Question

The mechanism for the electrophilic addition to alkenes involves the formation of a brominium ion (has a 3-membered ring composed of the two carbon atoms from the alkene and the bromine atom). The accepted mechanism states that after the formation of the brominium ion, another Br- (the electrophile) ion attacks on the carbon atoms attached to the bromine, thereby breaking the the 3-membered ring and relieving the strain.

My question is: Why doesn't the Br- ion attack the clearly positively-charged bromine ion as it is clearly more electron deficient and therefore more electrohilic. In other words, shouldn't this reaction be reversible?

Answer 1

The electrophilic addition mechanism to alkenes involves the formation of a bromonium ion, which is a 3-membered ring composed of two carbon atoms from the alkene and a bromine atom. The Br- ion does not attack the positively-charged bromine ion because the attack occurs at the region of increased electron density above and below the alkene plane.

Step-by-step explanation:

The electrophilic addition mechanism to alkenes involves the formation of a bromonium ion, which is a 3-membered ring composed of two carbon atoms from the alkene and a bromine atom. According to the accepted mechanism, the electrophile in the form of Br- attacks the carbon atoms attached to the bromine, breaking the 3-membered ring.

The reason that the Br- ion does not attack the positively-charged bromine ion is because the attack occurs at the region of increased electron density above and below the alkene plane, which is more favorable for the reaction.