Acid Halide Addition to Double Bonds
A Crystal Clear Chemistry Reactions Tutorial
The fundamental reaction between a strong acid and a carbon-carbon double bond (an olefin) is:
Reaction Specifics
The acid is often an acid halide, such as HCl, HBr, and HI, and the reaction usually needs to be heated strongly. The reaction conditions are very harsh.
The reaction always forms in an arrangement that follows Markovnikov's Rule (that is, the A substituent is always on the more substituted carbon, while the hydrogen adds to the less substituted carbon). There is no specific stereochemistry involved with the reaction, however. The acid can add on the same or opposite face relative to the hydrogen, and the hydrogen can add to either side of the double bond.
This reaction is conducted in an non-aqueous soluent; if water is present, a different reaction occurs — alkene hydration.
The Mechanism
The addition proceeds by the following stepwise arrow-pushing mechanism:
The mechanism has a carbocation intermediate, which is what causes the Markovnikov arrangement in the final product. The carbocation forms on the more substituted carbon because the substituents stabilize the carbocation. Then, the acid attacks the carbocation, giving a Markovnikov-substituted product. (The brackets are optional, and just show that the carbocation is an intermediate and not a product of the reaction).
An Example
In the example below, the acid is hydrogen bromide, and the alkene is 2-methyl 1-butene. The product is 2-methyl 2-bromobutane. The reaction starts by having the double bond electron pair attack the more positive hydrogen in the hydrogen bromide. This causes the bond in hydrogen bromide to break, leaving the bromide anion. The double bond and the hydrogen ion together form a carbocation, where the positive charge resides on the more substituted carbon (this is much more stable). The negatively charged bromine ion then attacks the carbocation, forming a single bond and giving a Markovnikov-substituted product.
Summary
The main thing to remember is that the anion substitutes onto the more substituted side, and the hydrogen on the less substituted side. The reaction follows Markovnikov's rule because of the carbocation intermediate. Water in the reaction does not result in this product, but instead in an alkene hydration product. 