Introduction to Electrophilic Addition Reactions of Alkenes

Alkenes are electron-rich molecules with a filled π orbital (HOMO) that can donate electrons. This π orbital interacts with the empty LUMO of an electrophile like bromine. The electron density of the double bond allows it to act as a weak nucleophile, initiating the addition reaction by transferring electrons to the electrophilic center.

During electrophilic addition, one π bond breaks while two σ bonds form. The electrophile accepts electrons from the π bond, creating a σ bond with one carbon. A carbocation forms on the second carbon, which then reacts with a nucleophile to complete the addition and form the final product.

Addition reactions are exothermic because σ bonds are stronger than π bonds. When one π bond breaks and two σ bonds form, the energy released from forming stronger bonds exceeds the energy required to break the weaker π bond. This results in a negative enthalpy change, making the reaction thermodynamically favorable at low temperatures.

Addition and elimination reactions exist in temperature-dependent equilibrium governed by Gibbs free energy. At low temperatures, the negative enthalpy term dominates, favoring addition reactions. At high temperatures, the entropy term becomes significant, shifting equilibrium toward elimination. This relationship explains why addition is favored at lower temperatures and elimination at higher temperatures.

Halogenation, dihydroxylation, halohydrin formation, and epoxidation are oxidation reactions because carbon bonds with more electronegative atoms, increasing oxidation state. Hydrogenation is a reduction reaction producing alkanes. In hydration and hydrohalogenation, one carbon is oxidized while the other is reduced, so they are classified as neither oxidation nor reduction reactions.

During hydrobromination of but-2-ene, the acidic proton in HBr accepts electrons from the π bond and transfers to one carbon of the double bond. This creates a secondary carbocation intermediate on the other carbon. The bromide ion then attacks this positive center, yielding a racemic mixture of 2-bromobutane as the final product.

The filled π orbital (HOMO) of the alkene interacts with the empty orbital (LUMO) of an electrophile. This bonding interaction occurs when the electrophile attacks between the two carbons of the double bond. The π electrons transfer to the electrophilic center, forming a σ bond and generating a carbocation that proceeds to the addition product.