7.6:
Introduction to Electrophilic Addition Reactions of Alkenes
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.
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.