Organic Chemistry - "Change", Part 6: The Addition Reaction; How HCl Adds to Isobutylene

 Greetings,

This post explains the particular way HCl will add to a Branched Terminal Alkene.We'll use Isobutylene (an important industrial compound) for our example.  We'll also define C1 as the double-bonded terminal carbon and C2 as the carbon atom across the double bond.  An analysis of the reactants leads us to the question, "Are there equal chances of H+ and Cl- adding to C1 and C2 or is there a preference, and if so, what is the preference (specifically H+ adding to C1 or C2?), and why?"  The Hydrogen ion will add first (therefore controlling which carbon, C1 or C2, Cl- will add to). H+ adds first because it is a strong electrophile (attacking a region rich in electrons - namely, the double bond). In the process of H+ addition, a pair of electrons moves from the double bond, onto an adjacent carbon, and a partial positive charge forms on the other carbon across the double bond. The hydrogen atom will bond to the carbon atom which produces a partial positive charge on the more substituted carbon across the double bond - A partial positive charge on a more substituted carbon forms a more stable carbocation. In short, the H+ ion bonds to the less substituted carbon, as a rule for the type of reaction discussed here. The following image illustrates the mechanism of the reaction.

The first step is slow because the activation energy, for the formation of a carbocation, is relatively high.  Step #1 controls the overall rate of the reaction.  Step #2 happens fast because a carbocation is a highly reactive entity because the chloride ion readily bonds to a positively charged carbon atom.

That's all for this post.

As always, thank you for reading!