THERMOCHEMISTRY OF CYANOCARBONS
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Thermochemistry of Cyanocarbons. 11. The Heats of Combustion of Pyridinium Dicyanomethylide, Malononitrile, and Fumaronitrile
by Richard € Boyd, I. K. Ranjan Guha, and Richard Wuthrich Department of Chemistry, Utah State' Universdty, Logan, Utah 84391 (Received December 19, 1986)
The heats of formation of solid and gaseous pyridinium dicyanomethylide, malononitrile, and fumaronitrile have been determined v i a the heats of combustion of the solids and their heats of vaporization (from vapor pressure curves). From these data, the thermochemical bond energy of the N+-C- bond in the ylide is calculated to be 67 kcal mole-'. The data for malononitrile and fumaronitrile are used to revise and extend a discussion of the electrostatic and electronic effects of cyano group substitution. The pK, for the dissociation in water of the conjugate acid of the ylide was determined to be 1.55.
-
Introduction Ylides are compounds in which the formal structure has a positively charged atom covalently bonded to negatively charged carbon. There are a number of examples of phosphorus ylides, whose stability may perhaps be promoted by d-orbital participation rendering the actual structure less ionic. With nitrogen, where this possibility is remote, stable ylides are rare. How-
In order to provide auxiliary data to aid in the interpretation of the stability of I we have also measured the heats of combustion and vaporization of malononitrile (dicyanomethane) for which there are no modern values. Malononitrile is an interesting compound thermochemically in its own right as it is one of the simplest compounds in which the phenomenon of electrostatic repulsion between highly polar cyano groups can manifest itself. We have previously investigated this effect in tri- and tetra~yanoethylene.~In order to make the study of cyano group interaction more complete we have also included fumaronitrile (trans-dicyanoethylene) in this study.
Experimental Section
I ever, recently the compound pyridinium dicyanomethylide (I), an example of a stable nitrogen ylide, has become knownl1S2and its structure was confirmed by X-ray diffraction.a In the present work we have measured the heat of combustion and heat of vaporization of I in order to provide quantitative information about the stability of this structure. Also of interest in assessing the stability of the ylide is the ease or diflticulty with which it protonates, ie., the position of the equilibrium
+
B H + s B H+ (1) where B represents I and BH+ its protonated form (presumably protonated on the N carbon). Consequently, we have measured the pK. of reaction 1.
The pyridinium dicyanomethylide furnished to us had been synthesized via the reaction of pyridine and tetracyanoethylene oxide.112 It was purified by three recrystallizations from acetone and then further purified by twice subliming along a temperature gradient5 and keeping the center fraction. It was necessary to keep (1) W. J. Linn, 0. W. Webster, and R. E. Benson, J. Am. Chem. SOC.,85, 2032 (1963). (2) A. Rieche and P. Dietrich, Chem. Ber., 96,3044 (1963). (3) C. Bugg, R. Deaiderato, and R. L. Sass, J . Am. Chem. Soc., 86,
3167 (1964). (4) R. H. Boyd, J . Chem. Phya., 38, 2629 (1963); paper I of this series. (6) G. J. Sloan in "Physica and Chemistry of the Organic Solid State," D. Fox, M. M. Labes. and A. Weiasberger, Ed., Interscience Publishers, Inc., New York, N. Y., 1963, Chapter 2.
Votunw 71, Number 7 June 1967
R. H.BOYD,K. R.GUHA,AND R. WUTHRICH
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Table I : Energy of Combustion at 25""
- AEoo, M
MF
AR
AR'
PI
cai/g (air va. atainlesa steel)
PI
PI
Qd
7.6 7.3 7.4 7.4 7.4 7.4
1.5 1.5 1.5 1.5 1.5 1.5
-0.2
6.0 6.0 5.8 6.1 6.2
1.5 1.5 1.5 1.5 1.5
Malononitrile 0.294024 0.285834 0.287224 0.280415 0.286695 0.287360
0.002191 0.002136 0.002182 0.002247 0.002372 0.002391
0.198924 0.193452 0.194601 0.189836 0.193838 0.194540
0.197911 0.192464 0.193592 0.188797 0.192742 0.193435
0.257870 0.258884 0.256558 0.259922 0.269708
0.002122 0.002086 0.002134 0.002170 0.002148
0.190108 0.190822 0.189053 0.191256 0.198478
Fumaronitrile 0.189126 0.5 0.189857 0.5 0.188066 0.5 0,190253 0.4 0.4 0.197485
0.257673 0.240911 0 240165 0.252531 0.240709 0.244217
0.002303 0.002012 0.002200 0,002020 0.002359 0.002302
0.206981 0.193557 0.193046 0.202727 0.193338 0.195997
Pyridinium Dicyanomethylide 0.205916 0.6 5.7 0.192627 0.5 5.3 0.192029 0.4 5.3 0.201793 0.4 4.0 0.192247 0.6 5.3 0.194933 0.5 5.2
I
0.5 0.5 0.5 0.4
0.4 0.4
-0.2 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
1.4 1.4 1.4 1.4 1.4 1.4
-0.4 -0.4 -0.3 -0.5 -0.4 -0.3
5988.3 5990.5 5995.6 5988.6 5980.5 5988.2 5988.6 Av u = 4.3 5984.3 cal/g (vacuum) 6528.6 6528.3 6525.8 6515.8 6518.9 6523.5 Av 0 = 5.2 6518.7 cal/g (vacuum) 7118.9 7123.2 7123.0 7126.0 7114.6 7111.2 7119.5 Av u = 5.2 7114.8 cal/g (vacuum)
6 M = mass of sample (g, in air u8. stainless steel). M F = mass of cotton thread fuse (g, in air us. stainless steel). A R = corrected temperature rise (ohms). AR' = A R - M~(0.4624)(ohms). q1 = electrical ignition energy (cal). qz = nitric acid correction (cal). q3 = correction to standard states (Washburn correction) (cal). q4 = unburned carbon correction (cal). -AE," = [EAR' q1 - q2 - pa - q4] /M;the energy of combustion corrected to standard states. & =8944.0 cal ohm-'.
-
the hot end of the sublimer at -110" or less to avoid deco;nposition (dark residue) during sublimation. The final melting point was 245O, in agreement with the literature values.'J hlalononitrile was furnished to us purified by repeated zone refining.6 It was then sublimed through a temperature gradient from the zone-refining tube. Fumaronitrile was purchased from Chemical Intermediates and Research Laboratories, Inc. It was purified by repeated zone refining and sublimed through a temperature gradient from the zone-refining tube. The heats of combustion were measured by previously described technique^.^ The calorimeter differed from that previously described in having a difThe Journal of Phgeical Chemistry
ferent resistance thermometer. The technique also differed in that the electrical energy was supplied from the capacitor bank charged to 20-23 v rather than 45 v as previously described.' This resulted in a smaller electrical energy correction and lessened the pitting of the contacts on the firing switch due to arcing which waa noticed previously. A calibration baaed on 12 combustions of benzoic acid (National Bureau of Standards sample 39i) by two different operators resulted in an energy equivalent of 8944.0 f 3.1 cal ohm-', where -~
~~
~~
~~
~~
(6) R. H.Boyd, R. Christensen, and R. Pua, J. Am. Chem. SOC.,87, 3554 (1965). (7) R. H.Boyd, Rev. Sci. Inetr., 35, 1086 (1964).
THERMOCHEMISTRY OF CYANOCARBONS
2189
the uncertainty is the 95% confidence level for the sample mean. Weights were reduced to in vacuo values with the following densities (g/ml) estimated from the dimensionsof weighed pellets : malononitrile, 1.1; fumaronitrile, 1.15; and pyridinium dicyanomethylide, 1.27. The results of the combustions are recorded in Table
I. The vapor pressures were determined by an effusion cell technique.6 The results are given in Table 11.
P 2X
A
c
C
.-U4) .-
-
c
a, 0
u
C
0 .c
Table 11: Vapor Pressures' Temp,
OC
W t loss, mg
Effusion time X 101, sec
X
w
p, Ir
-17.50 -12.55 -7.85 -2.35 3.15 8.15
Malononitrile 1.240 76.7 1.103 24.85 1.860 21.07 2.933 18.80 6.477 21.58 10.550 20.53
0.637 1.765 3.55 6.33 12.31 21.26
8.15 2.60 -1.95 -7.70 -12.70 -17.60 -22.85 -27.75
Fumaronitrile 19.868 19.30 10.955 19.27 6.841 18.95 3.796 20.19 1.885 19.62 1.053 20.50 1.050' 39.43 2.000 169.17
39.00 21.43 13.49 6.95 3.52 1.864 0.956 0.420
130.5 137.1 140.9 149.9 154.2 160.2 166.8 133.3
Pyridinium Dicyanomethylide 1.262 91.69 2.497 67.35 0.912 21.60 1.584 18.00 1.894 12.37 2.219 11.50 2.158 6.06 1.256 66.03 Log P ( r ) =
Malononitrile Fumaronitrile Pyridinium dicyanomethylide a
U C
.-+
@/TI
+B
0.465 1.256 1.441 3.04 4.53 5.73 12.55 0.643
Figure 1. Absorption spectrum of pyridinium dicyanomethylide. Solid curve is neutral form in water; dashed curve is protonated form in strong aqueous hydrochloric acid; dashed-dot curve is in hydrochloric acid of intermediate strength where both protonated and neutral forms are in equilibrium.
The pK, for the dissociation of the conjugate acid of I (eq 1) was measured spectrophotometrically in hydrochloric acid solutions. A Beckman DU spectrophotometer with the cell compartment thennostated at 25" was used for the quantitative measurements. The neutral form, B, has an absorption maximum at 374 mp (e 1.74 X lo4). This absorption decreases (reversibly) in solutions of increasing acid strength. A slight residual absorption at this wavelength due to BH+ remains in strong acid (see Figure 1). Prolonged exposure to strong acid solution results in irreversible changes in the spectrum. The pK, was determined* from the intercept at CH+= 0 of a plot of log R
A
B
-4139 -3758 -6549
16.076 14.970 15.955
Orifice diameter, 0.333 mm.
+ log CH+ = log K
where R = CB/CBH+, C is the molar concentration (CH+ refers to hydrogen ion concentration of the aqueous hydrochloric acid solvent), and f is the molar activity coefficient, vs. C H +. The protonation ratio, R, was determined from
R
+
The constants in the equation log P = ( A / T ) B were calculated by least squares, and the heat of vaporization was calculated from the relation AH,,, = -2.303RA. A summary of the derived thermochemical data is given in Table 111.
- log VH+~B/~BH+)
=
[D - Do(BH+)]/[Do(B)- D ]
where D is the optical density of a given solution (at A,, 374 mp of B), D"(B) is the optical density of a given solution of same concentration in pure water (entirely in B form), and D o(BH+) is the optical density of (8) M. A. Paul and F. A. Long, Chem. Rev., 57, 1 (1957).
Volume 71, Number 7 June 1967
R. H.BOYD,K. R. GUHA,AND R. WUTHRICH
2190
Table 111: Summary of Thermochemical Data AH,', koal mole-'
Compound
Malononitrile
(c) (I3)
Fumaronitrile
(C)
-395.03 -413.9 -508.63 -525.8 -1018.3 -1048.3
(g) Pyridinium dicyano- (c) (g ) methylide
f 0.3b f 0.5c f 0.46
f0.6' f 0.8* f 3.OC
AHvapl" koal mole-'
18.9 f 0 . 2 17.2 f 0 . 2 30.0 f 0 . 3 (403-440'K)
AHP, kcal mole-]
44.56 i 0 . 3 63.5 f 0 . 5 64.11 f 0.4 81.3 f 0 . 6 95.1 AO.8 125.1 f 3 . 0
+
b Uncertainty is fuAH,", where u = (UP u2)'/' and u1 and uz are relative standard 0 Uncertainties are arbitrary estimates. deviations of calibration and sample runs. Uncertainty is summed uncertainty of a and 6, plus added uncertainty for pyridinium dicyanomethylide due to experimental AHvap range being far from 25'.
a solution of same concentration in strong acid (residual absorption due to shoulder of BH+ at A,, for B). The log R values a t various acid concentrations are listed in Table IV. Table IV : Determination of the pKa for the Dissociation of the Conjugate Acid of Pyridinium Dicyanomethylide"
+
dl
Log R log CH*
1,006 1.506 2.00 2.61 3.00 3.52
1.211 1.034 0.881 0,684 0.405 0.210
W+l,
a From intercept at CH+ = 0, pKa = -(log R -1.55.
+ log CH+)O=
Discussion Probably of most direct interest in discussing the stability of pyridinium dicyanomethylide would be the thermochemical average bond energy of the C--N+ bond in this compound. We have assigned this by considering the average thermochemical C-CN bond energies to be the same as in malononitrile. Then the enthalpy of the reactiong CN=c-GN
+ I
+
€I2
-
0
3- CNCHLN,
AH=-28.0 kcalmole"
(2)
Using'oE(H-H) = 104 kea1 mole-' and E(C-H) = 99.5 kcal mole-', we arrive a t E(C--N+) = 67 kcal mole-'. This considerable stability of the formal C--N+ bond is no doubt related to the presence of the cyano groups which stabilize the structure through delocalization of CN / the negative charge. The stability of the -C- group \ CN is well known and, for example, accounts in part for the strengths of cyanocarbon acids." The stability of the structure I is further evidenced by the negative value for the pK, for the dissociation of its conjugate acid (-1.55); i.e., the protonated form is quite a strong acid which in turn implies considerable stability of the ylide. The thermochemistry of cyano-substituted hydrocarbons was discussed in paper I4 in terms of electrostatic repulsion between cyano groups and, where appropriate, the n-electron conjugation of a cyano group with the carbon-carbon double bond. A table was presented comparing the actual heats of formation with those expected in the absence of the cyano group and conjugation interactions. We present here a revised table (Table V) with the results of the present work incorporated. The calculated heats of formation of the reference structures (column 2) are computed from Franklin's12 group contributions and a value for the (9) Calculated from our present data and using hHr'[pyridine(g)] = 33.55kcal mole-': W.N. Hubbard, F. R. Frow, and G . Waddington, J. Phys. Chem., 65, 1320 (1961). (10) F. D. Rossini, et al., "Selected Values of Chemical Thermcdynamic Properties," U. S. National Bureau of Standards Circular 600, U. 9. Government Printing Office, Washington, D. C., 1952. This value of the C-H thermochemical bond energy is the average bond energy in methane and is used in simple bond energy schemes for hydrocarbons. Its use here therefore assigns any differences in the C-CN bonds in malononitrile and the ylide to the C--N bond in the ylide. (11) R. H. Boyd, J. Phys. Chem., 67, 737 (1963). (12) J. L. Franklin, I d . Eng. Chem., 41, 1070 (1949). +
is related to thermochemical bond energies as AH = E(C--N+)
+ E(H-H)
The Journal of Physical Chemiatry
- 2E(C-H)
(3)
THERMOCHEMISTRY OF CYANOCARBONS
2191
Table V : Stabilization Energies of Cyanocarbons
Compound
Malononitrile Succinonitrile Cyanogen Acrylonitrile Fumaronit'rile Tricyanoethylene Tetracyanoethylene Tetracyanoquinodimethane
(1)
(2)
AHf0(g), obsd
AHi'(g), calcd
63.5 53 73.6" 43.7b 81.3 123.9 168.5 184.0
53 48 58 44 76 107 140.6 205.1
(3) StabilizationC energy, (2)
- (1) - 10
-5 -6 0 -5 - 17 -28 21
(4)
(5)
Dipolwdipolec energy (calcd)d
Delocalization energy (calcdId (63 4)
-9 -4 -24 0 -3 - 19 -37 - 16
0 0 1 2 3 5 7 4 . O,!?'
-
(6) Sum (4)
+ (5)
-9 -4 -23 2 0 -14 -30
Reference 9. * H. S. Davis and 0. F. Weideman, I d . Eng. Chem., 37,482 (1945). The sign of the stabilization energy is taken to be positive when the actual molecule is more stable than the reference structure. For comparison with stabilization energy, dipoledipole energy is taken to be negative for a repulsive configuration contrary to usual usage. d The carbon-carbon bond resonance inis taken to be equal to 4. ' The value ,!? = 4 would be questionable here and the delocalization energy is reported in units tegral, ,!?, of ,!?. 0
group contribution of -CN of 29 kcal mole-' that we have selected. The latter is based on Evans and Skinner'slS data for propyl cyanide, isopropyl cyanide, and phenyl cyanide and data reported in Kharasch's comp i l a t i ~ n 'for ~ acetonitrile. I n passing, it may also be noted that the older data in Kharasch's compilation for malononitrile (AH" = 394.8 kcal mole-') and succinonitrile (AHo = 545.7 kcal. mole-I) (from Berthollet and Petit (1889)) are in remarkable agreement with our present results and a recent heat of combustion of succinonitrile.'5
Acknowledgments. The authors are indebted to the U. S. Army Research Office (Durham) for financial sup
port of this work. They are also very grateful to Professor R. C. Anderson of this department for furnishing the pyridinium dicyanomethylide and to Dr. G. J. Sloan of the Central Research Department, Du Pont Co., for furnishing the purified malononitrile. R. W. is grateful to the National Science Foundation for assistance from the Undergraduate Research Participation Program. (13) R. W. Evans and H. A. Skinner, Tram. Faraday SOC.,55, 256 (1959). (14) M. 8.Kharaaoh, J . Res. NdZ. Bur. Std., 2,359 (1929). (15) AH.,' = -546.22 f 0.035 kcal mole-': N. Rapport and E.
Westrum, Jr., private communication.
Volume 71, Number 7 June 1967
