Aqueous HX (X = Cl, Br, and I) solutions are the most ubiquitous HX source in the laboratories. However, the hydrohalogenation of an alkene or alkyne by an aqueous HX solution is difficult to perform under standard conditions. Because of the acid’s high ionization potential in water, it is held in the aqueous phase and has a relatively low concentration in the organic phase; the alkene or alkyne, on the other hand, being highly nonpolar, exists primarily in the organic phase. Because the two reactants occupy different phases, their interactions with one another are limited, and, as such, aqueous HX is rendered a generally ineffective reagent. By introducing a hydrogen bond donor (HBD) such as 1, 1, 1, 3, 3, 3-hexafluoroisopropanol (HFIP), the nucleophilicity of water can be reduced such that HX is liberated from the aqueous phase and allowed to enter the organic phase where it may interact with the substrate; the result is an environment in which aqueous HX can serve as an effective hydrohalogenation reagent. Through this mechanism we have achieved high-yielding hydrohalogenation of various alkenes and alkynes using aqueous HCl, HBr and HI, mediated by HFIP.

HFIP promoted hydrohalogenation using aqueous HX solution

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