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Enthalpy of mixing of hexafluorobenzene with some pyridines
323 Thermochimica Acra, 9 (19743 323-326 a Elscvier !ScientiiicPublishingCompany, Amsterdarrk - Printedin Belgium
Note
Enthalpy
of mixing
of hexafluorobenzene
with
some pyiidines
ROGER MEYER”. MONIQUE MEYER AND JACQUES METZGER Luboraroire associe’au C.N.R.S., Strucrure er rtfacriritdcn s&ie hPt&ocyclique, Unirersire’de Procence, Arenue Poincad, 13397 Marseille Cedex 4 (France)
(Received 16 January 1974)
Recent studies of fluorocarbons with various compounds indicate the possibility of a complex formation’-6. In a previous work Armitase and Morcom6 have determined the excess pyridine and 54,6enthalpy of hexafluorobenzene with pyridine, Z$-dimethyl trimethyl pyridine. This work has been extended to a mixture of hexafluorobenzene with 2-methy! pyridine, 4-methyl pyridine, 3,5-trimethyl pyridine. These pyridines were chosen in order to study the ef?ect of the position of a substituant on the ring EXPERIMENTAL
The calorimeter used was a CR-M-T_ already described7_ The hexafiuorobenzene obtained from the Imperial Smelting Corporation was distilled and stored over freshly molecuiar sieve. Pyridines were dried, distilIed and stored out of contact with air in the dark. All samples showed a purity greater than 99 moIes per cent. RE!XILTS AND
DISCUSSION
The enthalpies of mixing were fitted by a least squares computer program to the expression: HE/X,(l--r,)
= i:
Ai(l-22x,)‘-’
U)
i=l
where xl is the mole fraction of hextiuorcbenzene. The values of the coefficients Ai together with the standard deviation Q of the results are given in Table 1. The experimental results are given in Table 2. The excess enthalpies of mixtures of pyridine, 2$-dimethyl pyridine, have been measured by Armitage
2,4,6_trimethyl
pyridine with hexafiuorobenzene
and Morcom6
at different temperatures_
+To whom sll correspondenceshould be addressed.
Their
324 TABLE VALUES
I OF THE
LEAST
.w_rfure
SQUARES
PARAMETERS
AZ
-4, IN
AS
EQN (1) Standmd P/x,
C,F, + ‘-methyl - _pyname Standard dcviatiorr of paxmetcrs
3.7
ChFe i 4-methyl pyridine Standard deviation of Pm C,F,+3,5dimethyl pyxidine Standard deviati0n of parameters
l
Tboughout
-510
319-l
96.6
71.9
14
x2.9
2.5
-43-I
355.3
2
8.8
-4!3525
1309
330.5
1.3
32
I.7
this paper 1 Cal,,
deriation of
(1 -x,1.
G (Cal’
moi- ‘J
7.9
- 162.3
51.5
20
12.5
3.8
z9
= 4.184 J.
results showed that pyridine and its homologues act essentiahy as n donors, the heat evoIved increases with substitution_ T&is work has now been extended to mixtures of hexafluorobenzne with Zmethyi pyridine, 4-methyl pyridine, 3,Zi-dimethyl pyridine, The behaviour of pyridine mixtures is very similar to that of the corresponding aromatic compounds, except that the 2 position of heterocychc compounds appears to be privileged. We have considered hV0 sets of mixtures: 2-methyl pyridine, 2&dimethyi pyridine, 2,4,6-trimethyl pyridine and pyridine, 4-methyl pyridine, 3,Sdimethyl pyridine. For each set the HE becomes more negative as the number of methyl groups on the pyridine ring increases. If we pIot HE (x1 = 0.5) against the number of methyl groups we obtain approximately a straight line. In every case the excess enthalpies for the C,F,-pyridines mixtures are less negative than for C,F,-aromatic hydrocarbon mixtures. While the curves for C,F,+Zmethyl pyridine and C,F6+3,5-dimethyl pyridine are negative over the entire mole fraction range, the excess entbalpy for 4-methyl pyridine is S-shaped with HE negativeat low mole fraction of base and positive at low mole fraction of hexafhrorobenzcne. In recent publications1*4*8*git was suggested that the decrease in HE which accompanies the addition of methyl groups on the benzene ring is caused by the increase in polarizabiiity Ieading to stronger l-2 interactions, a similar situation is evident in the present study. There is evidence that weak complex formation does take place in C6F6 + pyridines systems6*10*11,butthenatureofthecomplexesformedisa
325 TABLE
2
EXCESS Xl
EhTHALPY
HE
OF
CexP-1
*
(cd mot - ‘)
&F,
(1)
WITH
PYE2IDlNES
HE (cak) (Cal mol- ‘)
Xl
0.5923 0.6244 0.8140 0.9550 0.9743
(2) AT 31815K
p bP.) (co? f?iOl-
‘)
HE (caIc_) (Cal mol - ‘)
C, F6 i- 2-me fhyl pynidine
O.I5_34 0.2352 0.4230 0.4365 0.4620
- 34.57 - 58.99 -111.27 - 112_ss -117.69
- 34.57 -55.37 -111.97 -ilS.ll - 120.59
OS362
-
- 132-43
134.6
c = (Z NE (exp.)
-HE
(cak.)/m-
E)~‘* = 1.95 cal mol-
-
138.31 137.8 - 97.46 - 28.49 - 16.75
-
136.45 136.57 -99.10 - 28-58 - 16.61
I
where m = number of measurement and n = number of parameters into eqn (I) CeFe +&nerhyZ pyriciine 0.5267
-31.97
-31.58
0.3914
7.73
7.74
0.8473
- 28.62
-228.89
0.3474
13.02
13.86
0.717 0.50 0.4655
-37.01 -9.6 - 523
- 36.97 - 10.77 -4.64
0.2821 0.165 O-1389
20.66 22.3 i937
20.28 21.53 19-89
2.87
3.16
0.4202 a-=0.73
cal mol-*
C, Fe + 35-dimerhyI pyridine
0.08 I6
0.5905
- 130.36
- 130.10
O-1691 0.44-%3 0.4544
-2ft.81 -110.04 -111.97
-88.61
- 28.99 - 109.59 - 111.87
0.6875 0.733 0.861
- 126.17 -119.46 - 77.63
-
0.4887 0.5603
-
-118.69 - 128.19
0.8727 0.892
- 72.52 - 64.24
0=0_47
- 8.55
1 IS.69 127.58
126.37 119.02 - 78.08 - 72.83 - 63.47
Cal moI-l
matter of speculation. The lack of confirmatory non thermodynamic evidence has led to the suggestion that the interactions between the unlike molecules arise principally from electrostatic forces’ s6-* 2 o f type C-F bond dipoles ICelectrons. ACKNOWLEDGEMENT
The authors wouId like to thank Dr. K. W. Morcom for his interest in this work. REFERENCES 1 P. J.
Howell, B. J. Skilleme de Bristowe and D. J. Stubley,
J. Chem. 27xermadyn., 4 (1972)
2 J. Rex Goates, 0. H. Bevan and J. Reeder, L Chem. Thermodyn., 5 (1973) 135. 3 B- J- Skilleme de Btistowe and D_ Stubley. J. Chem. Thermdyn_, 5 (1973) 121. 4 D. V. Fenby and S. Ruenkrairergsa, 1. Chem. Thermdyn.. 5 (i973) 227. 5 S. Ruenkraircrgsa. D. V. Fenby and D. E. Jones, J. Chem. ZXermodyn~, 5 (1973) 347.
6 D. A. Armitage
and K. W.
Morcom,
Trans. Faraaby Sac., (1969) 688.
225.
326 7 M. JLafKte. M. Cotm and F_ Cam& 23rd Ann. CaIorimetty Conf. U.S.A.. C_N.R_S_, Brexct No. 1527 _ 156. 1965. S A_ Andreus, K_ W_ Morcom, W_ A_ Duncan. F_ L- Suinton and J_ M. PoIlock, L Chem. T&rl?wffyn_, 2 (1970) 95. 9 R. J. PowcIX and F. L. Sainton, J. Chcm. Thcrmocf~-tr_, 2 (1970) 87_ IO R. Meyer, A_ Barlaticr and J. Metzger, J_ C&m_ Phys, 68 (1971) 417. 1 I A_ Barbtier. R. Meyer, H. Desplanches and J. Metzger, C.R. Acad. SC:. Paris, 273 (1971) 1473. I2 M_ W_ Hanna, J. Amer- Chpm. SOL. 90 (1968) 28.5.
Note
Enthalpy
of mixing
of hexafluorobenzene
with
some pyiidines
ROGER MEYER”. MONIQUE MEYER AND JACQUES METZGER Luboraroire associe’au C.N.R.S., Strucrure er rtfacriritdcn s&ie hPt&ocyclique, Unirersire’de Procence, Arenue Poincad, 13397 Marseille Cedex 4 (France)
(Received 16 January 1974)
Recent studies of fluorocarbons with various compounds indicate the possibility of a complex formation’-6. In a previous work Armitase and Morcom6 have determined the excess pyridine and 54,6enthalpy of hexafluorobenzene with pyridine, Z$-dimethyl trimethyl pyridine. This work has been extended to a mixture of hexafluorobenzene with 2-methy! pyridine, 4-methyl pyridine, 3,5-trimethyl pyridine. These pyridines were chosen in order to study the ef?ect of the position of a substituant on the ring EXPERIMENTAL
The calorimeter used was a CR-M-T_ already described7_ The hexafiuorobenzene obtained from the Imperial Smelting Corporation was distilled and stored over freshly molecuiar sieve. Pyridines were dried, distilIed and stored out of contact with air in the dark. All samples showed a purity greater than 99 moIes per cent. RE!XILTS AND
DISCUSSION
The enthalpies of mixing were fitted by a least squares computer program to the expression: HE/X,(l--r,)
= i:
Ai(l-22x,)‘-’
U)
i=l
where xl is the mole fraction of hextiuorcbenzene. The values of the coefficients Ai together with the standard deviation Q of the results are given in Table 1. The experimental results are given in Table 2. The excess enthalpies of mixtures of pyridine, 2$-dimethyl pyridine, have been measured by Armitage
2,4,6_trimethyl
pyridine with hexafiuorobenzene
and Morcom6
at different temperatures_
+To whom sll correspondenceshould be addressed.
Their
324 TABLE VALUES
I OF THE
LEAST
.w_rfure
SQUARES
PARAMETERS
AZ
-4, IN
AS
EQN (1) Standmd P/x,
C,F, + ‘-methyl - _pyname Standard dcviatiorr of paxmetcrs
3.7
ChFe i 4-methyl pyridine Standard deviation of Pm C,F,+3,5dimethyl pyxidine Standard deviati0n of parameters
l
Tboughout
-510
319-l
96.6
71.9
14
x2.9
2.5
-43-I
355.3
2
8.8
-4!3525
1309
330.5
1.3
32
I.7
this paper 1 Cal,,
deriation of
(1 -x,1.
G (Cal’
moi- ‘J
7.9
- 162.3
51.5
20
12.5
3.8
z9
= 4.184 J.
results showed that pyridine and its homologues act essentiahy as n donors, the heat evoIved increases with substitution_ T&is work has now been extended to mixtures of hexafluorobenzne with Zmethyi pyridine, 4-methyl pyridine, 3,Zi-dimethyl pyridine, The behaviour of pyridine mixtures is very similar to that of the corresponding aromatic compounds, except that the 2 position of heterocychc compounds appears to be privileged. We have considered hV0 sets of mixtures: 2-methyl pyridine, 2&dimethyi pyridine, 2,4,6-trimethyl pyridine and pyridine, 4-methyl pyridine, 3,Sdimethyl pyridine. For each set the HE becomes more negative as the number of methyl groups on the pyridine ring increases. If we pIot HE (x1 = 0.5) against the number of methyl groups we obtain approximately a straight line. In every case the excess enthalpies for the C,F,-pyridines mixtures are less negative than for C,F,-aromatic hydrocarbon mixtures. While the curves for C,F,+Zmethyl pyridine and C,F6+3,5-dimethyl pyridine are negative over the entire mole fraction range, the excess entbalpy for 4-methyl pyridine is S-shaped with HE negativeat low mole fraction of base and positive at low mole fraction of hexafhrorobenzcne. In recent publications1*4*8*git was suggested that the decrease in HE which accompanies the addition of methyl groups on the benzene ring is caused by the increase in polarizabiiity Ieading to stronger l-2 interactions, a similar situation is evident in the present study. There is evidence that weak complex formation does take place in C6F6 + pyridines systems6*10*11,butthenatureofthecomplexesformedisa
325 TABLE
2
EXCESS Xl
EhTHALPY
HE
OF
CexP-1
*
(cd mot - ‘)
&F,
(1)
WITH
PYE2IDlNES
HE (cak) (Cal mol- ‘)
Xl
0.5923 0.6244 0.8140 0.9550 0.9743
(2) AT 31815K
p bP.) (co? f?iOl-
‘)
HE (caIc_) (Cal mol - ‘)
C, F6 i- 2-me fhyl pynidine
O.I5_34 0.2352 0.4230 0.4365 0.4620
- 34.57 - 58.99 -111.27 - 112_ss -117.69
- 34.57 -55.37 -111.97 -ilS.ll - 120.59
OS362
-
- 132-43
134.6
c = (Z NE (exp.)
-HE
(cak.)/m-
E)~‘* = 1.95 cal mol-
-
138.31 137.8 - 97.46 - 28.49 - 16.75
-
136.45 136.57 -99.10 - 28-58 - 16.61
I
where m = number of measurement and n = number of parameters into eqn (I) CeFe +&nerhyZ pyriciine 0.5267
-31.97
-31.58
0.3914
7.73
7.74
0.8473
- 28.62
-228.89
0.3474
13.02
13.86
0.717 0.50 0.4655
-37.01 -9.6 - 523
- 36.97 - 10.77 -4.64
0.2821 0.165 O-1389
20.66 22.3 i937
20.28 21.53 19-89
2.87
3.16
0.4202 a-=0.73
cal mol-*
C, Fe + 35-dimerhyI pyridine
0.08 I6
0.5905
- 130.36
- 130.10
O-1691 0.44-%3 0.4544
-2ft.81 -110.04 -111.97
-88.61
- 28.99 - 109.59 - 111.87
0.6875 0.733 0.861
- 126.17 -119.46 - 77.63
-
0.4887 0.5603
-
-118.69 - 128.19
0.8727 0.892
- 72.52 - 64.24
0=0_47
- 8.55
1 IS.69 127.58
126.37 119.02 - 78.08 - 72.83 - 63.47
Cal moI-l
matter of speculation. The lack of confirmatory non thermodynamic evidence has led to the suggestion that the interactions between the unlike molecules arise principally from electrostatic forces’ s6-* 2 o f type C-F bond dipoles ICelectrons. ACKNOWLEDGEMENT
The authors wouId like to thank Dr. K. W. Morcom for his interest in this work. REFERENCES 1 P. J.
Howell, B. J. Skilleme de Bristowe and D. J. Stubley,
J. Chem. 27xermadyn., 4 (1972)
2 J. Rex Goates, 0. H. Bevan and J. Reeder, L Chem. Thermodyn., 5 (1973) 135. 3 B- J- Skilleme de Btistowe and D_ Stubley. J. Chem. Thermdyn_, 5 (1973) 121. 4 D. V. Fenby and S. Ruenkrairergsa, 1. Chem. Thermdyn.. 5 (i973) 227. 5 S. Ruenkraircrgsa. D. V. Fenby and D. E. Jones, J. Chem. ZXermodyn~, 5 (1973) 347.
6 D. A. Armitage
and K. W.
Morcom,
Trans. Faraaby Sac., (1969) 688.
225.
326 7 M. JLafKte. M. Cotm and F_ Cam& 23rd Ann. CaIorimetty Conf. U.S.A.. C_N.R_S_, Brexct No. 1527 _ 156. 1965. S A_ Andreus, K_ W_ Morcom, W_ A_ Duncan. F_ L- Suinton and J_ M. PoIlock, L Chem. T&rl?wffyn_, 2 (1970) 95. 9 R. J. PowcIX and F. L. Sainton, J. Chcm. Thcrmocf~-tr_, 2 (1970) 87_ IO R. Meyer, A_ Barlaticr and J. Metzger, J_ C&m_ Phys, 68 (1971) 417. 1 I A_ Barbtier. R. Meyer, H. Desplanches and J. Metzger, C.R. Acad. SC:. Paris, 273 (1971) 1473. I2 M_ W_ Hanna, J. Amer- Chpm. SOL. 90 (1968) 28.5.