Reactions of Alkenes: Addition and Oxidation Mechanisms

Reactions of Alkenes

Introduction to Alkene Reactivity

Alkenes contain a π bond, making them unsaturated and susceptible to addition reactions that convert them into saturated compounds. These reactions are generally thermodynamically favorable and often exothermic. The enthalpy change ($\Delta H^\circ$) for addition reactions can be estimated using bond strength data, with the π bond of ethene having a bond strength of approximately 65 kcal mol-1. Although the entropy change ($\Delta S^\circ$) is negative, the exothermic enthalpy usually dominates.

Additions to the π Bond

Alkenes react with various reagents (A–B) to form saturated products. The following table summarizes estimated enthalpy changes for common addition reactions to ethene:

Reaction	ΔHo (kcal mol-1)	Product	ΔHo (kcal mol-1)
Hydrogenation (H2)	65 + 104	2 × 98	-33
Bromination (Br2)	65 + 46	2 × 68	-29
Hydrochlorination (HCl)	65 + 103	98 + 99	-17
Hydration (H2O)	65 + 119	98 + 104	-11

Additional info: These values are estimates and do not account for all bond changes, but illustrate the exothermic nature of alkene additions.

1. Catalytic Hydrogenation

Mechanism and Catalysis

Catalytic hydrogenation involves the addition of H2 across the double bond of an alkene, producing an alkane. This reaction requires a metal catalyst such as Pd/C or PtO2, as alkenes and hydrogen do not react appreciably without it. The catalyst accelerates the reaction by providing an alternative pathway with a lower activation energy ($E_a$), but is not consumed in the process.

• Mechanism: Hydrogen molecules adsorb onto the metal surface, breaking the H–H bond. The alkene also adsorbs, and hydrogen atoms are transferred to the same side (syn addition) of the double bond, forming the alkane.

• Stereochemistry: The addition of H2 is syn, meaning both hydrogens add to the same face of the double bond, leading to cis products in cyclic systems.

Example: Hydrogenation of cyclohexene with H2 and a catalyst yields cis-1,2-dihydrocyclohexane.

2. Electrophilic Additions

General Mechanism

Alkenes are electron-rich and react with polar reagents (A–B) via electrophilic addition. The π electrons attack the electrophilic part (Aδ+), generating a carbocation intermediate, which is then attacked by the nucleophile (Bδ–).

• Syn vs. Anti Addition: Depending on the mechanism, addition can be syn (same side) or anti (opposite sides).

A. Hydrohalogenation by HX

Hydrohalogenation is the addition of hydrogen halides (HCl, HBr, HI) to alkenes, forming alkyl halides. The reaction is regioselective and follows Markovnikov's Rule: the hydrogen atom adds to the less substituted carbon, generating the more stable (more substituted) carbocation intermediate.

• Regioselectivity: The major product results from the most stable carbocation intermediate.

• Rearrangements: At higher temperatures, carbocation rearrangements can occur, leading to different products.

Example: Addition of HBr to propene yields 2-bromopropane as the major product.

Origin of Markovnikov's Rule

The rule is explained by the relative stability of carbocation intermediates. Secondary carbocations are more stable than primary, so the transition state leading to a secondary carbocation is favored.

B. Halogenation (Addition of Halogen)

Halogenation involves the addition of X2 (Cl2, Br2) to alkenes, forming vicinal dihalides. The reaction is characterized by anti addition (opposite sides of the double bond).

• Mechanism: The halogen molecule becomes polarized upon approaching the alkene, forming a cyclic halonium ion intermediate. The nucleophilic halide ion then attacks from the opposite side, resulting in anti addition.

• Bromine Test: The disappearance of the red color of Br2 indicates the presence of an alkene.

Example: Addition of Br2 to 2-butene yields racemic 2,3-dibromobutane.

3. Hydroboration-Oxidation

Mechanism and Regioselectivity

Hydroboration-oxidation is a two-step process that converts alkenes to alcohols via anti-Markovnikov addition. In the first step, borane (BH3) adds across the double bond in a syn fashion, with boron attaching to the less hindered carbon. In the second step, oxidation with hydrogen peroxide (H2O2, OH–) replaces boron with a hydroxyl group.

• Regioselectivity: The reaction is anti-Markovnikov, with OH attaching to the less substituted carbon.

• Stereochemistry: The addition is syn, so H and OH add to the same face of the double bond.

Example: Hydroboration-oxidation of 1-methylcyclopentene yields trans-2-methylcyclopentanol with syn addition of H and OH.

4. Electrophilic Oxidation

A. Epoxidation with Peroxycarboxylic Acids

Epoxidation is the addition of an oxygen atom across a double bond to form an epoxide. Peroxycarboxylic acids (e.g., mCPBA, peroxyacetic acid) are commonly used oxidants. The reaction proceeds via a concerted syn addition mechanism.

• Mechanism: The peracid transfers an oxygen atom to the alkene, forming an epoxide and a carboxylic acid.

• Regioselectivity: More electron-rich alkenes react faster.

Example: Reaction of cyclohexene with mCPBA yields cyclohexene oxide.

B. OsO4 Syn-Dihydroxylation

Osmium tetroxide (OsO4) adds across alkenes to form vicinal syn-diols (glycols). The reaction proceeds via a cyclic osmate ester intermediate, followed by hydrolysis.

• Mechanism: OsO4 forms a six-electron transition state with the alkene, adding two hydroxyl groups to the same face (syn addition).

Example: OsO4 treatment of cyclohexene yields cis-1,2-cyclohexanediol.

Additional info: These notes summarize the key addition and oxidation reactions of alkenes, including mechanisms, regio- and stereoselectivity, and representative examples, as covered in a typical Organic Chemistry course.