The Electrophilic Addition to Alkynes Hilton M. Weiss Bard College, Annandale-on-Hudson,NY 12504 Electrophilic addition to alkenes has been studied exhaustively and the carbocation mechanism explains most material quite well. Rates of reaction, regiochemistry, stereochemistry, rearrangements, side products, etc. are easily acwunted for in most introductory organic chemistry books. Electrophilic addition to akynes, however, usually gets less attention and often is introduced with statements such as 'The triple bond reacts with HC1 and HBr in much the same manner as does the double bond" (1)or "Addition Figure 1. The vinyl cation. of acids like the hydrogen halides is electrophilic addition, and it appears to follow the same mechanism with alkynes ing. electrons to overlap effectivelvwith the cationic center. as with alkenes, via an intermediate carbonium ion" (2). he 120' bond angle aiso may make the overlap less effecThe latter textbook (3)goes on to explain that the "unstative. This vinvl cation is therefore ex~ectedto be "about as ble vinvlic cations" can be formed because thev are orostable as a mkthyl cation" (8).~roton'ationof a substituted duced &om generically unstable alkynes. This "same text alkyne can produce only a monosubstituted vinyl cation (4) lists the heterolvtic bond dissociation enerw of vinvl that should be "about as stable as aprimary akyl cation" halides to be 15 ~ c a k m o l greater e than those &he cork(8).Neither methyl nor primary carhocations are formed sponding ethyl halides and ethyl cations are rarely postueasily, and they are postulated rarely in reaction mechalated. Since the heat of hydrogenation of 1-hexyne is only nisms. Similarly, vmyl cations are intermediates 9 Kcallmole greater than the heat of hydrogenation of 1,5only undcr forcing condit~onsor when stabilized bv an uhexadiene, alkyne instability cannot justify the formation heteroatom or a-phenyl group. of vinyl cations. ' If the vinyl cation is not involved in the electro~hilic adThe same textbook (3)also notes that the addition ofHC1 ditions of alkynes, what mechanism is operati;e? Many to 3,3-dimethyl-1-butyne gives rearranged products a s studies (9) on these reactions have lead to the idea that a might be expected from a cationic intermediate. This retermolecular mechanism predominates. This may occur in sult (51,however, was obtained when the reaction was perone step or via an initial complex of the alkyne with the formed in anhvdrous liouid HC1 under nressure and - - is -~ ~ a -~ ~ electrophile. In either case, the electrophile is presumed to rare exceptionto the rut that electrophiiic additions to albond more strongly to the less substituted end of the rrkynes give unrearranged products. This generalization is bond, leading to the observed Markovnikov addition. This never mentioned explicitly in spite of many examples result, along with the anti addition and the lack of rearwhere a vinvl cation mieht be exoected to rearranee.' rangements, are reminiscent of the omercuration reacAnother textbook (6)Gotes that, in the addition i f HX to tion that is commonly ( l 0 , l l ) shown to proceed through a alkynes, "Trans stereochemistrv of H and X is normallv rr-complex. A rr-complex between HC1 and &butme re(though not always) found in the product!'. In spite of thfs centlyhas been isolated (12) and its structure detlrmined stereoselectivity, this book shows the reaction proceeding by X-ray crystallography. through a "vinylic cation". It should be clear that either version of this mechanism All textbooks seem to agree that vinyl cations are partic(Fig. 2) can explain all of the results described above. This ularly unstable species yet they are presented invariably mechanism probably predominates in all cases where the a s intermediates in these reactions. The rationale for vipotential carbocation is less stable than a secondary alkyl nylic cation instability also seems varied. One book (7) notes that its 'vacant p-orbital belongs to an sp-hybridized cation. Additions to simple alkynes, allenes (and probably carbon rather than to a n so2-hvbridized carbon. Since a " carbon that has sp-hybridizat~onis more electronegative 'This author could find no examples of vinyl cation rearrangement than one havina it is less tolerant of the -sv2-hvbridization. . " in the addition of electrophiles to alkynes except in cases where a positive charge." I t is generally agreed that an sp-orbital nucleophile would have been required to attack the "protonated al(lower enerm, closer to the nucleus) is more electroneaakyne" adjacent to a quaternary carbon. Such steric hindrance to the tive than an-sp2-orbital but that the electronegativity of& Ad,3 may be a requirement to the formation of vinyl cations by this route. orthoponal D-orbital is relativelv unaffected bv atom hvbridizition.. A better rationale (8)for vinyl cation instability derives, "at least in part, (from) the lack of stabilizing hvperconiupative interactions with .. .R HX neighboring groups, such as oceurs H-c-c-~ / in alkyl cations." The vinyl cation is X assumed t o have t h e structure H-C=C-R shown in Figure 1, and the sp2carbon orbitals lined up with the vacant p-orbital are presumably too electronegative to allow their bond- Figure 2. The Ad~3mechanism forthe addition of HX to simple alkynes. ~
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Volume 70 Number 11 November 1993
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to ethylene) utilize this mechanism under most conditions and even substituted alkenes will follow this mechanism in nonpolar solvents (13).Fadors influencing the choice between the bimolecular or termolecular mechanism are discussed in reference 9a. 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. Literature Cited 1. Shitwieser, A,; Heathmck, C. H.:Kosower. E. M, hfmduelion to Organic ChemisLW. 441 hd.;MsemiUsn: New Y&, 1 9 8 2 , 312. ~
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Journal of Chemical Education
2. M O ~8 . T.: ~ ~ ~w d,R., N. Organic Chemlst~,6th ed.; ~ e n t i e + n ~ l Englewood l: Cliffs, NJ, 1992.0 434. ReL2, p435. 4. ~ ~ f .p222. . 5. ~ ~ ~ b ~ e h ~m a n~,~ ~ .~ h them. , .~sot. .IWO,%, ; 14161418. 6. MeMuny, J. OrgonicChomrstry. 3rded.:Bmo~iCole:PadficG~~ve,CA, 1992,p259. 7. Ref I, p 313. 8. n,f, 6 , , 260. 9. ( a j ~ a h e y ,c~. .; k e ,D.J. J A ~ c. h m sot. 1~1,89,2780-2781.(blFahey.8. C.; Lee,D.J.ilAm.Chm. Soc.lPB8,90.21262131. 10. Ref. 2, p 347. 11. Ref 6, p 223. 12. ~ m t zD.; , ueeg,J ~ Am. chm. sot 1 m . 1 1 4 . 5 8 8 ~ ~ ~ 8 8 8 . 13. Fahey R. C.; Monahan, M. W. J. Am. Chm. Soc 1970,92,28162820, Boyd, R. H.: TaR,R. W.:Wolf, A.P;Ch%tmtmn,D. R. J.Am C h . Soc 1960.82.4729-4736.
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