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 orbit…
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.
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Q1: Why are alkenes considered nucleophiles in electrophilic addition reactions?
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.
Q2: What happens to bonds during an electrophilic addition reaction?
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.
Q3: Why are addition reactions typically exothermic?
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.
Q4: How does temperature affect the equilibrium between addition and elimination reactions?
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.
Q5: What is the difference between oxidation and reduction in alkene addition reactions?
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.
Q6: What intermediate forms during the hydrobromination of but-2-ene?
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.
Q7: How does the HOMO-LUMO interaction initiate an electrophilic addition?
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.