Alkyl Halide Reactions: Alkene formation using Bulky Bases (E2 Hofmann Product)

Alkyl halides can sometimes undergo elimination reactions when exposed to bulky bases (tBuOK, DBN, etc…), and elimination reactions almost exclusively occur when the reaction is run with heat (Δ):

This reaction proceeds through an E2 elimination reaction, where the halide is removed and a double bond is formed with the least substituted carbon atom (Hofmann rule). An elimination is more likely to occur with secondary and tertiary alkyl halides as opposed to primary alkyl halides. Heat can be applied to cause this reaction to proceed faster:

tBuOK and DBN are BULKY

The reaction mechanism is depicted below using potassium tert-butoxide (tBuOK) however, it would proceed the same way using the other bulky strong bases:

One of the hallmarks of SN2 and E2 reactions is that they occur in 1 step! The free electrons from the negatively charged oxygen atom on the tBuOK molecule pull a proton off a the least substituted carbon because tBuOK is bulky and cant “reach” the proton on the more substituted carbon atom. The free electrons then form a bond between this carbon and the carbon bound to the halide (Br in this example). This pushes the chloride ion off, resulting in the formation of an alkene.

To learn more about SN2 and E2 reactions and how to distinguish them, be sure to check out our Lessons!

Practice this reaction using our Reaction Solver!