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The Electrophilic Addition to Alkynes Revisited Thomas T. Tidwell Department of Chemistry, University of Toronto, Toronto, ON M5S 1A1, Canada
A recent article in this Journal entitled “The Electrophilic Addition to Alkynes” (1) concludes with the statement “In the face of all this evidence, it is time for our textbooks to drop the (unstabilized) vinyl cation as the predominant intermediate in the electrophilic addition to alkynes.” It is argued instead “that a termolecular mechanism predominates. This may occur in one step or via an initial complex of the alkene with the electrophile.” The discussion actually centers on additions of HCl and HBr, and was illustrated by the figure below (Fig. 1).
X
δ– H
H–C≡C–R HX
H δ+
X–
R C
C
H
X
H–C≡C–R Figure 1. The AdE3 mechanism for the addition of HX to simple alkenes.
The evidence referred to in support of the mechanism in the Figure would appear to be the following: 1. There are qualitative differences between the reactivities of alkenes and alkynes, and since the former are agreed to react by rate-limiting proton transfer to carbon with carbocation formation, alkynes react by some other path. 2. Rearrangement is much less common in additions of protic acids to alkynes, compared to alkenes. 3. The reactions of alkynes with HBr and HCl in some cases are dependent on [HBr]2 or [HCl]2, and give predominantly or exclusively anti addition. 4. Vinyl cations are particularly unstable and are probably intermediates only under forcing conditions or when stabilized by an α-heteroatom or α-phenyl group. 5. The structure of a π-complex between HCl and 2butyne has recently been reported. It is concluded that “simple alkynes” (meaning alkyl substituted alkynes) and allenes utilize the mechanism of the figure.
This article highlights the fascinating chemistry of vinyl cations and alkyne additions. The AdE3 mechanism (or ANAHDxh mechanism in the IUPAC Recommendations for the Representation of Reaction Mechanisms) (2) is in fact currently accepted for addition reactions of 1,2dialkyl alkynes (3) (and alkenes) with HCl and HBr in acetic acid (3b). However, the process shown in Figure 1 is not the accepted representation of this mechanism, and the arguments given (1) are in many cases invalid and could lead textbook writers and students into the mistaken impressions that there is evidence that the complex shown is the principal intermediate or transition state in the solution-phase electrophilic addition to
alkynes, that vinyl cations are rare and difficult to form, and that there are major differences in the reaction mechanisms of alkynes and alkenes with electrophiles. The specific arguments cited above are considered below, in turn. First, contrary to the claim made in reference 1, many of the main features of electrophilic additions to alkenes and alkynes are actually very similar. Thus a comparison of the rates of acid-catalyzed hydrations of alkenes and alkynes shows a linear free energy correlation between the two, and this has been interpreted in terms of similar carbocation mechanisms for each (3a). The evidence adduced for AdE3 additions is very similar for both alkenes and alkynes (3b, 3c), and there is no suggestion of a difference between the two. Second, the energetics of rearrangement in vinyl cations have been examined by molecular orbital calculations (4), and it is found (4b) that even though the me+ thyl vinyl cation ( CH3C=CH2) is 9.8 kcal/mol less stable + than the isomeric allyl cation (CH2CH=CH 2) there is a 19.6 kcal/mol barrier to their interconversion by a simple 1,2hydride migration. By contrast, there is no barrier to hydride rearrangements in the ethyl and 2-butyl cations, as the hydride bridged structures are more stable than the classical open carbocations (4c, 4d). Thus there are barriers to migrations in vinyl cations, and an absence of arrangement is not evidence against the presence of vinyl cations. The theoretical arguments indicate that hydride bridging in alkenyl cations such as C2H3+ is less likely than for the analogous C2H5+ or the 2-butyl cation, which is the opposite of the premise of the first point. Third, alkenes also sometimes give anti addition of HBr and HCl in reactions that are second order in electrophile, and there is no fundamental difference in the behavior of alkenes and alkynes (3b, 3c, 5a). The occurrence of highly selective anti additions is not general, and there are also many examples where there is not a strong preference for anti addition. The evidence for the AdE3 mechanism (anti addition and a kinetic term in [HX]2) has been interpreted (5a) in terms of formation of a rate-limiting transition state as shown in eq 1, and
R R–C≡C–R
2HX
X C
X
C
H
R
R
X C
H
H
(1)
C R
not the representation in the figure. This evidence has been found only for disubstituted alkynes or alkenes, both for the practical reason that the stereochemistry is more difficult to establish for a terminal alkyne as de-
Vol. 73 No. 11 November 1996 • Journal of Chemical Education
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picted in the figure, and also because carbocation formation is more likely in such terminal alkynes, and the AdE3 mechanism is less likely to be competitive. Fourth, vinyl cations are not prohibitively destabilized relative to alkyl cations, as shown by gas phase isodesmic energy comparisons such as that in eq 2 (5b). In many cases the reactivity of alkynes may exceed that of the corresponding alkenes (3a, 5a).
+
CH3C=CH2 + CH3CH=CH2
Literature Cited
∆H° = –3 kcal/mol
(2)
+
CH3C≡CH + CH3CHCH3
Fifth, the finding of an HCl/acetylene π-complex in the gas phase does not establish this as an intermediate that lies on the reaction coordinate for solution phase electrophilic addition. Such complexes may occur in solution, but would occur before the rate-limiting transition state and would be difficult to establish experimentally. The question of whether such π-complexes occur as energy minima prior to the rate determining transition state is a subtle one that is not easily answered, and even graduate-level textbooks on organic reaction mechanisms usually do not delve into these questions. This is a valid scientific problem, but not one that is likely to be profitably pursued in a beginning course.
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Vinyl cations are well recognized intermediates that in a number of cases may be directly observed by NMR (6), and their formation has been proposed even for protonations of allene (7a) and acetylene itself (7b). Emphasis on these fascinating and well established species (8) is more likely to capture the imagination of students than complex and questionable arguments that at best only apply in a few specialized cases.
1. Weiss, H. J. Chem. Educ. 1993, 70, 873–874. 2. (a) Commission on Physical Organic Chemistry, IUPAC. Pure Appl. Chem. 1989, 61, 23–56; (b) Guthrie, R. D.; Jencks, W. P. Acc. Chem. Res. 1989, 22, 343–349. 3. (a) Allen, A. D.; Chiang, Y.; Kresge, A. J.; Tidwell, T. T. J. Org. Chem. 1982, 47, 775–779; (b) Fahey, R. C.; Monahan, M. W. J. Am. Chem. Soc. 1970, 92, 2816– 2820; (c) Fahey, R. C.; Lee, D.–J. J. Am. Chem. Soc. 1968, 90, 2124–2131. 4. (a) Radom, L.; Hariharan, P. C.; Pople, J. A.; Schleyer, P. v. R. J. Am Chem. Soc. 1973, 95, 6531–6544; (b) McAllister, M.; Tidwell, T. T.; Peterson, M. R.; Csizmadia, I. G. J. Org. Chem. 1991, 56, 575–580; (c) Carneiro, J. W. de M.; Schleyer, P. v. R.; Koch, W.; Raghavachari, K. J. Am. Chem. Soc. 1990, 112, 4064–4066; (d) Klopper, W.; Kutzelnigg, W. J. Phys. Chem. 1990, 94, 5625– 5630. 5. (a) Schmid, G. H. “Electrophilic Additions to Carbon-Carbon Triple Bonds”; In The Chemistry of the Carbon–Carbon Triple Bond, Part 1; Patai, S., Ed.; Wiley–Interscience: New York, 1978; (b) Mayr, H.; Gonzalez, J. L.; Lüdtke, K. Chem. Ber. 1994, 127, 525–531; (c) Tidwell, T. T. Angew. Chem. Int. Ed. Engl. 1984, 23, 20–32. 6. (a) Siehl, H.-U.; Kaufmann, F.-P.; Hori, K. J. Am. Chem Soc. 1992, 114, 9343– 9349; (b) Siehl, H.U.; Müller, T.; Gauss, J.; Buzek, P.; Schleyer, P. v. R. J. Am. Chem. Soc. 1994, 117, 6384–6387. 7. (a) Cramer, P.; Tidwell, T. T. J. Org. Chem. 1981, 46, 2683–2686; (b) Lucchini, V.; Modena, G. J. Am. Chem. Soc. 1990, 112, 6291–6296. 8. Nefedov, V. D.; Sinotova, E. N.; Lebedev, V. P. Russian Chem. Rev. 1992, 61, 283–296.
Journal of Chemical Education • Vol. 73 No. 11 November 1996
