Chapter 8: Addition Reactions of Alkenes

Introduction to Addition Reactions

Addition reactions are a fundamental class of transformations in organic chemistry, particularly involving alkenes. In these reactions, two atoms or groups are added across the carbon–carbon double bond, converting unsaturated hydrocarbons into saturated ones.

• General Reaction:

• Alkenes: Unsaturated hydrocarbons containing at least one carbon–carbon double bond.

• Products: Saturated hydrocarbons with new substituents added to the former double bond carbons.

Hydrogenation of Alkenes

Hydrogenation is the addition of hydrogen () across the double bond of an alkene, typically catalyzed by metals such as Pt, Pd, Ni, or Rh.

• Reaction:

• Example: Ethylene () + Hydrogen () → Ethane ()

• Heats of Hydrogenation: The enthalpy change () for hydrogenation reflects alkene stability. Lower heat of hydrogenation indicates greater stability.

Table: Heats of Hydrogenation of Some Alkenes

Mechanism and Stereochemistry of Hydrogenation

Hydrogenation proceeds via syn addition, meaning both hydrogen atoms add to the same face of the double bond.

• Mechanism: Alkene adsorbs onto the metal surface, hydrogen atoms are transferred from the surface to the same side of the alkene.

• Stereochemistry: Only the syn addition product is formed; steric hindrance can direct which face is attacked.

• Example: Hydrogenation of α-pinene yields only cis-pinane, not trans-pinane.

Electrophilic Addition of Hydrogen Halides (H-X) to Alkenes

Alkenes react with hydrogen halides (HCl, HBr, HI) to form alkyl halides via electrophilic addition. The reaction is regioselective, often following Markovnikov's rule.

• General Reaction:

• Markovnikov's Rule: The hydrogen atom adds to the carbon with more hydrogens; the halide adds to the more substituted carbon.

• Reactivity Order: (HI is most reactive)

• Regioselectivity: A reaction favoring one constitutional isomer over others is regioselective; if only one isomer forms, it is regiospecific.

Mechanism Example: Electrophilic Addition of HBr to 2-Methylpropene

1. Protonation of the double bond forms the most stable carbocation intermediate.

2. Bromide ion attacks the carbocation, yielding the alkyl bromide.

Carbocation Rearrangements

Carbocation intermediates formed during addition reactions can undergo rearrangements to form more stable carbocations.

• Types of Rearrangement: Hydride shifts, methyl shifts.

• Other Fates: Addition to nucleophile, elimination to form alkene or alkyne.

• Example: HCl addition to 3-methyl-1-butene yields two products due to hydride migration.

Anti-Markovnikov Addition: Peroxide Effect

In the presence of peroxides (ROOR) or light, HBr adds to alkenes via a radical mechanism, resulting in anti-Markovnikov addition.

• Br adds to the least substituted carbon.

• H adds to the carbon with the least hydrogens.

• Mechanism: Radical chain process (see Chapter 10 for details).

Acid-Catalyzed Hydration of Alkenes

Alkenes react with water in the presence of acid (usually H2SO4) to form alcohols. The reaction follows Markovnikov's rule.

• General Reaction:

• Mechanism: Protonation of alkene, nucleophilic attack by water, deprotonation to yield alcohol.

• Rate: More substituted alkenes hydrate faster due to more stable carbocation intermediates.

• Conditions: Excess water favors hydration; concentrated acid and little water favor E1 elimination.

Hydroboration–Oxidation Reaction

This two-step reaction converts alkenes to alcohols via anti-Markovnikov addition, with syn stereochemistry.

• Step 1: Hydroboration with (in THF or diglyme)

• Step 2: Oxidation with and

• Product: Alcohol with the OH group on the less substituted carbon.

• Mechanism: No carbocation intermediate; concerted syn addition.

• Developed by: Prof. Herbert C. Brown (Nobel Prize, 1979)

Addition of Halogens to Alkenes

Alkenes react with halogens (Br2, Cl2) to form vicinal dihalides via anti addition.

• General Reaction:

• Stereochemistry: Anti addition; only trans-dihalide products are formed.

• Mechanism: Formation of a bromonium ion intermediate, followed by nucleophilic attack.

• No carbocation rearrangements.

• Solvents: HC2H3O2, CCl4, CHCl3, CH2Cl2

Stereochemistry of Halogenation

• Anti addition to (Z)-2-butene gives meso-2,3-dibromobutane.

• Anti addition to (E)-2-butene gives a racemic mixture of enantiomers.

Halohydrin Formation

Alkenes react with halogens in the presence of water to form halohydrins (vicinal halide and alcohol).

• Mechanism: Bromonium ion intermediate; water attacks the more substituted carbon.

• Regioselectivity: Water attacks the more substituted carbon due to greater carbocation character.

Formation and Nomenclature of Epoxides

Epoxides are three-membered cyclic ethers formed by reaction of alkenes with peroxy acids.

• General Reaction:

• Peroxy Acids: Peroxyacetic acid, m-chloroperoxybenzoic acid

• Stereochemistry: Cis alkenes yield cis epoxides.

• Nomenclature: Named as epoxy derivatives of the parent alkane (e.g., 1,2-epoxycyclohexane).

Mechanism of Epoxidation

• Oxygen is transferred from the peroxy acid to the less crowded face of the alkene.

• Epoxidation results in formation of an epoxide and a carboxylic acid.

Ozonolysis of Alkenes

Ozonolysis cleaves alkenes to form carbonyl compounds (aldehydes, ketones, or carboxylic acids).

• General Reaction: ozonide intermediate carbonyl products

• Oxidative Work-up: H2O2 oxidizes aldehydes to carboxylic acids.

• Reductive Work-up: Zn or (CH3)2S yields aldehydes and ketones.

Synthetic Applications: Retro-Synthetic Analysis

Retro-synthetic analysis is a strategy for planning organic syntheses by working backward from the target molecule to simpler starting materials.

• Key Steps: Identify functional group transformations, break down complex molecules into simpler precursors.

• Example Problems:

  • Propose a synthesis to make 2,3-epoxy-2-methylhexane from 2-methylhexane.

  • Propose a synthesis to make tert-butylbromide from 1-bromo-2-methylpropane.

  • Propose a synthesis to make trans-1,2-dichlorocyclopentane from cyclopentane.

Summary Table: Key Addition Reactions of Alkenes

Key Terms and Concepts

• Markovnikov's Rule: In addition of HX to an alkene, the hydrogen attaches to the carbon with more hydrogens.

• Regioselectivity: Preference for one direction of chemical bond making/breaking over all other possible directions.

• Stereochemistry: The spatial arrangement of atoms in molecules and its effect on chemical reactions.

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

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

• Carbocation Rearrangement: Migration of hydride or alkyl groups to stabilize carbocation intermediates.

• Retro-Synthetic Analysis: Planning synthesis by breaking down the target molecule into simpler precursors.

Additional info: For deeper understanding, students should practice drawing mechanisms and predicting products for each reaction type, considering both regioselectivity and stereochemistry.