene (178.1 K) and liquid nitrogen. The 1- chloro-2-trifluoromethyl-hexafluorocyclobutane was purified to 99.5% by two passes through a 6-mm outer diameter 2-m-long Poropak R gas-chro- matography column.

Apparatus

Gases were handled in a conventional vacuum system. The furnace, reaction vessel, and sampling system have been described previously [7]. Two 500-cm3 calibrated flasks, linked to a pressure transducer (Bell and Howell, type 4-366-0002), were used for preparing chlorotrifluoroethene- hexafluoropropene mixtures. Each flask was equipped with a 7-cm-long condensing nipple and a glass-covered magnetic stirrer. Sovirel greaseless valves were used throughout the mixing section.

Procedure

Pyrolyses were performed by the static method. In the preparation of mixtures pure compounds a t known pressures in calibrated volumes were condensed into one flask, allowed to evaporate, and mixed by magnetic stirring. The procedure was checked by gas-chromatographic analysis. Reactions were followed by gas-chromatographic analysis of samples taken into a gas sample valve. To ensure that the sample analyzed came from

CYCLODIMERIZATION OF HEXAFLUOROPROPENE 1031

the middle of the reaction vessel, the first two samples taken at each sam- pling time were discarded. The pressure in the reaction vessel was recorded continuously.

Analyses

Analysis of reaction mixtures by gas-solid chromatography was per- formed with a Pye model 104 flame ionization chromatograph fitted with a 32-cm X 6-mm glass column packed with Poropak Q. For calibration of the chromatograms a series of mixtures of hexafluoropropene (standard, peak area A,) and the other compounds (peak area A;) was prepared and analyzed under conditions identical to those used for the mixtures after reaction. Calibration graphs were constructed by plotting ratios of peak areas AilAs against ratios of pressures PiIP,. The slope A;P,IA,P; gave the calibration factor f ; .

Spectra

Infrared Spectra. Infrared spectra were recorded on a Perkin-Elmer spectrophotometer model 457 using a 10-cm gas cell.

Nuclear Magnetic Resonance Spectra. 19F resonance spectra were measured at 94.1 MHz and a sample temperature of 308'K with a Varian Associates spectrometer model H.A. 100 using the continuous-wave tech- nique. All samples were diluted by Analar grade carbon tetrachloride. The concentration of solute ranged from 15 to 30% vlv. Spectra were in- ternally referenced to hexafluorobenzene and were calibrated by the usual side-band technique. Peak areas were measured using a planimeter.

Mass Spectra. The low-resolution mass spectra of the compounds were measured on an A.E.1 spectrometer model MS 9.

Properties and Identif ication o f t he Products

1,2-Dichloro-hexafluorocyclobutane [3] and 1,2-di(trifluoromethyl)- hexafluorocyclobutane [8] were characterized from their spectroscopic properties.

l-Chloro-2-trifluoromethyl-hexafluorocyclobutane. The relative mo- lecular mass of the compound estimated from vapor density measurements was 266 f 1 (calculated 266.4). The vapor pressure of the cis-trans iso- meric mixture measured over the temperature range of 242-313 K using an isoteniscope is given by log P(Pa) = (9.87 f 0.09) - (1570 f 18)lT. The infrared spectrum was determined a t 260 Pa.

Line positions and absorbancies are summarized in Table I. The bands 875 cm-l, 997 cm-', and 1285 cm-l were more intense in samples prepared at high temperatures, and the bands 795 cm-', 900 cm-', 1065 cm-', 1218

1032 ATKINSON AND TSIAMIS

TAHIX I. Infrared spectrum of l-chloro-2-trifluorornethyl-hexafluorocyclobutane, 260 Pa.

wavenurdber Absorbance % ,savenunher Absorbance %

(v /cm-l ) ( V

1382 45.3 1029 34.6

1345 1 1327

38.4

69.2

1323 70.4

1285 69.2

1247 100.0

1218

1190

1128

1083

55.3

39 .O

6.3

15.7

997 13.2

034 9.3

9 3 5 48.3

875 59 .7

ti33 29.6

795 7.1

737 30.2

733 36.7

729 27.2

1065 15.1

cm-l, and 1345 cm-l were weaker. These sets of bands are probably from the trans and cis isomers, respectively.

The mass spectrum is given in Table 11. The molecular ion is not ob- served a t an ionization potential of either 16 or 70 eV. All the fragments expected from splitting the cyclobutane ring a t opposite bonds are obtained, except C:3FG+. The loss of CF3 is clearly an important step in fragmenta- tion.

In the high-resolution nuclear magnetic resonance spectrum 22 peaks could be distinguished (see Table 111). The peaks were allocated in two groups of 11 by correlation of both peak areas in a sample and changes in the ratio of peak areas observed in samples synthesized a t differing tem- peratures. The results are summarized in Table 111, which also gives a preliminary allocation of the peaks. The allocation is based on a com- parison with the spectra of octafluorocyclobutane, chloroheptafluorocy- clobutane [9], dichlorohexafluorocyclobutane [ 3 ] , and di-(trifluoro- methyl)-hexafluorocyclobutane [8 ] . In general substitution of one F in a CF2 group by C1 moves an tu-CF2 group to a lower field and causes a sep- aration between the components of the CF2. There is a lesser effect on a P-CF? group. Substitution of one F in CF2 by CF:3 causes a smaller shift of ( Y and 0 CF2 to low field and does not significantly separate the compo-

CYCLODIMERIZATION OF HEXAFLUOROPROPENE 1033

TABLE 11. Mass spectrum of 1 -chloro-2-trifluoromethyl-hexafluorocyclobutane, ionization potential 70 eV."

Ion

C5F8C1

c5r8ci

C4F6C1

C4F6C1

CqF7

C3P5C1

C3F5C1

c,r5

c 3 p

c3:,c1

c4F5

c i ' 3 5

m / e

- 249

247

199

1 9 7

181

1 6 8

166

1 6 2

1 4 9

1 4 7

1 4 3

1 3 1

relative

intensity %

1

3

7 . 5

22

7 . 5

3 .5

1 0 . 5

3

5

1 4 . 5

5

5 1

Ion

C2F3C1

C2F3C1

3F4

C F C 1 3 2

C3F2C1

'ZF4

c3F3

CF2C1

CF2C1

c3F2

cF3

118

116

1 1 2

111

109

100

9 3

87

85

74

69

relative

intensity 2

24

7 1

3

1

3 . 5

100

18

5

1 4 . 5

3 .5

3 1

a Single peaks below 3% and rnle < 45 excluded.

nents of the CF:! group. The position and intensities of the peaks confirm that the compounds are 1-chloro-2-trifluoromethyl-hexafluorocyclobutane. The compound with the lower field CF(C1) resonance is taken to be the trans isomer.

The composition of a mixture prepared a t 675 K was 41% cis and one prepared a t 648 K was 47% cis.

Cyclic Dimerization of Hexafluoropropene

In the range of 575-700 K pyrolysis of hexafluoropropene yields 1,2- di(trifluoromethy1)-hexafluorocyclobutane (I) and a very small proportion of 1,3-isomers [6]. The cis and trans isomers of (I) are initially formed a t similar rates, but subsequent isomerization increases the proportions of cis. A t higher temperatures perfluoroiso butene is formed [6,10,11]. A t 750 K ( I ) dissociates quite rapidly by symmetrical ring cleavage to hexa- fluoropropene [4]. In the present work thetemperatures used were suffi- ciently low to make the dissociation of (I) very slow, but not negligible. The low temperatures also made isobutene formation unimportant. A t the

1034 ATKINSON A N D TSIAMIS

TABLE 111. Analysis of nuclear magnetic resonance spectrum.

I (1)

trans F ( 6 )

d A

A t o m Frequencqa/Hz J/Hz 6/ppm

2950

3165 215 3 2 . 6 4

'6

F6

F 3856

F4 11067 2 1 1 4 1 . 9 7

4

3117

3 3 5 3 236 34 .90

3450

3684 234 37.39

2279 2 4 . 2 2

-1512 - 1 6 . 0 7

8 3 8 3 89 .09

F5

F5

F 3

F3

Fl

F2

cF3

B

A t o m Frequency /Hz J /Hz 6/pprn

'6

' 6

FI(

F4

F5

F5

r- F ' 3

F1

r2

w3

2999

3214 2 1 5

3787

4002 215

3055

3289 2 34

3502

3736 236

21'+1

-2337

8 3 6 0

3 3 . 1 7

4 1 . 2 2

3 4 . 0 6

38 .08

22 .75

- 2 5 . 9 0

8 8 . 8 4

a Frequency is given as - (Hi - Href).

subatmospheric pressures used dimerization of perfluoropropene was so slow that measurements had largely to be based conversions below 1%. The agreement between rate constants calculated for initial pressures ranging from 7.8 to 45 Pa at 681.5 K (see Table IV) confirmed that the dimerization is second order.

The method of calculation was based on the following assumptions. The reactions are

(1)

with rate constants defined bv

k z

k - z 2C#6 * C6F12 (2a * b )

- k-2Cb 1 d C , deb - k2Ca2; 2 d t dt

CYCLODIMERIZATION OF HEXAFLUOKOPROI'ENE 1035

TAHLE IV. Analyses and rate constants for C3Fc cyclodimerization at 681.4 K.

Initial Cxper imen t Period Pressure Time of Press ratio k-

1 1 16 288 4.10 3.09 7.82~10-~

1 2 16 036 7.00 5.14 7.97x10-'

1 3 15 829 9.85 6.99 7.78x10-'

4 1 7 835 7.15 7.92x10-'

4 2 7 745 22.70 4.80 8.37~10-~

6 1 45 149 2.00 4.25 7.97~10-~

6 2 44 416 4.30 8.64 8.03~10-~

6 3 43 632 6 .OO 12.83 8.33xlO-'

k-2 was taken as the weighted average of the values for cis and trans iso- mers [4]

(3) k-2 = 2.54 X 1015 exp(-268,620 J/mol RT) sec [4 ]

The rate equation is

(4)

For changes in Ca of only about 1% C, can be taken as constant. Integration then gives

2 = k-2(Cb exp(k-2t) - CbO) exp(k-2t) - 1 (5) k2Ca

for C b = CbO a t t = 0. Cb was calculated from areas of chromatographic peaks using

C b = AbCa/A$i

In applying eq. (5) to samples after the first, a correction was applied for changes in concentrations caused by sampling. Table IV shows the re- producibility obtained in some experiments a t 681.4 K.

About three samples were taken for analysis during each experiment. The average rate constants for each experiment are given in Table V. In- dividual values of k2 have an uncertainty up to about f 5%, depending on the reaction conditions.

1036

2 K

ATKINSON AND TSIAMIS

TABLE V. Rate constants for the cyclodimerization of hexafluoropropene.

5 7 4 . 2

5.71.. 2

574. 2

5 8 6 . 3

5 8 6 . 3

598.6

5 7 6 . 6

6 1 2 . 8

612.8

6 2 1 . 8

6 2 2 . 4

6 2 2 . q

622.4

6 4 5 . 2

645.2

6 4 5 . 2

t 6 2 . 7

6 6 2 . 7

662.7

6 7 3 . 7

6 7 3 . 7

(;Gl.,,

t 8 l . 1;

581.5

581.5

681.5

6 8 1 . 5

6 8 1 . 5

6 9 8 . 1

698.:

6 5 8 . 5

7 0 3 . 'I

40 0 7 8

46 525

52 8 1 5

2 6 270

45 9 4 3

46 660

36 4 7 8

31 4 4 1

2 1 7 9 0

18 186

41 G28

2C 289

25 834

25 562

1 3 3 1 1

4 Y 8 3 8

25 764

2 3 910

20 1 2 9

24 087

18 133

1 6 288

i: 4 4 1

2E 2 3 3

7 835

6 1 165

4 5 149

UG 6 3 5

1 8 6 4 1

24 2 7 3

25 545

51 453

2 1 . 3 8

qb .33

2b .95

2 3 . 5 5

7 % . 0 8

46 .14

2 3 . 2 6

1 2 . 5 3

2 2 . 7 0

22.77

23.18

22.08

2 1 . 3 2

22 .85

21.48

13.C3

8.1C

2 . 5 2

13.50

2 3 . 6 0

2 4 . 6 8

9 . 8 5

1 3 . 5 3

12.50

2 2 . 7 0

4.oc 6 .oo 6.30

8.00

11.2c

6 . 5 0

5 . 4 5

2

4

?

3

>

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

L

2

3

3

2

-I

3

2

9.153

' 2 . l U 4

3.1u5

8 . 8 6 5

d.812

8 . 6 2 6

a . 6 4 3

0 . 3 0 7

a .3 ,7

8.151

7 . 7 3 0

7 . 1 4 2

7 . 7 2 8

7.413

1 . 1 2 2

7 . 4 3 c

7 .23;

7.230

7 . i b 5

7 . 1 3 3

7.vy1

7.5'35

7 . 3 9 1

7 . 0 8 9

7.C83

6 . 8 3 7

t . 828

6 . 8 2 1

6 .835

CYCLODIMERIZA'rION OF HEXAFLUOROPROPENE 1037

Fitting by the method of least squares gave the following Arrhenius equation (standard errors are given):

logk2(m:'/mol-sec) = 3.91 f 0.02 - (7505 f 14) K/T

[with A = (8.2 f 0.4) X lo3 m:3/mol-sec and E = 143.7 f 0.3 kJ/mol.]

C'ycloaddition of Hexafluoropropene to Chlorotrifluoroethene

The rate constant for cyclodimerization of chlorotrifluoroethene at 600 K is higher [3,7] than that for dimerization of hexafluoropropene. By using a low proportion of chlorotrifluoroethene mixed with perfluoropropene the lormation of 1-chloro-2-trifluoromethyl-hexafluorocyclobutane can be made to predominate over both of the cyclodimerizations. Percentages of chlorotrifluoroethene from 2 to 16% were used. The composition of the mixture after reaction was determined by chromatographic analysis with the Poropak Q column a t 358 K. The reactions and rate constants in the system are as follows:

k 1 and k-1 are defined by

When the product yields are low and the temperature is below 670 K, the back reactions can be neglected. The differential equations for the system then become

(11)

1038 ATKINSON AND TSIAMIS

To a first approximation C, is constant because the reaction was rarely taken beyond consumption of 5% of the hexafluoropropene. As a closer approximation the loss of hexafluoropropene was taken to be proportional to the loss of chlorotrifluoroethene,

(12) Cao - Ca = g(Cf0 - Cf) where the proportionality factor g is a function of initial temperature and initial concentrations.

Substitution of eq. (12) into eq. (9) and integration gives

where

(14) CY = 2 k 1 + gk3, P = k3Ca0 - kgCfO

Combination of eqs. ( l l ) , (12), and (13) gives dC,/dt as a function of t , Ca0, and Cfo. Integration then gives

Ce 1 CfO 7

(15) - = -In [l + y - y exp(-Pt)]

where

(16) Y = Cfo(2kl + gk3)/k3(Ca0 - gcfo)

Equation (15) was fitted to the experimental measurements by iteration, using a computer. To obtain an initial value of g,Cf was first calculated from an approximate form of eq. (13). A trial value of k3 was obtained from CeICaCft. The values of C,, Cf, and g were recalculated a t each step. It- eration continued until the differences for (a) the two sides of eq. (15) and (b) successive values of g were not more than 1%. The results were accepted only if the calculated values of the yields C b and C d were in concordance with the experimentally measured values. Deviations only occurred after long reaction times. Details of some typical experiments are given in Table VI. The rate constants obtained are given in Table VII.

A t temperatures over 670 K the dissociation of l-chloro-2-trifluoro- methyl-hexafluorocyclobutane (11) proceeds at a significant rate, and the method of calculation given above gives only an “apparent” rate constant k3a. The true rate constant was obtained by plotting k3a against t using the equation k3a = k 3 - 0.5k-3k3t. Approximate values of k-3 obtained in this way were in agreement with those calculated from measurements on the pyrolysis of (11). They were not true values of k-3 because the py- rolysis of (11) yielded unidentified major products in addition to hexaflu- oropropene and chlorotrifluoroethene. Fitting of the rate constants to the

CYCLODIMERIZATION OF HEXAFLUOROPROPENE 1039

I u ,

0 0 3 4 x x u , f

1

m m . . 3 3

0 I

0 3 X 0

N -!

o m o m -i N

N r - s m o w 4

0 m P

" 0 I I

2 2 x x s m o m . . N r l

> u ,

0 0 3 3 x x

I

o m ? ? N N

> u ,

0 0 3 3 x x J l u ,

I

o m . . - 3

O N N r n 3 N

>

0 -l

X m m 4

z 2 3 ji O N N o m m

N S -l

i 0 4 o m N r n

2 3

w m 4 s r n m

o m r l N 3 N

o r l m w

N 3 m m

o r - w u , r - m 4 3

I

m f N o

W t a r - rl

m 4

m N N O 0 N P r -

r l o m

m m m r - n o N 3

0 I-

0

3

m u , P P

- 3

m m

4 4 O N

3 d m s

m cr m r - 3 m

d u , 3 m N f

m f -4

m o s m S N

m 3 m - l f P

N N m w

m m m m m m m r - f f

3 r- m

f 3

o o 0 2 2 / I

3

o m O f N 3 d r n

r i d m m m m

m m . . "

3

I, II !I I,

am a" I am a- ll II

am a- l l I1 m u - a a

1040

c.::.;' , ~. r.13.i

t 7 3 . 5

L.!.j..,

547.i

i . , , 7 . : .

t-7."

547.6

GL7.6

fL'7.C

Cb7.G

ATKINSON A N D TSIAMIS

i? 102

2c 2c3

i 5 -38

11 2 3 7

31. :9'

2 3 925

17 26'4

2? 697

21 226

15 163 U9 581

TABLE VII. Rate constants for the cycloaddition of hexafluoropropene to chlorotrifluoroethene.

~ ,. .,>6.L

5 i d . l

j9S.1

538.1

573.7

57?.7

573.:.

573.3

573.L

21 2 1 5

29 356

21 012

1 4 9i.i

47 4 5 3

33 8 ' . i

23 162

215 L i 5

32 3G4

___ 'm P a - -.

601

4 GO

1296

971

732

749

5GO

380

ilO4

827

614

2 215

1 339

967

2 970

2 120

1 516 3 651

i G49

1 999

2 157

1 736

2 749

1 927

1 365 2 292

3 157

2 259

1 609 ,

7 135

5 094 j 3 4iiG 1 91t I

4 187 i

Tota l time t / r n i n

373

522

536

4 8 5

503

521

403

1424

532

535

&2',

303

47G

300

321

3'3 3

361

314

579

u 6 :

5:a

680

307

330

1080

3 5 5

78C

37c

;2co

1294

1945

1845

1833

1817

2

3

3

3

3

3

2

2

3

3

2

2

2

1

2

2

2

J

2

3

3

1

1

1

1

2

2 ?

3

3

3

4.9 85*

4.971;:

q.980::

5.001"

4.976"2

5 .346:':

5.364:;

5.318"'

5.322"

5.332"

5.355"

5.580

5.71c

5.677

5.695

5.729

5.711

5.680

E . C,6 7 t .Z62

6.070

6.063

6.477

6.495

6.504

6.491

6.5C8

6.498

6 ,u96

6 . 8'J?

6.886

6.91u

6.935

6.909

* Corrected for back-reaction.

CYCLODIMERIZATION OF HEXAF1,UOROPROPENE 1041

Arrhenius equation by the method of least squares gave the following constants and standard errors:

logh3(m3/mol.sec) = (3.86 f 0.03) - (6184 f 15) K/T

with A = (7.25 f 1 ) X 103 m3/mol.sec and E = 118.4 f 0.3 kJ/mol.

on the reaction conditions. Individual values of k 3 have an uncertainty up to about f8%, depending

Discussion

Enthalpies and entropies of activation for the reactions studied and for two similar reactions are given in Table VIII, and where possible enthalpies and entropies of reactions are given. When the reactions yield a mixture of cis and trans isomers, these are formed in approximately equal amounts, not in the proportions corresponding to isomeric equilibrium [3,8]. Values of AH" and ASo for reactions (1) and (6) refer to formation of equilibrium isomeric mixtures of 1,2-isomers.

In considering the entropies of activation, influences that should be taken into account include (a) variations in entropy of reaction, (b) variations in relative position of the transition state along the reaction path, as indicated by variations in AH*, (c) steric hindrance, and (d) statistical factors. There is a need for caution arising from the sensitivity of the figures to the method of treatment of experimental results.

For the formation of an open transition state with terminal difluoro- methene groups the reaction of tetrafluorethene has a symmetry-factor ratio four times higher than that for the other dimerizations giving a AS* higher by 11.5 J/K-mol. The low AS* for chlorotrifluoroethene is ac- counted for by this, but AS* for perfluoropropene is even lower. On sta- tistical grounds the intercombination reaction (7) is expected to have an additional AS* contribution of 5.8 J/K-mol in comparison with the di- merizations. A simple prediction of AS* for reaction (7) from AS* for reactions (1) and (6) gives AS* (7) = 159.4 J/K.mol, well above the observed value.

There are no significant variations in the entropies of reaction. As the dimerization of perfluoropropene has a notably high AH*, the transition state for this reaction can be expected to be further along the reaction path, stiffer than that for-the other two dimerizations listed in Table VIII, with the observed especially low AS*.

Although the three dimerizations have widely differing enthalpies of reaction, this is not reflected proportionately in the enthalpy of activation. The change from tetrafluoroethene to perfluoropropene gives an increase of 78.8 kJ/mol in AH", matched by an increase of 37.4 kJ/mol in AH*. On the other hand the increase of 40.7 kJ/mol in AH" in going from tetrafluo- roethene to chlorotrifluoroethene gives no more than an increase of 4 kJ/mol

TA

BL

E VI

II.

Ent

halp

ies

and

entr

opie

s of a

ctiv

atio

n, c

alcu

late

d at

700

K.a

2 C2

F4 +

C4

F8

94

.7

-154.7

-209.5

-199

.7

12

98.4

-153.0

5 4

3.7

> 2 C3

F6 +

1,

2-C4

F6(C

F3)2

(6)

132.1

-170

-1

30.7

-2

02

This

pap

er

tr

2 C2

F3C1

+ 1,

2-C4

F6C1

2 (1

) 98.6

-163.5

-168.8

-197.9

z 8

C3F6

+ C

23

F

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CYCLODIMERIZATION OF HEXAFLUOHOPROPENE 1043

in AH*. Replacement of F by CFS strengthens the double bond in the al- kene (see [13]), but has only a small effect on the single bonds in the cy- clobutane. Hence the equilibrium in the dimerization is moved toward the alkene when F in tetrafluoroethene is substituted by CF3. Replacement of fluorine by chlorine reduces the stability of the cyclobutane ring, but has little effect on the strength of the double bond in the alkene. The transition state is apparently loose in structure with AH* more related to the ease of breaking the double bond than to any influences that substituents exert on ring stability. AH* for the combination of CSFG and CzFBCl is about 9 kJ/mol lower than the average of the AH* values for the two dimeriza- tions. This suggests that particular structural difficulties or unfavorable charge distributions arise when there are two CF3 groups to accommodate in the transition state.

The reactions described here yield 1,2-isomers. Calculations by Epiotis [14] indicate that 1,3-isomers are favored in concentrated (2, + 2,) addition. With the approximations involved in these calculations it would be unwise to conclude without reservation that 1,2-isomer formation must be via a biradical mechanism. The lack of stereo selectivity as between cis and trans isomers does fit with a biradical mechanism. Other evidence favoring this conclusion has been given by Bartlett and Wheland [15].

We have used the MIND0/2 program [16] together with a program for modifying geometry to explore the stability of the di-(trifluoromethyl) cyclobutanes. The 1,2-isomers were found to be of higher stability than the IJ-isorners, but the prediction regarding the relative stability of the cis and trans 1,2-isomers conflicted with the experimental evidence [4].

Bibliography

[ 1 I J . D. Roberts and C. M. Sharts, Org. Reactions, 12, 1 (1962). 121 1’. D. Bartlett, Science, 159,833 (1968). [ 3 ] B. Atkinson and M. Stedman, J . Chem. Soc., 512 (1962). [4] B. Atkinson and B. Stockwell, J . Chem. SOC. R, 984 (1966). [5] J. R. Lacher, G . W. Tompkin, and J. D. Park,J . Am. Chem. Soc., 74,1693 (1952). IS] M. Hauptshein, A. H. Fainberg, and M. Baird, J . Am. Chem. Soc., 80.842 (1958). [7] B. Atkinson and C. Tsiamis, Int. J . Chem. Kinet., 11,585 (1979). [8] B. Atkinson and B. Stockwell, J . Chem. Soc R, 740 (1966). 191 M. Stedman, Ph.D. thesis, University of London, 1960.

[ 101 B. Atkinson and V. A. Atkinson, J . Chem. Soc.. 2086 (1957). 11 11 R. A. Matula, J . Phys. Chem., 72,3054 (1968). [ 121 B. Atkinson and A. B. Trenwith, J . Chem. Soc., 2082 (1953). 11.11 H. E. O’Neal and S. W. Benson, J . Phys. Chcm.. 72,1866 (1968). [ 141 N. Epiotis, J . Am. Chem. Soc., 95,5624 (1973). 1151 P. D. Bartlett and R. C. Wheland, J . Am. Chem. Soc., 94,2145 (1972). 1161 M. d. S. Dewar and D. H. Lo, J . Am. Chem. Soc., 94,5296 (1972).

Received October 17,1978 Revised December 28,1978 Accepted April 9,1979