7.6: Introduction to Electrophilic Addition Reactions of Alkenes
The double bond in a simple, unconjugated alkene is a region of high electron density that can act as a weak base or a nucleophile. The filled π orbital (HOMO) of the double bond can interact with the empty LUMO of an electrophile. A bonding interaction occurs when the electrophile attacks between the two carbons; the electrophile then accepts a pair of electrons from the π bond and undergoes addition across the double bond, yielding a single product.
Addition and elimination reactions can be considered to exist in a temperature-dependent equilibrium, which can be better understood from the change in Gibbs free energy (ΔG) of the reaction. In addition reactions, one π bond is broken, and two σ bonds are formed. These reactions are usually exothermic because σ bonds are stronger than π bonds; thus, the enthalpy term (ΔH) is negative. The entropy term (−TΔS) is always positive: the number of molecules decreases, leading to a negative ΔS, and T is always positive on the kelvin scale, so the negative of that product is a positive term overall. Consequently, the value of ΔG is dependent on the temperature of the system, and addition reactions are favored at low temperatures.
When an alkene undergoes halogenation, bonds are formed between carbon and the more electronegative halogens; thus, the carbon atoms are oxidized. Dihydroxylation, halohydrin formation, and epoxidation are also oxidation reactions. Conversely, the addition of hydrogen across the double bond in alkenes is a reduction reaction that yields the corresponding alkanes. In hydration and hydrohalogenation reactions, one of the carbon atoms is oxidized while the other is reduced; as a result, they are not classified as oxidation or reduction reactions. In the hydrobromination of but-2-ene, the acidic proton in HBr accepts a pair of electrons from the π bond. The proton is transferred to one of the carbons in the double bond, while the other carbon acquires a positive charge, resulting in a secondary carbocation intermediate. The bromide ion then reacts with the positive center to yield a racemic mixture of 2-bromobutane.