106
F. E. ROGERS
Thermochemistry of the Diels-Alder Reaction. 11. Heat of Addition
of Several Dienes to Tetracyanoethylene by F. E. Rogers Department of Chemistry, University of Dayton, Dayton, Ohio 46409
(Received J u n e SO, 1971)
Publication costs assisted by The University of Dayton
The heat for the Diels-Alder addition of tetracyanoethyleneto 2,3-dimethyl-l,3-butadiene, 1,3-~yclohexadiene, cyclopentadiene, and anthracene has been determined by solution calorimetry. Corrections for heats of solution and vaporization gave the hest of the gas phase reaction at 25". Comparison is made with the analogous ethylene reactions.
Introduction
Dimethyl-1,3-butadiene (K and K Laboratories) showed two impurities on glpc analysis which were We have been interested in the thermochemical removed by slow distillation through a 16 theoretical aspects of the Diels-Alder (D-A) reaction and recently plate fractionation column. reported on the heat of addition of tetracyanoethylene The adducts were prepared according to the pre(TCNE) to isoprene.' The heat of addition was viously reported procedure. Except for the anthradetermined for the solution, standard state, and gas cene adduct, each was further purified by two phase reaction, and comparison was made with the sublimations. The anthracene product was recrystalanalogous ethylene reaction. This initial work suglized twice from dichloromethane. The melting point, gested that the heat of any D-A reaction of TCNE nmr spectrum, and elemental analysis of the adwill approximate the value for the corresponding ducts are as follows. 1 ,Z-Dimethy1-4,4,5,5-tetracyanoethylene reaction. This conclusion has been investicyclohexene, mp 124.5" (lit.2 mp 136-137"), nmr: CH3, gated further, and we now wish to report the heat of addition of TCKE to 2,3-dimethyl-1,3-butadiene, s, 1.756, 6H; CH2, s, 3.006, 4H. Anal. Calcd for C12HloN4: C, 68.55; H, 4.79; N, 26.65. Found: C, 1,3-cyclohexadiene, cyclopentadiene, and anthracene 68.71 ; H, 4.89; lX, 26.60. 2,2,S,S-Tetracyanobicycloto form the following adducts. [d.Z.I]heptene,mp 215-217" dec (lit.2 mp 223"), nmr: CH2, S, 2166, 2H; CH, t, 4.276, 2H; =CH, t, 6.786, 2H. Anal. Calcd for C11HeN4: C, 68.03; H, 3.11; N, 28.86. Found: C, 68.09; H, 3.14; N, 28.77. I I11 d,d,S,S-Tetracyanobicyclo[b.Z.Z]octene,mp 264.5-266" (lit.z mp above 300"), nmr: CH2, m, 1.336, 4H; CH, m, 3.436,2H; =CH, m, 6.226, 2H. Anal. Calcd for C&AT4: C, 69.21; H, 3.87; N, 26.91. Found: C, 69.22; H, 3.93; N, 26.78. 9,lO-Dihydro-9,10-tetracyanoethanoanthracene, mp 11 IV 266" dec (lit.2 mp 268-270"), nmr: CH, S, 5.956, 2H; =CH, m, 7.656, 8H. Anal. Calcd for C ~ O H I O N ~ : Corrections for the heats of solution and vaporC, 78.41; H, 3.29; N, 18.29. Found: C,78.42, H, ization gave the heat of the Diels-Alder reaction in 3.29; N, 18.30. These compounds were used in the the solution, standard state, and gas phase. The heat of solution determinations. heats of formation of the adducts were calculated and The addition reactions were run a t 25.0 f 0.5" in a examined in terms of strain energy. solution calorimeter by previously described techExperimental Section niques.' The reactions are quantitative and complete The TCNE (Aldrich Chemical Co.) was sublimed in less than 10 min when an excess of one component and stored in amber bottles under nitrogen. It wm is used. To compensate for the slow dimerization sublimed again immediately before using. Anthracene reaction of cyclopentadiene both reference and reaction (Aldrich Chemical Co., puriss, 99.9%) was used without further purification, Cyclohexadiene (Aldrich (1) F. E. Rogers, J . Phys. Chem., 75, 1734 (1971). Chemical Co.) showed only one component on glpc (2) W. J. Middleton, P. E. Heckert, E. L. Little, and C. G. Krespan, analysis suitable for detecting 0.1% impurity. 2,3J . A m e r . Chem. Soc., 80, 2783 (1958). The Journal of Physical Chemistry, Vol. 76, N o . 1, 1978
HEATO F
ADDITION O F
SEVERAL DIENESTO TETRACYANOETHYLENE
107
~~~
Table I : Heats of Solution, Vaporization, and Reaction (kcal/mol) r------AHeoln Addenda
TCNE 2,3-Dimethyl-1,3butadiene Cyclopen tadiene 1,3-Cyclohexadiene Anthracene
(25.0
5.60 f 0.13 (0.13225)O 0.25 ( 0 .450)a 0.1 ( 0 .080)a 0.1 (0.080)" 5.99 f 0.26 (0. 1724)a
* 0.5°)----
AHv (T, 'C)---
7 -
Adduct
Addends
Adduct
AH.(exptl)'
19.4 fl , O b 3.21 f 0.04 (0.07209)" 4.28 f 0.15 (0.11003)" 3.58 i.0.18 (0.10046)" 2.76 i 0.15 ( 0 .10250)a
7.4"
25.8 f 1.0" (105) 28.0 It 1 . 3 e (135) 26.7 f 1 . 3 e (160)
6.SC 7.9d
-41.96 f 0.27[D] -21.28 f.0.55[T] -30.8 i 0.46[D] -12.92fO.lO[T]
a Average amount of component (grams) injected into 200 ml of dichloromethane. * R. H. Boyd, J. Chem. Phys., 38, 2529 (1963). 1). R. Stull, E. I?. Westrum, Jr., and G. C. Sinke, "The Chemical Thermodynamics of Organic Compounds," Wiley, New York, N. Y., 1969. Estimated from Trouton's rule using 22.47 as constant. e Estimated uncertainty. These values were used without correction to 25". Heat of reaction when bracketed component is added to dichloromethane solution of other component: [TI = TCNE, [D] = diene. c
'
Table I1 : Heats of Diels-Alder Reaction and Heat of Formation of Adducts (kcal/mol) Adduct formed
AHrO(aoln)
- 42 Is.
AHr'
AHr0(d
-39.57f0.44
-40.57zk2.4
"J
-39.65
.28.90 f 0.77
IIa.
&
111.
-32.9
-26.88f0.68
&
-29.5 f 3 . 1
r---AHfo Exptl
138.7 -16.4
164.3
4.88"
(g)----
Est.
127.1 -19.9
Strain energy
11.6 3.5
139.4
25.0
-7.66
12.5
-25.56 f 0.70
-23.8 f 3 . 0
176.6
144.5
32.1
-22.6
-23.8
20.6
-2.53
23.1
-9.69f0.50
(-10)
(CNh
(CN),
IV* a
-18.52 i 0.23
S. S. Wong and E. F. Westrum Jr., J. Amer. Chem. SOC.,93, 5317 (1971).
calorimeter contained the same amount of the diene. The reactions were initiated by the injection of about 0.5 mmol of one component into 200 ml of dichloromethane containing a five- to tenfold excess of the other component. The results in Table I are the average of a t least two runs. With the exception of the anthracene adduct which decomposes, the vapor pressures were determined by the McLeod gauge method previously described.
The vapor pressures (vp) were measured over a 20-30" temperature range at 4 or 5" intervals. For low vp measurements, 30-100 p , an inert gas pressure of 200-250 p was used; for vp's above 100 p the inert gas pressure was increased to ca. 500-600 p. The heat of sublimation was calculated in the usual manner from the least-squares slope of a plot of log (vp) against 1/T. The results appear in Table I. The heat of solution was determined by the injection The Journal of Physical Chemistrg, Vol. '76, N o . 1 , 19759
F. E. ROGERS
108
of three successive portions of the diene, dienophile, or adduct into 200 ml of dichlsromethane at 25.0 f 0.5". Each injection represents the approximate amount involved in the Diels-Alder reaction. The three runs were averaged and reported in Table 11.
Discussion The heat of the Diels-Alder reaction in the solution, standard state, and vapor phases, was calculated according to the following cycle. diene(g)
+
*H,f
AHro(d
TCKE(g) --+
adduct(g)
AH4J
TCNE(s)
AH,'
4
adduct(s)
+ TCNE(so1n) -+adduct(so1n) AH,(soln)
When TCNE was added to the reaction solution AH,(exptl> = AH,"(soln) AH^; when the diene AH4. In was injected AH,(exptl) = AH,(soln) the former case the AH,' = AH,(exptl) - AH6 AH4 and AHro(g) = AH,' AH3 - AH1 - AHz. Similar expressions can be derived for the case when the diene is injected to initiate the reaction. These results are shown with the analogous D-A reaction of ethylene in Table 11. The instability of the anthracene made it impossible to determine the heat of sublimination but the closeness of the gas and standard state heats of addition for the other three reactions suggest a value of -10 kcal/mol. In comparison to the "normal" D-A reaction which forms a monocyclic product, the decrease in AHro(g) on going down the table parallels the increase in strain energy in the products. In addition to ring strain the anthracene adduct forms at a cost of 11.3 kcal/mol resonance energy in anthracene, combining to make this reaction the least exothermic of the group. How closely the reaction heat [AH,' (g) ] follows the increase in ring strain can be seen by comparing the increase in ring strain of the analogous hydrocarbons (last column, Table 11) with the difference in AHr"(g).
+
+
AIAHr'k)
- 9.0 (Ia-IIa) - 19.6 (Ia-IIIa)
I
- 11.1 (1-11)
- 16.8 (1-111) CU.
-30.6 (I-IV)
The heats of addition for the reference reactions of ethylene were calculated from the heat of formation It was necessary, hoxever, data of Cox and P i l ~ h e r . ~ to estimate the heats of formation of dimethylcyclohexene (Ia) and norbornene (IIIa). Benson's group increment scheme6gives - 16.5 kcal/mol for the former, and the structural relationship below also indicates the same value. AHf'k):
[ -1.8G
0 ; y'OJ -1.08
-10.12
-10.34
-16.42
The Journal of Physical Chemistrg, Vol. 76, N o . 1, 2972
&
+
4
AH = -0.9 kcal/mol (1)
~~~~~~
+
TCNE
+
+
TCNE
+
+
+
A(strain energy)
TCNE
H2C=CH2
JA H e
AH5J
diene(so1n)
+
f AH8
AHz?
diene(1)
Combining a recent value for the gas phase heat of formation of norbornane, - 12.42 kcal/molle and the heat of hydrogenation of norbornene, - 33.0 kcal/mol gives 20.6 kcal/mol for the heat of formation of norbornene. The reported value of 24.7 kcal/mo17 for this compound was based on a high estimate for the heat of sublimation which has sincc been measured. Comparison of the TCNE and ethylene reaction with the same dienc is equivalent to subtracting one reaction from another. The resulting equations are
-1G.4(cst.)
n HIC=CHz
AH = 3.4 kcal/mol(3)
i ') : :&
These disproportionation reactions may be represented as ae
+d
e
+ ad
AH
=
AH,'(g) - AHre(g)
where e = ethylene, d = any other dienophile, ae and ad the corresponding adducts, and AHre(g) the heat of addition for the analogous ethylene reaction. When d is an acyclic dienophile, the number of rings in ae equals the number in ad. For cyclic dienophiles the number in ad is larger than in ae by the number of rings in d. In the usual type of rings formed in the D-A reaction we may assume to a first approximation that ring strain is additive. Therefore the difference AHre - AH," will be largely determined by the relative behavior of the dienophiles e and d in losing a double bond. The similarity between the D-A reaction and hydrogenation (3) Taken as the difference in the heat of hydrogenation of anthracene to 9,lO-dihydroanthracene (- 17.1 kcal/mol) and cyclohexene
t o cyclohexane (-28.4 kcal/mol). Heat of formation data from ref 4. (4) J. D. Cox and G. Pilcher, "Thermochemistry of Organic and Organometallic Compounds," Academic Press, New York, N. Y.,
1970. (5) S. W. Benson, "Thermochemical Kinetics," Wiley, New York, N. Y . , 1968. (6) P. v . R. Schleyer, J. E. Williams, and K. R. Blanchard, J . Amer. Chem. SOC.,92, 2377 (1970). (7) R. B. Turner, P. Goebel, B. J. Mallon, W. von E. Doering, J. F. Coburn, Jr., and M. Pomerantr, {bid., 90, 4315 (1968).
HEATOF ADDITIONOF SEVERAL DIENESTO TETRACYANOETHYLENE H2
+e
-
eH2
HB+d=dHz e
+ dH2
eH2
A'" A"
+ d, AHH' - AHH
allows the useful approximation that
- AHH'V -32.8 - A"
AHre(g) - AHro(g) 15: and AHro(g)
=
AHre(g)
+ AHH + 32.8
the equation which fits reactions 1, 2, and 3 (above) 1.3 f 2.2. This equation is AHro(g) = AHre(g) suggests a small difference in the heat of hydrogenation of ethylene and TCNE. This equation can be applied to the D-A reaction of cyclopentadiene with itself and ethylene.
+
From Table 11, AH,' = -23.5 kcal/mol and the heat of hydrogenation of cyclopentadiene to cyclopentene (A") calculated from the heats of formation is -23.7 k ~ a l / m o l . ~Substitution gives AHro(g) = - 15 com-
109
pared with a literature value of -17.0 i1.1.* If as Table I1 suggests, AH,' - AH," is essentially constant for a series of D-A reactions then the estimation possibilities are broadened considerably. Further work in this area is underway. The heats of formation of the TCNE adducts were calculated from the heats of formation of the reactants and AH,"(g) are shown in Table 11. The strain energy in a molecule is generally assigned as the difference between this experimental value and the heat of formation predicted on the basis of some empirical scheme. We have adopted the olefinic increment values of Benson5 and the "single conformation" values of the saturated CHI, CH2, CH, C groups from Schleyer.6 The predicted heats of formation and the strain energy are given in Table 11. Since the value for the C(CN)2 group (68.4 kcal/mol) was calculated from malononitrile, it contains the nonbonded interaction energy of two geminal cyano groups. Tetracyanodimethylcyclohexene (I), tetracyanobicyclooctene (11), and tetracyanobicycloheptene (111) are more strained than the parent hydrocarbon by 8, 12.5, and 9 kcal/mol, respectively. This strain is assigned to the repulsive interaction of two adjacent C(CN)2 groups and agrees closely with the predicted value of 10 kcal/moL1 (8) A. Wasserman, "Diels-Alder Reaction," Elsevier, Amsterdam, 1965. (9) Calculated by the formula AH[>C(CN)z] = AHr[CHz(CIV)21 AH[C-(H)2(C)2], where A H f [CH2(CN)z]is 63.5 andAH[C-(H)z(C)zl is -4.95 kcal/mol.
The Journal of Phgsical Chemistry, Vol. 76, N o . 1, 1972
