Sept., 1 9 w
Alssoci\TIOX E Q C I L ~ iBix ~ ZI I' i - ~ o ~ ~ i5~TRICHLORIDE-TRIMETHTL~~~~I~J', oiz~ 1296
ASSOCIATION EQUILIBRIA COMPOUND FOR114ATIOS IS THE PHOSPHORUS TRICHLORIDE-TRIJ4ETHYL~4MIKESYSTEM BY ROBERT R.HOLMES I h p a r t m e n t of ('hemistry, ('ai neqze Instztute o,f Il'echnologil, Schenley Pal k , Pztlsburgh, P a . Recciicd .Ilnrrh 28. 1980
Examination of the binary system phosphorus trichloritle-t8riniethylainine a t - 46.5" showed the formation of tmhe solid, Cl,P.K(CH,),. Interpretation of pressure composition data shows that it, has an exist.ence in the liquid state and is, hence, not a simple lattice compound. In solution a dissociat'ion equilibrium is established and an interaction energy of 6.4 kral.jmole calculated. I n the vapor the complex loses its esist,enee. The heat of dissociat,ion of t,he solid to the gaseous romponents was found to be 22.4 kcal./mole. P3I magnetic resonance measurements made on the liquid phase support the phase diagram interpretations. The presence of a phosphorus-nitrogen linkage is indicated.
Introduction Examination of known complexes of trihalides of Group Vb reveals that arsenic, antimony and bismuth trihalides form complexes with donor molecules, whereas phosphorus trihalides do not. For example, donor molecules such as ethers and aldehydes form 1:l complexes with antimony trihalides, compounds of the type, CeH6XH +-4sX4-, are known with arsenic halides2 and compounds of the general formula, R3N.BiC13, are formed between amines and bismuth trichloride.' In view of the above if such a complex involving a phosphorus trihalide molecule is possible, it might be anticipated that the resultant stability mould be low. With this in mind a detailed investigation of the interaction of phosphorus trichloride with a typical basic molecule which would show no tendency for side reactions and possess volatility sufficient for convenience in a complete examination of the system using vacuum techniques waq undertaken. Trimethylamine was chosen to fulfill these requirements. Results -1ddition of (CH,)??; to PC13 at -78" where both components are liquid resulted in the formation of a white solid which was easily sublimable and was observed to liquefy on warming to 0". To eqtablish its composition the system was examined more thoroughly a t -46.5". The PC1,-(CH3)3N System at -46.5" .-The data for the pressure-compositioii diagram are shown 111 Table I and plotted in Fig. 1. The data are represented a' pressure of the hystem wrsus molt fraction of amine, the latter applying to the (wiidei)sed phase. The first addition of amine, corresponding to a mole fraction of 0.189, caused the appearance of ;I d i d phaqe and a resultant drop in the vapor pre+ wre of pure liquid PC1, from 2.0 to 1.2 min The ure remained constant oil further additions of e up to a mole fraction of 0.5, the latter plateau corresponding t o the vapor pressure of a saturated wlution of the solid in liquid PClj (T'LC curve). The Sharp rise in pressure at 0.5 mole fraction serves to indicate the existence of a 1 : 1 complex hetween the romponents. Further additions of amine caused solution of the complex and the appearance of a semiid plateau a t 64.5 mm.,the \ sirIn!tich Tlir ( IieinlraI Elements a n d 1 heir ( oin\ 01 I Olford I nil er-itx ( 1 ) I' 1' I G~~~ v i i ,
rul,u\
vapor pressure of the satmated solution of complex in (CH3),S (CLT' curve). Solution m-as complete at a mole fraction of 0.81. One sees that dilution of the solution between the mole fractions of 0.87 to 1.0 gives a linear relat'ioiiship which closely follows Raoult's law (the dotted line). The light line (V) represents the vapor line and will be discussed aft,er presentation of Table 11. TABLE I THE PHOSPHORUS TRICHLORIDE-TRIMETHYLANISE SYSTEM AT
3Iole fractiono (CH3)aS
Press., 111111.
-46.5" Mole fractioria (CH3)3S
Press., rnm.
0,819 57.2 2.0 ,844 til .4 1.2 64.2 1 3 ,869 ,884 65.6 1. 3 897 titi. 3 ti.? 67.3 52.8 . 91 0 ,514 67.8 54.7 ,910 ,656 68.7 54.5 930 ,710 ,938 69.4 54.5 ,735 . !I44 70.5 54.5 ,780 In this experiment 0.493 niniole of PC1, wts ustd. 0 0.189 ,288 ,467 ,499
a
The Sublimation of Solid ClaP.N(CH:~)a.-To obtain the sublimation pressures of C;l;3P.?;(CII3);< as a function of temperature, t'he following experimental procedure was employed : The compound was formed at low temperatures using a slight excess of amine and warmed to -16.5", where m a l l arnouiit,s of amine were removed until a pressure minimum was reached. The system had a very small vapor volume (35.8 cc.) to niinirn:zc m y change in composition in the coiidensed phase. Vor example, for the sample size used (2.87 mmoles). if it rould be sublimed ent'irely iiito the gaseous phase mit,hout dissociat,ion to the cwmpoiieiith a pressure of about 2000 mm. would be exert'ed. The data reported in Table I1 show a p m w m vwriat'ioii from 0.9 mm. at -46.5" to 8.6 inin. ut -21 .Oo. To ensure pressure independence of t>he relat,ivc amount of vapor, the det'erminatioii \ w s repettted using a sample size of 1.42 ninioles and a vapor volume of 45.8 ml. The results wcrc the s:iiiiie a:: those obtained with the smaller vapor volume. Also a sinal1 ainouiit of the vapor wis removed from the system at the highest temperature, -21.0", in the two experiments (the measuremeiits heing made st,art,irig at -16.5" and progressively increasing the t,emperature). The latter variat'ion did not affect the nieasuremeiits. T u iiiterpret the
ROBERT R.IIOLMLS
1296
Vol. 64 TABLE I1 TRICHLORIDE-TRIMETHYLAMIXE
SUBLIMATION PRESSURES O F SOLID PHOSPHORUS Temp.,
'C. -46 5 P, mm. 0 9
-39 5 14
-37.5
18
-32.7 2.6
-28 0 4.3
7 7 2'
-22
-21.0 8.8
pressure of 120 mm. Hence, the sublimation pressures of Table I1 are taken to correspond to the equilibrium CI,P.N(CH,),(s) = PC13(g) (CHS)BN(g). The heat for the above dissociation reaction may be obtained assuming K = ( 0 . 5 ~ ) ~This . is done simply by plotting log p us. 1jT and equating twice the slope to - AH/2.303R. The resultant heat of dissociation is 22.1 kcal./mole. The sublimation pressure of 0.9 mm. at -46.5" was used to construct the vapor line in Fig. 1. Of the 0.9 mm., 0.45 mm. corresponds to gaseous PCls at a mole fraction of 0.5. Using the latter value as a maximum for PC13 in going to higher amine concentrations the mole fraction of gaseous amine was calculated and the vapor line established. The measurements on the 2.87-mmole sample (sample 1) described in Table I1 were continued up to -3.6" and listed in Table I11 with accompanying information from visual observation.
+
/ /
/
/ /
TABLE I11 VAPOR
0
l).4 0.6 0.8 1.0 Mole fraction, (Cki3)3X. phosphorus trichloride-triinct~i~-lamiiiiii~ system at -46.5".
il.2
Fig. l.--Tlic
PRESSURE
1C I I I ~ ) . ,
P ,
OC.
-18.0 -15.4 -13.5 - 10.6 - 7.0 - 3.6
OF
THE
PHOSPHORUS
.
12.8 20.5 28.1 46.8 50.4 72.9
Solidismoistlooking Looks entirely liquid Liquid Liquid
I .it?
I .;(I
\':tpor volrirnc W I ~ S 35.8 ml. volunic IV:LS 45.8 ml.
* V:qor 3:
TRICHLORIDE-
TRIMETHYLAMIKE SYSTEM Sample 1 Sample 2 I'ress.,a Observation, Temp., Press., b mm. condensed phase 'C. mm.
-20.8 -19.0 -17.0 - 15.3 -13.6 -11.8 - 7.8 0.0 4.9
8.9 11.5 14.8 19.9 28.5 32.4 43.4 67.7 84.4 11.0 115.2 15.8 142.6
1.2.$
*i.il
I 7'
x
-i,ll
4.-
IV.
Fig. 2.-J'aristion n i t h temperature of the vapor pressure of phosphorus trichloride-trimethylamine: sampIe 1 ;
0, sample 2 ; snmple.
'I'hc results coinbiiied with thosc of Table TI :ire plotted in Fig. 2 . The log p us. l/[r plot shows ;L sh:irp increase in slope at -21' and a further slope change a t approximately - 10'. The slope diaiiges are assumed to be caused by the appearance of a liquid phase a t A (previous to this point the equilibrium being C = V and afterwards, LC\') and the disappearance of t'Iie solid phnse :it 13(LCV going to L V) in qualitative agreemerit, with visual observation. Using a smaller sample size, 1.42 mmoles (sample 2 treated in the same manner as that used in obtaining the data of Table 11) and a larger vapor volume, 45.8 ml., the measurements were repeated. The results (Table 111) plotted in Fig. 2 show the same slope between points A and B but the slope change occurs at point C corresponding t'o - 13.6" instead of - 10'. The latter serves to indicate that this point is a function of the ratio of vapor to liquid which is usual for a binary system of this t y p e 3 The range
e,
a,represents two points, same vnlue for each
sublimation pressures further, studies 1%-eremade on mixtures of gaseous PC1, and (C€13)& a t 25'. It was found that the components followed Dalton's law quite closely showing only a slight tendency for association in the gas phase up to a total
( 3 ) .T. E. Ricci, "The I'haee Rule a n d Heterogenroiis Equilibriuni," I).Van Xostrand Co., Inc., New York, N . Y., 1931, y p . 158-160.
ASSOCIATION EQUILIBRIA IN PHOSPHOI~US TRICHLORIDE-TRIMETHYLAMIKE 1297
Sept., 1960
A to B or A to C then may be called the “melting range under the vapor pressure of the system.” The PCI3-(CHJ3N System at O”.-To understand more fully what is occurring in the liquid phase (above point B of Fig. 2), the phase diagram was examined in the liquid region. Additions of (CH3)3N to liquid PC13 a t 0’ resulted in the pressure composition diagram shown in Fig. 3; the data are listed in Table IV. The total pressure curve obtained is of the type expected for a homogeneous, liquid system in which the components are associating to some degree. In order to establish the type of association taking place in a more quantitative fashion, calculations were carried out using a modified treatment based on Dolezalek’s method4 in which it is assumed that the components, (CH3),X, PC13 and any complexes present, obey Raoult’s law. Using the latter assumption the activities are equated to the calculated mole fractions for each. It is also assumed that the partial pressure of the complex is negligible as measurements of the total vapor pressure of mixtures of gaseous PC13 and (CH3)3K a t 25.0’ indicate. To illustrate, assume an equilibrium in which only a 1:1 adduct is considered to be present and derive an expression for the equilibrium constant for the association PCL(1)
+ N(CH3)d(soln.) =
C13P.N(CH&(soln.)
The definition of symbols to be used are activities of (CH3)3S and PC13, resp. partial vapor pressures of (CH3)J and PCls, resp. p t 0 and pa0 vapor pressures of pure (CH,I~N AT CONSTAKT T E X P E R A T ~(0’) ~RE hlde fraction,O
Total
I’C1a.N-
press.,
(Clia)i,
I(CH3)IN
111111.
0.128 ,251 ,305 .Xl9 .:181 ,408 ,429 ,455
The above expressions may be combined to give K
=
OLVl
+ PlOSZ) [(p*OA-*+ P1OAVrl)- p t (Nl +
Pt2.VlLV*-
Pt(lV*
- N1)(p,~S1- P,O-VVs) ~ i ~ ~ z~ (*VI)’ Nz
N2)I
(6)
Similar expressions may be derived for other (4) J . H. Hildebrand and It. L. Srott. “The Solubility of Nonelectrolytes,” 3rd ed., Reinhold Publ. Corp., New York, Pi. Y., 1960,p. 176.
0.1
types of complexes assumed to be present. Using the data of Table IV and considering iiidividual equilibria involving the components and these compositions, PC13,r\;(CH3)3,PC13.2h-(CI13)3,PCla,
.18:i
Ihprcwions for S,, ar and I< then follow
t
25
34.8 :I 4 . 4 34.4 35. 2
K
I’C13.2NI’CIa..?U(CHa):j, (CHB)~, I< X 10--1 h’ X 10-4
31.7 512 ”9.0 32.t; 3:1 ,:I 203
ii0
21’(”1:
