The Reactions of Alkenes

Introduction to Alkene Reactivity

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their reactivity is largely due to the presence of the π bond, which is more reactive than a σ bond. This chapter focuses on the mechanisms and stereochemistry of addition reactions to alkenes.

• Alkene Structure: The double bond consists of one σ bond and one π bond. The π bond is formed by the sideways overlap of p orbitals and is susceptible to attack by electrophiles.

• General Reactivity: Alkenes undergo addition reactions where the π bond is broken and new σ bonds are formed.

Mechanism of Electrophilic Addition Reactions

General Features of Electrophilic Addition

Electrophilic addition is the most common reaction of alkenes. In these reactions, an electrophile attacks the electron-rich π bond, followed by nucleophilic attack to complete the addition.

• Step 1: The π bond breaks as the alkene reacts with an electrophile (Y+).

• Step 2: A carbocation intermediate may form, which is then attacked by a nucleophile (Z-).

• Result: Two new σ bonds are formed, converting sp2 carbons to sp3.

General Equation:

Addition of Hydrogen Halides to Alkenes

Regioselectivity in Addition Reactions

When a hydrogen halide (HX) adds to an alkene, the reaction can produce different constitutional isomers depending on which carbon receives the hydrogen and which receives the halide.

• Example: Addition of HBr to 2,3-dimethyl-2-butene yields 2-bromo-2,3-dimethylbutane.

• Regioselectivity: The major product is determined by the stability of the carbocation intermediate.

Which sp2 Carbon Gets the H+?

The hydrogen adds to the sp2 carbon that leads to the most stable carbocation intermediate.

• Markovnikov's Rule: In the addition of HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens (less substituted), and the halide attaches to the more substituted carbon.

• Example: tert-butyl chloride (major) or isobutyl chloride (minor)

Mechanism of Hydrogen Halide Addition

The formation of the carbocation is the rate-limiting step in the mechanism.

• Step 1: Protonation of the alkene to form a carbocation intermediate.

• Step 2: Nucleophilic attack by the halide ion on the carbocation.

• Carbocation Stability: Tertiary > Secondary > Primary > Methyl

Mechanism Example:

• (via tert-butyl cation) (major product)

Carbocation Stability and Rearrangement

Relative Stabilities of Carbocations

Carbocation stability is influenced by alkyl substitution and hyperconjugation.

• Order of Stability: Tertiary > Secondary > Primary > Methyl

• Hyperconjugation: Delocalization of charge via adjacent C-H bonds increases stability.

Carbocation Rearrangement

Sometimes, carbocations rearrange to form a more stable intermediate via hydride or alkyl shifts.

• 1,2-Hydride Shift: A hydrogen atom with its bonding electrons moves to the carbocation center.

• 1,2-Alkyl Shift: An alkyl group migrates to the carbocation center.

• Result: Formation of a more stable carbocation and, consequently, a different product.

Regioselectivity and Stereochemistry of Addition Reactions

Regioselectivity

Regioselective reactions favor the formation of one constitutional isomer over another.

• Highly regioselective: One product predominates.

• Moderately regioselective: A mixture forms, but one is favored.

• Non-regioselective: Products form in equal amounts.

Stereochemistry of Addition

Addition reactions can also be stereoselective, forming one stereoisomer in greater amounts, or stereospecific, where the stereochemistry of the reactant determines the product.

• Stereoselective: More of one stereoisomer is formed.

• Stereospecific: Each stereoisomer of the reactant gives a different stereoisomer of the product.

Example: Addition to a chiral center can produce a racemic mixture if both faces are equally accessible.

Summary Table: Carbocation Stability

Key Equations and Concepts

• General Electrophilic Addition:

• Markovnikov's Rule: "The electrophile (H+) adds to the less substituted carbon of the double bond."

• Carbocation Rearrangement: (hydride or alkyl shift)

Additional info: These notes are based on textbook slides and cover the foundational mechanisms and selectivity principles for alkene addition reactions, including carbocation stability, rearrangement, and the role of Markovnikov's rule. More advanced topics such as stereochemistry, anti/syn addition, and specific reagents (e.g., hydroboration, halogenation) are covered in subsequent sections of the chapter.