Addition of HCl to Alkenes to Give Alkyl Chlorides
Description: Treatment of alkenes with hydrochloric acid (HCl) will result in the formation of alkyl chlorides. The chlorine always ends up at the more substituted carbon of the alkene, since protonation of the alkene will result in the more stable carbocation.
Notes: Note that this reaction is “Markovnikoff selective”. Since it goes through a carbocation, rearrangements are possible.
Notes: When a secondary carbocation is formed adjacent to a tertiary or quaternary carbon, rearrangements are possible. Note the third example – where Markovnikoff’s rule gives no clear preference a mixture will be obtained.
Mechanism: Electrons from the C1-C2 π bond attack the hydrogen of HCl, expelling the Cl anion and leading to the formation of a carbocation (Step 1, arrows A and B). Note that the most stable carbocation is formed preferentially. The chloride anion then attacks the carbocation, leading to formation of the alkyl chloride (Step 2, arrow C).