Simple, unconjugated alkenes are electron-rich and can function as weak bases or nucleophiles. The filled π orbital of the double bond, which is the HOMO, can interact with the LUMO of an electrophile, such as bromine.
An addition reaction begins with the transfer of a pair of electrons from the π bond to the electrophilic center. A σ bond is formed between the electrophile and one of the carbons, while the other carbon acquires a positive charge. The carbocation then reacts with a nucleophile to form a σ bond, yielding the addition product.
An addition reaction and the corresponding elimination reaction can be represented as a temperature-dependent equilibrium.
In an addition reaction, one π and one σ bond are broken and two σ bonds are formed. Because σ bonds are stronger than π bonds, addition reactions are usually exothermic.
Thus, in the equation for Gibbs free energy change, the enthalpy term is negative. The decrease in the number of molecules indicates that the entropy term is always positive.
Consequently, for low values of T, the value of ΔG is negative, and addition reactions are thermodynamically favored at low temperatures.
In the halogenation of alkenes, bonds are formed with more electronegative atoms; thus, the oxidation state of carbon changes from −2 to −1.
Addition reactions such as halogenation, dihydroxylation, halohydrin formation, and epoxidation are all oxidation reactions.
The addition of hydrogen to alkenes yields the corresponding alkanes and is a reduction reaction.
In hydration and hydrohalogenation reactions, one of the carbons is oxidized while the other is reduced.
When but-2-ene undergoes hydrobromination, the acidic proton in HBr accepts a pair of electrons from the π bond.
The proton is transferred, resulting in a secondary carbocation intermediate. The bromide ion then reacts with the positive center to yield a racemic mixture of 2-bromobutane.
