Photolysis of perfluoropropylene-methanol mixture in vacuum ultraviolet

Article abstract of DOI:10.1023/A:1020956826633

The products of reaction of C₃F₆ with CH ₃OH, initiated by vacuum ultraviolet, were determined. The kinetics scheme of the process was proposed. The parameters of the kinetic processes were calculated. The feasibility of p

Full text of DOI:10.1023/A:1020956826633

Russian Journal of Applied Chemistry, Vol. 75, No. 8, 2002, pp. 1275 1279. Translated from Zhurnal Prikladnoi Khimii, Vol. 75, No. 8, 2002,
pp. 1301 1305.
Original Russian Text Copyright
2002 by Fokanov, Pavlov, Semenov, Maksimov, Morozov.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Photolysis of Perfluoropropylene-Methanol Mixture
in Vacuum Ultraviolet1
V. P. Fokanov, A. B. Pavlov, V. N. Semenov, B. N. Maksimov, and A. V. Morozov
Prikladnaya Khimiya, Russian Research Center, St. Petersburg, Russia
Received January 17, 2002
Abstract The products of reaction of C3F6 with CH3OH, initiated by vacuum ultraviolet, were determined.
The kinetics scheme of the process was proposed. The parameters of the kinetic processes were calculated.
The feasibility of photochemical preparation of 2,2,3,4,4,4-hexafluoro-1-butanol was analyzed.
One of methods for introducing perfluoroalkyl
groups in molecules of organic compounds is addition
of these groups to unsaturated hydrocarbons in the
course of radical telomerization [1, 2]. Many fluori-
was 1 and 20 W, respectively. The photoreactor was
equipped with a vacuum system used to fill the re-
actor with the initial reagents and to sample the reac-
tion mixture. The photolysis products were analyzed
nated organic compounds exhibit absorption only in on an HP5972 chromato mass spectrometer with
the range of vacuum ultraviolet (VUV). A great num- an HP-PLOT capillary column coated with Al2O3.
ber of compounds with required properties can be pre- The mass spectra were assigned with the aid of
pared by photoreactions of fluorinated organic com-
pounds under exposure to VUV. In this work, we ex-
amined a new route to polyfluorinated 1-butanol: irra-
diation of a C3F6 CH3OH mixture with 185-nm light.
WILEY.138.L library.
To understand the kinetics of synthesis of poly-
fluorinated 1-butanol, we studied perfluoropropylene
photolysis. Initially we studied C3F6 photolysis at
a pressure of 4 kPa and exposure time of 15 min.
Under these conditions, C3F6 is quantitatively con-
verted into perfluorocyclobutane derivatives (up to
45%) and perfluoroparaffin isomers.
Perfluoroolefins, as well as lower alcohols, do not
absorb in the range above 200 nm. Photolysis of C2F4
in VUV was studied in [3, 4]. Continuous absorption
of C3F6 in VUV, determined in our works, suggests
radical photolysis of this compound. Reactions of fluo-
roolefins with alcohols, usually initiated by peroxides
or -radiation, yield telomers [5]. Photochemical initia-
tion, unlike these methods, allows control of the pro-
cess owing to selective activation of definite bonds.
The aim of this study was do develop a new photo-
chemical synthesis procedure. In particular, we studied
possible reaction pathways, determined the yield of
the reaction products and the irradiation efficiency, and
constructed a mathematical model of the photolysis.
Formation of perfluoromethylcyclobutane, 1,2-, and
1,3-bisperfluoromethylcyclobutane (in the ratio 1.6 :
1.4 : 1) can be described by the following scheme:
C3F6 + h
C3F6*(+M)
C3F6, 2:CF2
CF3CF: + (M) C2F4 ,
C2F4 + h 2:CF2 ,
C2F4*(+M)
C2F4* + (M)
2:C2F4 ,
C5F10 (perfluoromethylcyclobutane),
C2F4* + C3F6
C5F10
:CF2 + CF3CF:,
C2F4 ,
C3F6* + (M)
EXPERIMENTAL
C2F4 + C3F6*
C3F6 + C3F6*
Photochemical studies were performed in a cylin-
drical quartz reactor. A 70 W low-pressure mercury
lamp [6] 0.9 m long was fixed at the reactor axis.
The power of this lamp, measured at 185 and 254 nm,
,
C6F12 (perfluorodimethylcyclobutane
isomers),
1
Reported at the Third International Conference Chemistry,
where perfluoropropylene in the electronically excited
state is marked with an asterisk. This is probably the
Technology, and Application of Fluorine Compounds , St. Pe-
tersburg, June 6 9, 2001.
1070-4272/02/7508-1275 $27.00 2002 MAIK Nauka/Interperiodica

1276
FOKANOV et al.
Table 1. Chromatogram* of the products of C3F6 pho-
tolysis and calculated component concentrations c
Photolysis of C2F4 under the same conditions occurs
simslarly. The main product, perfluorocyclobutane, is
formed after irradiation of perfluoroethylene for 3 min
(the C2F4 conversion is 15%). The process can be
described by the above kinetic scheme, since, in both
cases, predissociation degradation yields difluorocar-
bene. However, C3F6 photolysis is a more complex
process and yields a series of paraffin isomers, from
perfluoropropane to perfluorononane. This process in-
volves an additional pathway initiated by the :CF2
biradical and atomic fluorine formed in the primary
photolysis step.
,
ret
S, %
Component
c, %
min
2.99 0.192 C2F6
3.71 0.751 C2F4
5.99 1.087 C3F8
15.39 5.312 i-C4F10
15.47 4.082 C4F10
1.28
0
0.65
8.0
17.45 12.537 C5F10 (methylcyclobutane)
18.55 0.015 CF3CF=CFCF3
19.49 4.521 i-C5F12
12.0
0
The weakest bonds of a molecule excited by a
VUV quantum are ruptured owing to energy redistri-
bution after molecular collisions. Several pathways of
C3F*6 dissociation are possible. The excess energy of
photodissociation products was estimated on the basis
of the energy balance and analysis of fluorocarbon
fragments of saturated polyfluorinated products of
C3F6 photolysis performed at a large excess of hy-
drocarbon. Based on the results of this estimation, we
suggest the following scheme of C3F*6 dissociation:
8.9
19.54 3.486 C5F12
19.66 10.734 C6F12 (1,2-dimethylcyclo-
butane)
19.97 7.774 C6F12 (1,3-dimethylcyclo-
butane)
18.2
22.11 17.820 C2F5CF2CF(CF3)CF3
22.19 5.945 C6F14
22.28 5.898 CF3CF(CF3)CF(CF3)CF3
30.1
(Sum of
C6F14
isomers)
C3F*6 + (M)
158 kJ mol , respectively,
CF=CFCF3 + F, Eexc 22 and
25.09 3.644 CF3CF(CF3)CF2CF(CF3)CF3
25.27 6.241 C2F5CF2CF2CF(CF3)CF3
25.39 0.154 C2F5CF2CF(CF3)C2F5
25.47 2.608 C7F16
9.2
(Sum of
C7F16
1
1
C3F*6 + (M)
CF=CF2 + CF3, Eexc 31 and 36 kJ mol ,
isomers)
respectively.
30.26 0.688 CF3CF(CF3)CF2CF(CF3)C2H5
30.57 2.189 C2F5CF2CF2CF2CF(CF3)CF3
30.93 2.337 C2F5CF2CF2CF(CF3)C2F5
31.41 1.169 C8F18
4.8
(Sum of
C8F18
The secondary processes can involve reactions with
hot radicals CF3 and F , with subsequent photode-
gradation of the fragments formed. The first photoly-
sis step can be described by a superposition of the
above reactions:
isomers)
36.57 0.764
i-C9F20
2.8
*
(
) Retention time and (S) peak area.
ret
C3F6 + h
C3F*6(+M)
CF3 + F + C2F2,
triplet state that is involved in deactivation, predisso-
ciation degradation into radicals, and reactions with
other molecules, ultimately resulting in cyclodimeri-
zation.
which agrees well with the kinetic scheme of forma-
tion of fluoroparaffin isomers presented in Table 1.
Subsequent transformations involve addition of
fluorine atom to double bonds to form RF radicals
( RF, C3F7, C2F5, and CF3) recombining to give a
series of paraffin isomers. The rate constants of radi-
cal recombination decrease appreciably in going from
linear to branched structures and with increasing num-
ber of carbon atoms [8]. Therefore, recombination of
branched radicals, C4F9 and higher, was disregarded
in the kinetic scheme. The C7 C9 isomers may arise
in the photolysis products in the step of formation
of methylcyclobutanes, when RF radicals recombine
with the :C6F12 biradical, which has no time to form
a four-membered ring.
The presence of trace amounts of perfluoro-2-bu-
tene ( 0.01%) and C2F4 (<1%) in the reaction mixture
agrees with the reaction scheme, since C2F4 is con-
verted incompletely and the perfluoro-2-butene for-
mation by recombination of methylcarbene radical is
considerably slower than the C2F4 formation. Birad-
ical formation was detected in C3F6 pyrolysis [7],
i.e., at significantly lower excitation energy of perfluo-
ropropylene. Although cyclodimerization proceeds
mainly by degradation of compounds containing a
double bond to form saturated compounds and is typ-
ical of C3F6 pyrolysis (T 200 C), it occurs in the
course of C3F6 photolysis too.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

PHOTOLYSIS OF PERFLUOROPROPYLENE-METHANOL MIXTURE IN VACUUM ULTRAVIOLET 1277
Photolysis of C3F6 to form detectable products is
initiated to the same extent by difluorocarbene and
atomic fluorine generated in primary photolysis steps.
Generally C3F6 is photolyzed under the conditions
when the rate of reactions of excited C3F*6 molecules
is comparable with that of predissociation degradation
followed by secondary and tertiary radical reactions,
which strongly complicates the photolysis mechanism.
To understand the photolysis mechanism, mathemat-
ical modeling can be used.
of C3F*6 with CH3OH is more intense than C3F6 di-
merization. Side recombination reactions yield up to
1.5% fluorinated ethers [(C3F6 + F) + CH3O
C3F7OCH3, (C3F6 + CF3) + CH3O
C4F9OCH3]
and up to 6.2% RFH ( RF = CF3, C4F9, C5F11,
C6F13). Compounds RFH are formed mainly by re-
combination with the CHF2 radical
CHF2 + (C3F6
+
F)
C4HF9,
CHF2 + (C3F6
CHF2 + (C3F6
+
CF3)
C2F5)
C5HF11,
C6HF13,
C6HF13.
A mathematical model of C3F6 photolysis was de-
veloped by comparing experimental and calculated
data on the composition of the reaction products. The
kinetics scheme includes 50 reactions whose constants
were taken from the literature or were estimated by
extrapolation of the known rate constants of homologs,
taking into account the steric factors. A system of
differential kinetic equations of these reactions was
constructed and integrated by the Gear procedure [9].
+
(C3F6
+ H) + (C3F6 + F)
This system of three-step consecutive reactions ac-
counts for the absence of C2HF5 and C3HF7 in the photo-
lysis products. The radical CHF2 is effectively formed
by the reactions :CF2 + H
CHF2 + CH2O. Formation of C2HF3 traces is due to
the reactions C2F2 + F C2F3, C2F3 + CH3OH
C2HF3 + CH3O , or CF3 + CH3O C2HF3 + H2O.
CH3CF3 is formed by the reactions CF3 + CH3
CH3CF3 or CF3 + CH3O CH3CF3 + O: ( CF3 is
CHF2, :CF2 + CH3O
The time dependeces of the concentration of the
photolysis products are adequately described by the
kinetic model (Table 1). The calculations show that
C3F6 is completely decomposed after 100s of irradia-
tion. The convergence of the calculated and exper-
imental concentrations can be improved by varying
the rate constants of the reactions and quantum yields
of the products and by recording more exactly the ab-
sorption spectra of the radical products. A model cal-
culation confirmed the occurrence of the main reac-
tions of C3F6 photodissociation.
hot radical). Methanol is involved in photolysis and
in reactions with fluorine radicals:
CH3OH + h
CH3OH + h
H + CH3O (79%),
CH2O H2 (20%),
Synthesis of polyfluorinated 1-butanol by contin-
uous irradiation of a C3F6 CH3OH mixture with VUV
for 15 min was studied. Synthesis was performed at
three ratios of the partial pressures of the components
C3F6 : CF3OH = 4 : 0.8, 4 : 4, and 0.8 : 4. Under these
conditions, the ratio of the absorption coefficients
of C3F6 and CH3OH at 185 nm are 75, 15, and 3, re-
spectively. This allows evaluation of the effect of the
CH3OH photolysis on the formation of the target
product.
CH3OH + h
CH3 + OH (1%) [10],
RF + CH3OH
RF + CH3OH
RFH + CH3O (85%),
RFH + CH2OH (15%) [11].
The main reactions competing with the formation
of the fluornated alcohol are cyclodimerization of C3F6
and predissociation decomposition of C3F6. Photolysis
of CH3OH has virtually no effect on the synthesis.
Chromatograms of the products formed in photolysis
of the C3F6 CH3OH system are presented in Table 2
in order of increasing retention time.
The fact that the chromatograms of the photolysis
products of C3F6 and the initial mixture C3F6
:
CH3OH = 4 : 0.8 are similar indicates that CH3OH
is not involved appreciably in the radical process and
is converted mainly by addition to the double bond
The yield of 2,2,3,4,4,4-hexafluoro-1-butanol de-
creases to 11.5% with increasing CH3OH content in
the initial mixture ( C3F6 : CH3OH = 4 : 4). The total
content of the products of chemical reactions of the
photolysis products increases to 60%, and the selec-
tivity of the alcohol formation decreases from 90 to
19%. Among the photolysis products, 2,2,3,4,4,4-
C3F*6 + CH3OH
CF3CHFCF2CH2OH.
The yield of 2,2,3,4,4,4-hexafluoro-1-butanol is
20%. This reaction is responsible for formation of
about 90% of the photolysis products. Telomerization
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

1278
FOKANOV et al.
Table 2. Chromatograms of identified products of photolysis of C3F6 CH3OH mixture at various partial pressures of
the components
Composition, %, at indicated C3F6 : CH3OH ratio, kPa : kPa
,
ret
min
4 : 0.8
4 : 4
0.8 : 4
5.67
14.21
14.74
16.52
17.13
19.35
19.38
19.63
20.02
20.24
20.38
21.60
21.62
22.22
22.30
22.35
23.77
23.84
24.40
25.75
CH2=CF2 (3.2%)
C2HF3 (3.9%)
CHF3 (2.3%)
C2HF3 (0.13%)
CHF3 (0.27%)
CF3CH3 (0.13%)
C3F7OCH3 (0.32%)
C4HF9 (1.65%)
CHF3 (6.9%)
C2H3F3 (12.1%)
C2H3F3 (3.1%)
C4HF9 (10%)
CF3CHFCF2C(O)H (30.7%)
C2F5CH3 (4.6%)
C4HF9 (+C3H2F6) (8.1%)
C3H3F5 (13%)
CF2=CFCHF2 (1.5%)
C4F9OCH3 (1.25%)
CH2=CFCF3 (2%)
C5HF11 (4.9%)
C4H4F6 (1.1%)
C3F7CH3 (3.4%)
C5HF11 (1.2%)
CF3CHFCF2CH3 (0.6%)
C5HF11 (5%)
C4H3F7 (6.7%)
CHF2CF2CH2F (5%)
CF3CHFCF2CH2OH (90.8%)
C4H4F6O (19%)
CHF2CF2CH3 (10.5%)
C6HF13 (11.2%)
C6HF13 (3.1%)
CHF2CF2CF(CF3)CHF2 (0.5%)
C6HF13 (6.5%)
C5H2F10 (3.5%)
CH3CF2CF2CH3 (10.7%)
hexafluorobutanal is characterized by the maximal
yield (30%). As the temperature increases, this com-
pound can be formed by the reaction
complicated, since CH3OH photolysis and some tem-
perature-dependent reactions should be taken into ac-
count.
No products of addition to the double bond of C3F6
were detected in the reaction mixture of photolysis at
C3F6 : CH3OH = 0.8 : 4. All the products of the reac-
tion of C3F6 with CH3OH are formed by radical reac-
tions initiated by the photolysis products of C3F6 and
CH3OH. The content of RFH and RF CH3 increases
to 31 and 32%, respectively. The other products are
polyfluorinated paraffins and olefins. Trace amounts
of CH2O, CO2, HF, H2O, CF3OCF3, etc. are also
formed. The chromatograms of the main reaction prod-
ucts are presented in Table 2.
C3F*6 + CH3OH
CF3CHFCF2C(O)H + H2
or the secondary reaction
C3F*6 + CH2O
CF3CHFCF2C(O)H.
The number of the products of radical reactions
increases by approximately a factor of 5. The total
content of RFH compounds increases to 24%. New
compounds RF CH3 with total content of up to
12% are formed instead of ethers by recombination
RF + CH3
the reaction H + CH3OH
pounds can also be formed by reactions RF
CH3O RF CH3 + O:. The contribution of these re-
RF CH3, where CH3 is generated by
CH3 + H2O. These com-
CONCLUSIONS
+
(1) Photolysis of perfluoropropylene by vacuum
ultraviolet was studied. The photolysis products were
determined. The kinetic scheme of the process was
proposed.
actions increases with temperature. The heating of
the reaction mixture is due to a stronger absorption
in the system owing to an increase in its optical den-
sity. The increase to approximately 10% in the content
of polyolefins (CF2=CFCHF2, CH2=CFCF3, C2H2F2,
C2HF3 formed in reactions like CF3 + CH3
C2H2F2 + HF) that were not subjected to photolysis
is due to intensification of the pyrolytic processes.
The kinetic model of the process becomes severely
(2) Irradiation of a C3F6 : CH3OH mixture (4 : 0.8)
with vacuum ultraviolet yields 2,2,3,4,4,4-hexafluoro-
1-butanol in 20% yield. The yield of this compound
decreases with increasing CH3OH content in the mix-
ture. The calculated composition of photolysis prod-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 75 No. 8 2002

PHOTOLYSIS OF PERFLUOROPROPYLENE-METHANOL MIXTURE IN VACUUM ULTRAVIOLET 1279
ucts agrees well with the composition of the products
formed in the photochemical reactor.
Illumination Engineering), Aizenberg, Yu.B., Ed.,
Moscow; Energoatomizdat, 1983.
7. Buravtsev, N.N. and Kolbanovskii, Yu.A., Materialy
3-i Mezhdunarodnoi konferentsii Khimiya, teknolo-
giya i primenenie ftorsoedinenii - CTAF-2001 (Proc.
Third Int. Conf. Chemistry, Technology, and Ap-
plication of Fluorinated Compounds - CTAF-2001 ),
St. Petersburg, June 6 9, 2001, p. 123.
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