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The Thermodynamic Conjugation Stabilization of 1,3-Butadiyne Is Zero Donald W. Rogers,* Andreas A. Zavitsas, and Nikita Matsunaga Department of Chemistry and Biochemistry, Long Island University, Brooklyn, New York 11201 *
[email protected] Three generations of chemists have been taught that the configuration of two double bonds separated by a single bond CdC-CdC lends stability to a molecule relative to what we might expect from two isolated double bonds (1). There is good reason for us to believe this. In 1936, George Kistiakowsky (2) measured the enthalpy (heat) of hydrogenation of 1,3-butadiene H2 CdCH-CHdCH2 þ H2 f H3 CCH2 CH2 CH3 þ 57:1 kcal mol - 1 1, 3-butadiene n-butane
ð1Þ and compared it to twice the enthalpy of hydrogenation of 1-butene 2½H2 CdCH-CH2 -CH3 þ H2 f H3 CCH2 CH2 CH3 þ 30:1 ¼ 60:6 kcal mol - 1 1-butene n-butane
ð2Þ -1
and found a discrepancy of 3.5 kcal mol , which is well outside combined experimental error. Hydrogenation of the diene releases less than twice the heat output of the monoalkene; hence, it has less enthalpy (energy) than we might expect assuming its enthalpy to be a simple multiple of the smaller value. Kistiakowsky ascribed this difference to Pauling's resonance energies in compounds such as 1,3-butadiene that have conjugated double bonds (2). The molecular orbital picture widely used to illustrate this effect is shown in Figure 1, where the four p orbitals of 1,3butadiene become parallel and overlap, with delocalization of the electrons. Hence, one would expect that, with two sets of four overlapping p orbitals in 1,3-butadiyne, the thermodynamic conjugation stabilization would be considerably greater than the 3.5 kcal mol-1 of 1,3-butadiene, possibly even double that quantity. To our knowledge, no textbooks mention the apparently similar situation in 1,3-butadiyne. It was with some surprise that we found (3), as subsequently verified by several other groups (4), that the enthalpy of stabilization of 1,3-butadiyne is, within reasonable uncertainty, zero. Computational Thermochemistry Recently, computer hardware and software advances have not only verified, but in some cases corrected and even supplanted results from experimental thermochemistry (4d). In the present case, we have the following high-level ab initio computed enthalpies (Gaussian 03, ref 5) for comparison to the Kistiakowsky experiments. Enthalpies for molecules relative to isolated atoms are listed in Table 1.
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Figure 1. Overlapping parallel p molecular orbitals of 1,3-butadiene (left) and 1,3-butadiyne (right). Table 1. Enthalpies for Molecules Relative to Isolated the Atoms G3 Enthalpy/Eha
Molecule n-butane
-158.257932
1-butene
-157.046167
1,3-butene
-155.840332
1-butyne
-155.819234 -153.380838
1,3-butadiyne a
Eh = 627.51 kcal mol
-1
.
1,3-Butadiene Hydrogenation converts 1-butene into n-butane (for simplicity, only the carbon skeletons will be shown) CdC-C-C þ H2 f C-C-C-C and it also converts 1,3-butadiene into the same product state CdC-CdC þ 2H2 f C-C-C-C Subtracting the second reaction from twice the first reaction gets rid of problems associated with the computed enthalpy of formation of H2 and leaves 2ðCdC-C-CÞ - ðCdC-CdCÞ f C-C-C-C or, rearranging, 2ðCdC-C-CÞ f ðCdC-CdCÞ þ C-C-C-C Substituting the G3 enthalpies at 298 K values (5) into this equation, we get 2ð-157:046167 Eh Þ f - 155:840332 Eh þ ð-158:257932 Eh Þ
Δr H 298 ¼ - 0:00593 Eh ¼ - 3:72 kcal mol - 1 as compared to the experimental value of -3.5 kcal mol-1. The reaction we used in this calculation is an isodesmic reaction (Greek for equivalent bonds). In isodesmic reactions, the number of bonds and their hybridization is conserved on both
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sides of the thermochemical equation. If this isodesmic reaction had ΔrH298 = 0, we would have concluded that the system on the right has the same enthalpy as the system on the left, that is, that it is neither stabilized nor destabilized. In fact, the system on the right is -3.72 kcal mol-1 more stable than the system on the left. Following Kistiakowsky, we attribute this computed difference to resonance stabilization of the CdC-CdC conjugated system.
When examining trans-1,5-hexadiyn-3-ene, stabilization by flanking triple bonds is found; however, it is smaller than double -1 bond stabilization by about 4 kcal mol .
1,3-Butadiyne Comparable G3 calculations were carried out (3) for 1-butyne and 1,3-butadiyne (Table 1), yielding total enthalpies relative to the appropriate number of nuclei and electrons. If we carry out the same calculation as before but this time for the butynes, 2ð1-butyneÞ ¼ n-butane þ 1; 3-butadiyne
Δr H 298 ¼ - 0:004412 Eh ¼ - 2:77 kcal mol - 1
2ð-155:819234 Eh Þ f - 153:380838 Eh þ ð-158:257932 Eh Þ
Δr H
298
¼ - 0:000302 Eh ¼ - 0:19 kcal mol
- 157:046157 Eh - 117:777080 Eh f - 155:840332 Eh - 118:988320 Eh
Δr H 298 ¼ - 0:005415 Eh ¼ - 3:40 kcal mol - 1 CtC-C-C þ CtC-C f CtC-CtC þ C-C-C - 155:819234 Eh - 116:550504 Eh f - 153:380838 Eh - 118:988320 Eh
Δr H 298 ¼ 0:000580 Eh ¼ 0:36 kcal mol - 1 “Constructing” 1-buten-3-yne from 1-butene and 1-butyne and calculating its yne-ene stabilization energy, CdC-C-C þ CtC-C-C f CdC-CtC þ C-C-C-C - 157:046167 Eh - 155:819234 Eh f - 154:609033 Eh - 158:257932 Eh
we find some stabilization, but the result is considerably smaller than the ene-ene conjugation stabilization of -3.5 kcal mol-1. As early as 1991, Roth et al. pointed out, on the basis of experimental measurements (6), that the stabilization of the middle double bond in trans-1,3,5-hexatriene is substantial. By G3 computations (3b), we find a substantial stabilization energy: CdC-C-C-CdC þ C-C-CdC-C-C f
CtC-C-C-CtC þ C-C-CtC-C-C f CtC-CtC-CtC þ C-C-C-C-C-C
We find that, despite an apparent analogy to linear polyenes, the expectation of enhanced stabilization enthalpy in linear conjugated triple bonds is not supported by either thermochemical evidence where it exists or by high-level ab initio calculations. A possible rationalization for the lack of overall thermodynamic stabilization in polyynes is that stabilization of the overlapping p orbitals is counteracted by repulsions among the six electrons of each triple bond, which are too close together because the central C-C bond is only 1.37 Å long. An analogous situation is seen in the halogens. Bond lengths of diatomic halogens decrease in the sequence I2, Br2, Cl2, F2, all of which have three pairs of nonbonding electrons. One might anticipate that bond energies (indicative of bond strength) would increase in the same sequence. The expected sequence is found for I2, Br2, and Cl2 but not for F2, which is about 23.9 kcal mol-1 less stable than expected by extrapolation of the energies of the other three (7). Instability in F2 is usually ascribed to crowding of electrons within the confines of the short F-F bond. We ascribe the relative instability of diynes to the same sort of crowing in the shorter bonds of -CC-CC- relative to the corresponding dienes.
Although conjugative interaction may, and probably does, exist between the triple bonds in 1,3-butadiyne in terms of transmission of electronic effects, the net result of all the molecular energies in the 1,3-butadiyne molecule and of similar conjugated polyynes leads to a negligible thermodynamic stabilization compared to that of the corresponding polyenes. Acknowledgment
CdC-CdC-CdC þ C-C-C-C-C-C - 234:376074 Eh - 235:588852 Eh f - 233:178717 Eh - 236:797193 Eh
Δr H 298 ¼ - 0:010984 Eh ¼ - 6:89 kcal mol - 1
Vol. 87 No. 12 December 2010
Stabilization of the triple bond in 1,3,5-hexatriyne in a reaction that links up three isolated triple bonds to form the system of three conjugated triple bonds is similarly small
Conclusion
Δr H 298 ¼ - 0:001564 Eh ¼ - 0:98 kcal mol - 1
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- 231:919032 Eh - 235:588852 Eh f - 230:715103 Eh - 236:797193 Eh
Δr H 298 ¼ - 0:004630 Eh ¼ - 2:91 kcal mol - 1
CdC-C-C þ CdC-C f CdC-CdC þ C-C-C
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CtC-CdC-CtC þ C-C-C-C-C-C
- 231:919032 Eh - 235:588852 Eh f - 230:715103 Eh - 236:797193 Eh
-1
the conjugation stabilization enthalpy has nearly disappeared. Another pair of isodesmic reactions leads to more-or-less the same conclusion:
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CtC-C-C-CtC þ C-C-CdC-C-C f
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D.W.R. acknowledges the National Center for Supercomputing Applications and the National Science Foundation for allocation of computer time and the H. R. Whiteley Foundation of Friday Harbor Laboratories, University of Washington, for research time and facilities contributing to this work. Support
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from the Intramural Research Program of Long Island University is also acknowledged. 4.
Literature Cited 1. (a) Ege, S. Organic Chemistry: Structure and Reactivity, 3rd Ed. Heath, Lexington MA, 1994. (b) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry. Structure and Function, 5th ed. Freeman: New York, 2007. (c) Carey, F. A. Organic Chemistry, 7th ed., McGrawHill, New York: 2008. (d) Smith, M. B.; March, J. Advanced Organic Chemistry, 5th ed., Wiley-Interscience: New York, 2001. 2. Kistiakowsky, G. B.; Ruhoff, J. R.; Smith, H. A.; Vaughan, W. E. J. Am. Chem. Soc. 1936, 58, 146–153. 3. (a) Rogers, D. W.; Matsunaga, N.; Zavitsas, A. A.; McLafferty., F. J.; Liebman, J. F. Org. Lett. 2003, 69, 7143–7147. (b) Rogers, D. W.;
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5. 6. 7.
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Matsunaga, N.; Zavitsas, A. A.; McLafferty, F. J.; Liebman, J. F. J. Org. Chem. 2004, 69, 7143–7147. (a) Jarowski, P. D.; Wodrich, M. D.; Wannere, C. S.; Schleyer, P.; von, R.; Houk, K. N. J. Am. Chem. Soc. 2004, 126, 15036–15037. (b) Cappel, D.; Tullmann, S.; Krapp, A.; Frenking, G. Angew. Chem., Int. Ed. 2005, 44, 3617–2620. (c) Simmonett, A. C.; Schaefer, H. F., III; Allen, W. D. J. Chem. Phys. 2009, 130, 044301. (d) Bond, D. J. Org. Chem. 2007, 72, 5555–5566. Gaussian 03, Revision E. 01. Frisch, M. J., Pople, J. A. et al. , Gaussian, Inc., Wallingford CT, 2004. Roth, W. R.; Adamczak, O.; Breuckman, R.; Lennartz, H.-W.; Boese, R. Chem. Ber. 1991, 124, 2499–2521. Ebbing, D. D.; Gammon, S. D., General Chemistry, 6th ed., Houghton Mifflin Co.: Boston, MA, 1999.
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