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 and one π bond; the π bond is the site of reactivity.

• General Reaction: Addition reactions break the π bond and form two new σ bonds.

Mechanism of Electrophilic Addition Reactions

General Features of Electrophilic Addition

Electrophilic addition is the most common reaction of alkenes. The π bond acts as a nucleophile and reacts with an electrophile, followed by nucleophilic attack.

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

• Step 2: A carbocation intermediate forms.

• Step 3: The nucleophile (Z-) attacks the carbocation, forming two new σ bonds.

General Equation:

Addition of Hydrogen Halides to Alkenes

Regioselectivity in Addition Reactions

When a hydrogen halide (HX) is added to an alkene, the hydrogen and halide add across the double bond. The position where each atom adds is determined by the stability of the carbocation intermediate.

• Markovnikov's Rule: The electrophile (H+) adds to the less substituted sp2 carbon, forming the more stable carbocation.

• Major Product: The halide attaches to the more substituted carbon.

Example:

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

Mechanism of Hydrogen Halide Addition

Carbocation Formation and Rate-Limiting Step

The formation of the carbocation is the rate-limiting step in the addition of HX to alkenes. The more stable the carbocation, the faster it forms and the more it determines the major product.

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

• Step 2: Nucleophilic attack by the halide ion.

• Carbocation Stability: Tertiary > Secondary > Primary > Methyl

Example Mechanism:

• tert-butyl cation (major) tert-butyl chloride

Carbocation Stability and Rearrangement

Factors Affecting Carbocation Stability

Carbocation stability is influenced by alkyl substitution and hyperconjugation. More substituted carbocations are stabilized by electron-donating groups.

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

• Hyperconjugation: Delocalization of electrons from adjacent σ bonds into the empty p orbital of the carbocation.

Rearrangement: Carbocations may undergo hydride or alkyl shifts to form more stable intermediates.

Summary Table: Carbocation Stability

Regioselectivity in Electrophilic Addition

Major and Minor Products

Regioselectivity refers to the preference for forming one constitutional isomer over another in a chemical reaction.

• Highly regioselective: One product predominates.

• Moderately regioselective: Both products form, but one is favored.

• Non-regioselective: Products form in equal amounts.

Stereochemistry of Addition Reactions

Formation of Chiral Centers and Stereoisomers

Addition reactions can create chiral centers, leading to the formation of stereoisomers. The stereochemistry depends on the mechanism and the nature of the reactants.

• Racemic Mixture: If a chiral center is formed from an achiral reactant, both enantiomers are produced in equal amounts.

• Syn Addition: Both groups add to the same side of the double bond.

• Anti Addition: Groups add to opposite sides of the double bond.

Example: Addition of Br2 to an alkene forms a vicinal dibromide via anti addition.

Key Terms and Concepts

• Electrophile: Species that accepts an electron pair.

• Nucleophile: Species that donates an electron pair.

• Carbocation: Positively charged carbon intermediate.

• Regioselectivity: Preference for one direction of chemical bond formation over another.

• Stereoselectivity: Preference for formation of one stereoisomer over another.

Summary Table: Types of Selectivity

Learning Objectives

• Name and draw alkenes.

• Use curved arrows to show mechanisms.

• Predict products of electrophilic addition reactions.

• Identify carbocations that will rearrange.

• Explain differences in alcohol products from hydroboration-oxidation vs. acid-catalyzed addition.

*Additional info: The notes are based on textbook slides and cover foundational concepts in organic chemistry relevant to college-level study, including mechanisms, selectivity, and stereochemistry of alkene addition reactions.*