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Further Comments upon the Electrophilic Addition to Alkynes A Response to Criticism from Professor Thomas T. Tidwell Hilton M. Weiss Department of Chemistry, Bard College, Annandale-on-Hudson, NY 12504 Professor Thomas Tidwell has responded (1) to my recent article (2) in this Journal, which asserted that “it is time for our textbooks to drop the (unstabilized) vinyl cation as the predominant intermediate in the electrophilic addition to alkynes.” He takes the alternate view and asserts that “electrophilic additions to alkenes and alkynes are actually very similar.” In response, I would like to point out that alkenes capable of forming relatively stable tertiary carbocations regularly follow this AdE2 route. Alkenes capable of forming secondary carbocations are borderline, with many additions occurring via the cation, whereas concerted mechanisms often predominate under other conditions. This combination of factors dictates that the AdE2 mechanism should be taught to undergraduates. Furthermore, the common synthetic problems associated with cationic intermediates should be pointed out here much as they are in the SN1 and E1 mechanisms. The use of the mercuric ion to catalyze alkene hydrations is often shown as a way to avoid such cationic rearrangement. However, the situation with alkynes is very different. Alkynes cannot form tertiary carbocations, and even the secondary carbocation is formed somewhat more easily than an unstabilized vinyl cation. Professor Tidwell has published extensive research supporting his contention that alkynes are protonated in aqueous sulfuric acid, and it is difficult to argue with his work. The resulting cations, however, do not show the rearrangement potential or stereochemical properties that undergraduate students are taught to associate with such ions. More significantly, the most common and most useful electrophilic additions to alkynes appear to follow an AdE3 mechanism involving nucleophilic attack upon an alkyne–electrophile complex. As with most concerted reactions, the initial products are unrearranged, stereoselective products. If one is to choose which mechanism to show undergraduates, the AdE3 mechanism seems to be the likely choice. Professor Tidwell points out that vinyl cations have been detected in NMR experiments. There is no question that vinyl cations can exist under extreme conditions such as the FSO 3H/SbF5 solvents used in such NMR experiments. In such solvents, extensive rearrangement has also been shown to occur (3). In anhydrous liquid HCl, tert-butyl acetylene also appears to form a cation and rearrangement products are found (4). Under normal reaction conditions, however, I can find no report of rearranged products coming from this alkyne. Conversely, the corresponding alkene, tert-butyl ethylene, is known to rearrange in practically all electrophilic additions, including HCl in acetic acid (5). The same dichotomy exists for 1-hexyne and 1-hexene (6). It becomes clear that vinyl cations can exist and that they can rearrange; however, no rearrangements are found in aque-
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ous sulfuric acid or under the conditions used in other electrophilic additions to alkynes. I believe Professor Tidwell’s evidence that vinyl cations do occur during acid-catalyzed hydration but their existence has little experimental significance in any reactions. The prevalence of anti addition also suggests that the AdE3 mechanism is dominant with alkynes. Ion pairing in a very nonpolar solvent should favor syn addition. A free vinyl cation arising from the protonation of an alkyne would also favor syn addition for steric reasons.
H C H3
C
C + -
H
preferred attack syn to H
In response to Professor Tidwell’s assertion that anti additions have not been found for terminal alkynes, we have recently added DBr to terminal alkynes and found that this addition is essentially 100% anti; we hope to publish these results soon. Finally, Professor Tidwell appears to believe that the AdE3 mechanism occurs by a simultaneous attack of electrophile and nucleophile at opposite sides of the π bond. While this possibility has been suggested, the probability of such a well-aligned, three-body collision seems remote. Although I would like to take credit for the π-complex mechanism given in my article, it was originally postulated by Fahey and Lee in their original paper (7) on the addition of HCl to 3-hexyne. In defense of the πcomplex between HCl and acetylene, it should also be pointed out that this complex has been isolated as a solid (8) and not merely found “in the gas phase”. This initial association obviates the need for the termolecular collision, and the subsequent nucleophilic attack upon the π-complex is presumed to have a transition state similar to that proposed by Professor Tidwell. In any case, this point is moot, since both mechanisms reject the intermediacy of the vinyl cation. With so much experimental evidence that can only be explained by the AdE3 mechanism, it seems fallacious to present vinyl cations as the predominant intermediate in electrophilic addition to alkynes. Literature Cited 1. Tidwell, T. J. Chem. Educ. 1996, 73, 1081. 2. Weiss, H. J. Chem. Educ. 1993, 70, 873–874. 3. Barry, B.; Beale, W.; Carr, M.; Hei, S.-K.; Reid, I. J. Chem. Soc. Chem. Commun. 1973, 177. 4. Griesbaum, K.; Rehman, Z. J. Am. Chem. Soc. 1970, 92, 1416–1418. 5. Fahey, R. C.; McPherson, C. J. Am. Chem. Soc. 1969, 91, 3865–3869. 6. Touchette, K.; Weiss, H.; Rozenberg, D. J. Chem. Educ. 1994, 71, 534–536. 7. Fahey, R. C.; Lee, D.-J. J. Am. Chem. Soc. 1967, 89, 2780–2781. 8. Mootz, D.; Deeg, A. J. Am. Chem. Soc. 1992, 114, 5887–5888.
Journal of Chemical Education • Vol. 73 No. 11 November 1996
