RADIATIONS CHEMISTRY OF SOLUTIONS OF P4
1393
The Radiation Chemistry of Solutions of P4 in C Cl3Br:
the Form ation of Diphosphines
P. L. A ir e y *
Hahn-Meitner-Institut für Kernforschung Berlin, Sektor Strahlenchemie, Berlin-Wannsee
(Z. Naturforschg. 24
b, 1393— 1397
[1969] ; eingegangen am 30. J u li 1968)
The diphosphines CC13(Br) P2(Br) CC13 and CC13(Br) P2Br2 are formed when solutions of P4 in
CCl3Br are exposed to ionizing radiation or maintained at 100 °C for an hour. The radiation chemi­
cal yields (G values) are respectively 230 and 170 at RT. No “red” phosphorus is formed. The di­
phosphines react with Br2 to form CCl3Br; with Cl2 to form CCl3Br, Br2 and PC13; and with EtOH
to form CCl3P(O E t)2 and CC13P(H) (O) (OEt).
On absorption of U.V. light or ionizing radiation,
P 4 in many organic solvents is polymerized to a red
phosphorus which contains a large number of groups
from the solvent chemically bound to the phosphorus
skeleton (structure l ) 1-4.
R
R'
P A\ P
P XP '
/
I
R
In the radiation chemical process the initial step
involves the attack of a radicale from the solvent on
Sr p 4
R -+ R '‘
(1)
PP4-
(2 )
The solvents CC14 2, CHBr3 3 and c h 3s s c h 3 4
can, under the influence of ionizing radiation, react
further with the corresponding “red” phosphorus to
form the low molecular weight products CC13PC12,
CHBr 2PBr2 (and (CHBr2) 2PBr) and (CH 3S ) 3P,
respectively.
However, when CCl3Br is used as a solvent, no
“red” P is formed because the CCl3-Br bond energy
(49 kcal/Mole) 5 is so low, that the condensation
reactions (5) are dominated by competing degra­
dation reactions ( 6 ) and (7) which are steps in a
chain process propagated by the CCL radical. (R =
CC13 and R' = Br.)
P
/| XP - B r .
CCl3P4Br + •CC13 —> CCI3 —P
XL
C C I3
The radical RP 4 can either dimerize or propagate
the chain by reacting with another solvent molecule.
2 R P 4 - > (R P 4) 2
RP 4+ S-^RP4R + R''
n RP 4R'
n( RP4)
condensation
“red” P
(structure 1 )
2 + CCLBr -> CCloP
(3)
(7)
Br
CCL
The reaction products include C2C16 and the two
novel diphosphines
CC13X
/ C C l,
P-P
B r^
(5)
* Present address: Australian Atomic Energy Commission,
Sutherland, N.S.W. 2232, Australia.
1 a) R. S c h e n k , Ber. dtsch. diem. Ges. 36, 979 [1903]. b) A.
M ic h a e l i s a n d K. V. A r e n d , Liebigs A n n . Chem. 314, 259
[1901].
2 D. P e r n e r and A. H e n g l e i n , Z. N atu rfo rs ch g . 17 b, 703
[1962].
(2 )
P —Br + -CCL
P
(4)
Subsequent condensation of intermediates of the
type (RP 4) 2 and RP 4R' lead to the formation of a
highly substituted “red” phosphorus, part of which
is precipitated, and part of which remains in col­
loidal suspension.
(6)
and
N ßr
B r^
4
3
3 P.
A ir e y , H . D r a w e ,
CCl3\
/ ■ Br
P-P
and
A . H e n g l e in ,
^B r
Z. Naturforschg.
23 b, 916 [1968].
and M. S c h e f f l e r , Z. Natur­
forschg. 23 b, 911 [1968].
W . F r o s t , Can. J. Chem. 36, 1308 [1958].
4 H . D r a w e , A . H e n g l e in ,
5
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P. L. AIREY
1394
Experimental
CCl3Br was purified by washing successively with
concentrated H2S04 , water, NNaOH and water; dry­
ing with Na2S04 and distilling under reduced pressure.
Gaseous Cl2 was purified as described by P e r r i n et
a l.6. EtOH (spectroscopic grade), Br2 (analytical
grade) and white phosphorus were used without further
treatment.
Saturated solutions of P4 in CCl3Br (0.79 m ) were
prepared in amber flasks under an Ar atmosphere. In
the kinetic investigations 2 ml samples were degassed,
sealed in glass ampoules and irradiated with a 1500
curie 60Co y source. The C2C16 yield was estimated gaschromatographically. That of the unreacted P4 was
determined from the weight of precipitate formed on
adding a 5 fold excess of MeOH in which the diphosphine products 3 and 4 are soluble. The phosphorus
content of the polymerized material was shown to be
independent of dose (P = 29.5%, dose = 2.9 x 105
rad; P = 29.1%, dose = 1.6 x l0 6rad).
In the preparative work, degassed 50 ml samples
were irradiated and the products separated by vacuum
distillation. Because of their instability, the products
were handled in a glove box under a dry N2 atmo­
sphere.
The “red” P, formed by irradiation of P4 in PBr3 ,
CHBr3 and CC14 respectively was filtered off under dry
N2 , ground, and suspended in CCl3Br. The suspensions
were degassed and sealed in ampoules which were con­
tinuously rotated during the subsequent irradiation in
order to effect good mixing.
Reaction products were identified either by compari­
son with reference compounds in the gas-chromatograph
or from the enlementary analysis and infra-red spectra.
Radiation Chemical Investigations
A. N a t u r e
of the P r o d u c t s
When solutions of P4 in CCl3Br are irradiated, or
maintained for an hour at 100 °C, two phosphorus
compounds 3 and 4 and a small quantity of C2Cle
are formed.
pound is easily oxidized and ignites spontaneously
when brought in contact with tissue paper. Because
of this instability, the observed molecular weight
(505) had an uncertainty approaching 10 per cent.
The elementary analysis was C = 5.2% (theoretical
5.2%), P = 12.7% (13.4%), Br = 34.0% (35.0%),
Cl = 45.9% (46.4%). The theoretical values are
calculated from the proposed structure (3).
CC13X
/CC1,
P-P
B r/
3
^B r
Chemical evidence for this structure is presented
below.
b) Compound 4
Compound 4 is a solid, density 2.26 (23 °C),
which melts at 45 °C and boils at 46 °C (0.4torr).
It ignites when brought in contact with paper tissue
and shows the same absorption maximum in the
U.V. (290 m/0 as compound 3. From the observed
molecular weight (460) and elementary analysis
[C = 2.9% (theoretical 2.7%), P = 13.0% (14.7%),
Br = 56.0% (57.1%), Cl = 24.8% (25.6%)] the
structure 4 is proposed.
CC13\^
/B r
P-P
B r/
4
^B r
B. P r o d u c t Y i e l d s
From the dependence on dose of the concentra­
tions of unreacted P4 , and of product C2C16 , which
is illustrated in Fig. 1, it is estimated that G ( — P4)
= 400 ( ±10) and G (C 2C16) = 4 . The initial yield
of the diphosphines could not be determined since
a) Compound 3
Compound 3 is a colourless liquid which boils at
77 C (0.4torr), has a refractive index n,\ of 1.628 r^T
(25.0 °C) and a density of 2.00 (23 C). It ab- “
sorbs strongly in the infra-red between 740 and
850 cm-1. (CC13-P group) and at 1290 cm-1 (C-P
rocking frequency). The substance has an absorp­
tion maximum in the near U.V. at 290 m u (extinc­
tion coefficient = 113 ± 3 M _1 cm-1) . The comDOSE x 10‘ 5 ( r a d )
6
D. D. P e r r i n . W . A m a r e g o , and D. R. P e r r i n , Purification
of Laboratory Chemicals, Pergamon Press, London 1966.
ig. 1. The variation of [P4] and [C2C16] with dose (dose rate
= 1.8 X104 rad/min. Tep = 25 °C, [P4] initial = 0.8 m ) .
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RADIATIONS CHEMISTRY OF SOLUTIONS OF P4
the quantitative separation of base compounds was
not possible from solutions that still contained P 4 .
In order to calculate the yields of the diphosphines
3 and 4 in the presence of P 4 , it was necessary to
invoke the material balance equation ( 8 ), which is
justified because no other reaction products could
be detected at low dose.
[P4]total (0.8 M )
=
[P4] unreacted + 2 [3] + 2 [4] . (8)
If it is assumed that the ratio [3]/[4] ( = 1.3, dose
= 6.8 X 106 rad), which is found to be independent
of dose at high dose, remains unaltered at low
doses, it can be calculated that £ (3 ) = 2 3 0 and
G ( 4) =170. Experimentally it is found that the di­
phosphines 3 and 4 can be recovered in 41% and
43% respectively of the theoretical yield.
c) Irradiation of “ Red” P-CCl3 Suspensions
Samples of “red” P were prepared by irradiating
solutions of P 4 in (a) PBr3, (b) CC14 and (c)
CHBr3 . These samples of “red” P were suspended
in CCl3Br, evacuated, and irradiated at room tem­
perature.
(a) The “red” P prepared from solutions of P 4 in
PBr3 has the empirical formula (P 4B r)n . When sus­
pended in CCl3Br and irradiated with a dose of
1.1 X 107 rad, both diphosphines 3 and 4 were
formed, but the yield of 4 was in large excess
(G ( 4) = 9 0 ± 20, G(3) = 1 0 ).
(b) When CC14 was used as a solvent, the re­
sulting “red” P had the empirical formula
(CCl3P 60 ) n [c. f. ref. ( 2 )]. Following irradiation
of suspensions of this substance in CCl3Br, both
compounds 3 and 4 were formed, but in contrast to
above, in approximately equal amounts (G(3) =
30 ± 10, G (4) = 35 ± 10, Dose = 107 rad ).
(c) The “red” phosphorus from the P 4/CHBr 3
system has the empirical formula (CHBr 2P 3Br) n .
Following irradiation at room temperature (4-107
rad) CHBr3 and CHBr 2PBr 2 were identified by com­
parison with the standard compounds in the gaschromatograph. The diphosphine 4 was isolated by
vacuum distillation, as was the small yield of a
colourless liquid, CC13 —P —CHBr 2 , which boils at
Br
103 °C (0.3torr). Its structure was confirmed by
the I.R. spectrum, which shows strong absorptions
at 2910 cm - 1 (C —H), 1280 cm - 1 (P — C rocking),
160 cm - 1 (broad, CC13 —P) and at 650 cm - 1
(broad, C - B r ) .
1395
Discussion
I. The formation of the diphosphines
When solutions of P 4 in CCl3Br are exposed to
ionizing radiations, the 'CC13 and -Br radicals which
are formed in the primary processes, react rapidly
with P 4 to form CC13P4- and BrP4* respectively.
als
CCl3B r^ ^ - • CCI3 , Br,
(9)
• CCI3 + P4 —> CCI3P4•
•Br + P4 -^BrP4-
(10)
Since the CC13 —Br bond is very weak, chain pro­
pagating reactions of the type
CC13P4-+CCl3Br-> CCl3P 4Br+ •CCI3
(11)
occur very efficiently. As indicated in the introduc­
tion, the absence of “red” P as a reaction product
implies that reactions of the type (6 ) and (7) which
lead to the stepwise degradation of intermediates
such as CCl3P 4Br, and the formation of low mole­
cular weight products, completely dominate com­
peting condensation reactions (eqn. (5)).
Repetition of the more efficient processes of type
( 6 ) and (7) should finally lead to CCl3PBr 2 and
(CCl3) 2PBr. However, in fact the diphosphines 3
and 4 are formed, which indicates they are either
stable to CC13 radicals or are reformed after radical
attack according to the following reaction scheme.
/C C I3
CCI3(Br) P - P (Br) CC13+ •CC13
CCl3PBr + P
3
5
( 12 )
I
^B r
CCI3 5
+ CCl3Br —►CCl3PBr2+ *CC13
(CCI3) 2PBr + CCl3PBr2
3 + CCl3Br
(13)
(14)
11. Irradiation of suspensions of “ red” P in CCl3Br
The “red” P generated from irradiated solutions
of P 4 in (a) PBr3, (b) CC14 and (c) CHBr 3 has
the general structure 1. The degradation of this
polymer occurs by successive attack on the P —P
bonds by the ‘ CClg radical (eqn. 15) and sub­
sequent abstraction of Br from the solvent (eqn.
16)
R
R
CCI3
1
/ r\
r\
r\—P
P —r\
s\
/ p -|
CCI3—>• O A - P
P — Ay-i
\ p /
\p/
R'
R'
(15)
6
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P. L. AIREY
1396
R
Br
\ /
/P
6 + CCl3Br —>
for the synthesis of certain highly substituted phosphines.
^CC13
—P
P —'v' + •CC13 .
\P /
(16)
I l l . Chemical reactions of the products
Since the disphosphines 3 and 4 had not pre­
viously been described, it was decided to investigate
a few of their chemical reactions.
R'
In case (a), R = PBr 2 and R' = Br and so the
formation of a large excess of CC13 (Br) P 2Br 2 (com­
pound 4) can be explained. On the other hand,
since the “red” P from the CC14 solvent (case b)
contains CC13 groups attached to every 6 th phos­
phorus atom, the observed relative increase in the
yield of CCl3 (Br)P 2 (Br)CCl 3 (compound 3) is ex­
pected.
a) Hydrolysis
Both disphosphines are hydrolyzed in the pres­
ence of air to give a mixture of phosphoric and
phosphonic acids. The latter was identified from
characteristic IR absorptions (e.g. at 2390 cm-1,
indicating the H P 0 22G ion) of the salt mixture
formed on neutralizing with NaOH. C 0 2 and CHC13
were identified mass-spectromentrically as volatile
products of a hydrolysis performed under vacuum.
The following reaction scheme accounts for these
observations, assuming that the acids formed under­
go an oxidation.
In case (c) (CHBr3), R = CHBr 2 and R ’ = Br.
Degradation of this polymer leads to the formation
of CCl3 (Br) P 2Br2, CHBr 2 PBr2 and a small yield
of CCl3 (CHBr2) PBr. The detection of the last com­
pound indicates that “red” P is potentially useful
CCI3P -PCCI3+ 3 H 20-> CCI3P (OH) 2 +CCI3P (H) OH
I
I
Br Br
3 H 2O j/
I
H 20
3 H aO * /
\ h 20
3HC1 + CO, CHCI3 3 HC1 + C0 2 CHC13
44+
+
PH (OH) 2 P(OH ) 3 PH.,OH
PH (OH) 2
18(a)
18(b) 19(a)
19(b)
(17)
Analogous reactions have been observed during
the hydrolysis of (CF 3 P ) 4 7.
quasi phosphonium salt which is thought to be an
intermediate in such brominations 8.
b) Reaction with Br2
CCI3(Br) P 2 (Br) CC13 + Br2
3
When to a 0.3 M solution of compound 3 in CC14 ,
was added dropwise a 0.4 M Br2 solution, the only
detectible product was CCl3 Br, the yield of which
increased proportionally with the amount of Br2 ad­
ded. The formation of CCl3Br is anomalous because
normally Br 2 reacts with diphosphines to split the
P —P bonds and leave the C —P bonds intact8. At
the completion of the reaction (indicated by the
permanence of the bromine colouration) 1.9 moles
of Br2 had reacted with 1.0 mole of the diphosphine
to form 2.0 moles of CCl3Br and no PBr3 . In an
analogous experiment, 1.2 moles of Br 2 were ob­
served to react with 1.0 moles of compound 4 to
form 1.2 moles of CCl3 Br.
In both cases a brown solid which rapidly dis­
solved was initially formed. This is probably the
7
W. M a h l e r and A. G.
[1958].
B u r g , J.
Amer. chem. Soc. 80. 6161
(20)
->■[CCI3 (Br) P2 (Br) 2 •CCI3] ®Bre .
7
7
-> CCl3 (Br)P2Br2 + CCl3Br . (21)
4
The elimination ( 2 1 ) occurs either directly, or on
the condensation of bromophosphines formed on
splitting the P —P bond (cf. eqn. 14).
The second stage of the bromination of 3, which
is identical with the bromination of 4, yields 1 mole
of CCl3Br but no PBr 3 .
CCl3 (Br)P 2Br2 + Br2-> [CCl3P 2Br4] ®Bre .
4
8
-> [P2Br4] + CCl3Br .
8
W. S e i d e l and K.
113 [1963].
I s s l e ib ,
(22)
(23)
Z. anorg. und allgem. Chem. 325.
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RADIATIONS CHEMISTRY OF SOLUTIONS OF P4
Attempts to isolate P2Br4 by evaporating off the
solvent at — 5 °C led to the formation of PBr3 .
However, the first traces appeared only after 2/3 of
the solvent had been removed.
c) Reaction with Cl2
When Cl2 diluted with Ar was passed through
0.3 M solutions of the disphosphines 3 and 4 in
tetrachloroethane, the observed reaction products
were CCl3Br, PC13 and Br2 . No CC14 was formed
in either case. The yield of CCl3Br was quantitative
in the latter but only 48% theoretical in the former
case.
The CCl3Br may be formed either by the reac­
tion of the released Br2 on the disphosphines [eqns.
(20) — (23)] or by condensation of the substituted
phosphines formed on splitting the P —P bond [cf.
(14)].
d) Reaction with ethanol
When the diphosphines 3 and 4 were added to
EtOH at 77 °K and allowed gradually to warm up
in an inert atmosphere, two phosphorus compounds,
boiling at 37 °C (0.2 torr) and 70 °C (0.4torr),
and EtBr were isolated.
The lower boiling point compound has the fol­
lowing elementary analysis: C = 24.4% (theoreti­
cal 25.2%), H = 4.2% (4.2%), Cl = 42.2%
(44.1%), P = 13.1% (13.0%) and an I.R. spectra
characteristic of diethyl trichloromethyl phosphonite
CCl3P(OEt) 2 (9).
1397
The higher boiling point compound absorbes
strongly in the I.R. at 3070 cm - 1 (C — H) 2500
cm - 1 (P —H) 1310 cm “ 1 (P = O, C —P rocking
frequency) 1180 cm“ 1 (P —O —Et) and between
850 cm “ 1 and 740 cm- 1 (CC13 —P). The elemen­
tary analysis is also consistent with the proposed
structure:
0
/
CC13 —P —O E t.
\
(10)
H
It is believed that the phosphinate (10) is derived
from the phosphonite (9) by an A r b u s o v 9 reac­
tion catalysed by the generated HBr.
CCI3 - P - P - CCI3+ EtOH
I
I
Br Br
(24)
CCI3P (OEt) 2+ HBr + . . .
9
9 + HBr -> [CCI3PH (OEt) 2] ®Br0 -> 10 + EtBr .
(25)
The author is indebted to Professor A. H e n g l e i n
for his constant interest and many valuable suggestions,
and to the CIBA Fellowship Trust, U.K. for financial
support.
9 B. A. A r b u s o v , “Organo-phosphorus Compounds”, p. 307,
IUPAC Symposium, Heidelberg, Butterworths, London
1964.
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