Hydrofluoric acid can react with alkenes, but much slower than the reaction of HCl, HBr or HI Indeed, hydrogen iodide reacts rapidly with alkenes, as will HBr and HCl. Although strong acids such as HCl or HBr react with alkenes, weak acids such as water and alcohols do not react directly with alkenes. Carbocations, however, are highly reactive intermediates, and once generated they easily react with weak nucleophiles such as water or alcohols. If an alkene reacts with an acid catalyst in an aqueous medium the product is an alcohol, but in an alcohol medium the product is an ether. The reaction of cyclohexene and methanol in the presence of an acid catalyst initially gives the expected carbocation. Elemental fluorine was thought to be too reactive and too dangerous for reaction with alkenes.