Gas phase surface-catalyzed HCl addition to vinylacetylene: motion along a catalytic surface. Experiment and theory q Linda M. Mascavage a , Fan Zhang-Plasket b, c , Philip E. Sonnet b , David R. Dalton b, * a Department of Chemistry, Arcadia University, Glenside, PA 19038, USA b Department of Chemistry, Temple University, Philadelphia, PA 19122, USA c Process Analytical Technology, Merck and Co., Inc., WP78-110, PO Box 4, West Point, PA 19486-0004, USA article info Article history: Received 8 April 2008 Received in revised form 2 July 2008 Accepted 18 July 2008 Available online 24 July 2008 Keywords: Electrophilic addition Alkenes Alkynes FTIR abstract Gaseous mixtures of HCl and vinylacetylene were permitted to react in Pyrex IR cells (NaCl windows). Gaseous 4-chloro-1,2-butadiene and 2-chloro-1,3-butadiene (chloroprene) were the major products. Kinetic data (FTIR) generated a rate expression in concert with surface catalysis. Computational studies involving surface associated water provide a view that accounts for the experimentally determined or- ders and a bifurcated pathway producing both products. The results are in accord with wall-adsorbed reactant(s) as well as previously reported computational studies on the reactants. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction The addition of hydrogen halides to alkenes and alkynes has served as a paradigm for electrophilic additions in general since the dawn of organic chemistry. 1 Although it is commonly held that electrophilic addition to alkenes is the more facile 2 (a position which may be changing 2f ), ratios of the relative rate constants for various electrophilic addition reactions to unsaturated pairs of equally substituted alkenes and alkynes (k ene /k yne ) vary from 10 8 to 10 2 , apparently as a function of the nature of the addend, the substituent(s) on the unsaturated substrates and the solvent. 3 Di- rect comparisons where these effects have been intentionally minimized are rare (vide infra). Vinylacetylene (1-butene-3-yne [H 2 C]CH–C^CH]) is the sim- plest compound with both the carbon–carbon double and triple bonds. More than seven decades ago, Carothers et al. 4 reported that the addition of aqueous hydrogen chloride to vinylacetyl- ene initially yielded 4-chloro-1,2-butadiene (H 2 C]C]CHCH 2 Cl), which then isomerized to 2-chloro-1,3-butadiene (chloroprene [CH 2 ]C(Cl)CH]CH 2 ]) and some 1,3-dichloro-2-butene (ClH 2 CCH] C(Cl)CH 3 ), of unspecified geometry. The latter formed at significantly longer reaction times. They found that both calcium chloride [CaCl 2 ] and copper(I) chloride [CuCl] accelerated the disappearance of vinylacetylene. It was argued that in the presence of calcium chlo- ride, 4-chloro-1,2-butadiene formed via a 1,4-addition reaction and that this allene subsequently isomerized to 2-chloro-1,3-butadiene. In the presence of copper(I) chloride, only 2-chloro-1,3-butadiene formed 4b and when 4-chloro-1,2-butadiene was treated with aqueous copper(I) chloride alone it was recovered unchanged. Subsequently, Dolgopol’skii et al. 5 proposed that 2-chloro-1,3-bu- tadiene resulted from an ionized complex of copper(I) chloride with 4-chloro-1,2-butadiene. They, too, reported that 1,3-dichloro-2-bu- tene (ClH 2 CCH]C(Cl)CH 3 ), geometry unspecified, was obtained on addition of hydrogen chloride to vinylacetylene in the presence of copper(I) chloride. There appear to be no (further) suggestions as to the pathway(s) by which products might be forming from the starting materials. Weakly bound p-complexes are often considered as the first stage of an electrophilic addition reaction to unsaturated systems. The elegant work of Dubois et al. 6a,b on the bromination of alkenes has shown that product is commonly obtained via an initial p-complex intermediate. However, the situation with regard to electrophilic addition of protic acids, e.g., hydrogen chloride, to alkenes and alkynes is quite different. For the latter, while experi- mentally identifiable (and on occasion even isolable) 6c hydrogen chloridedalkene and/ordalkyne complexes are produced, there does not appear to be any evidence that those complexes actually lie on the pathway to product. 1g The gas phase p-complex of hydrogen chloride with vinyl- acetylene was examined by Legon and et al. 7 They reported a 1:1 q A portion of this work has been presented at the 236th meeting of the Amer- ican Chemical Society, Philadelphia, PA, USA on 17–21 August 2008. ORGN 627. * Corresponding author. E-mail address: david.dalton@temple.edu (D.R. Dalton). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2008.07.081 Tetrahedron 64 (2008) 9357–9367