Addition of hydrogen halide to simple unsaturated hydrocarbons

Addition of hydrogen halide to simple unsaturated hydrocarbons 

Olefins add hydrogen fluoride very readily, but polymerization of the olefins also occurs as a side reaction. Traces of acid and water, or temperatures of about 70°, lead to loss of HF from the alkyl fluorides produced. Preparative importance attaches to addition of HF to chlorinated olefins, whose tendency to polymerize in presence of HF is less the more Cl atoms are attached to the doubly bonded carbon atoms. HF adds readily to olefins with unsymmetrical halogen substitution, such as CH2=CHX, RCH=CX2, or RCX=CH2, but with greater difficulty when halogen is attached to both the doubly bonded carbon atoms. BF3, which assists removal of H+ from HF by complex formation, accelerates the reaction but also the exchange of halogen for fluorine and the resin formation by polymerization that occur as parallel reactions.169 

In general (exception CF3 next to C=C, see above), fluorine thus becomes attached to the carbon atom carrying the least hydrogen, according to Markov-nikov s rule. This is true also for fluorinated olefins. If an olefin contains other halogen atoms besides fluorine, then on addition of hydrogen halide by either the ionic or the radical mechanism the halogen becomes attached to the carbon 

The olefin is added, when possible without a catalyst (BF3), to liquid HF cooled to —40 to —60° the mixture is allowed to warm gradually and finally kept at 75-90° for 1-2 h. The reaction vessel may be made of copper, Monel metal, stainless steel, platinum, magnesium, or nickel. Reactions giving low-boiling fluorides must be carried out in an autoclave. No addition can be achieved when concentrated aqueous HF is used. 

In the following description of their addition reactions the other halogens will be treated as a group. 

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