Addition Reactions of Alkenes: Chapter 8 Study Guide

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Chapter 8: Addition Reactions of Alkenes

8.1 Introduction to Addition Reactions

Addition reactions are fundamental transformations in organic chemistry, where the π bond of an alkene is converted into two new σ bonds. This process is the opposite of elimination reactions.

Definition: An addition reaction involves the breaking of a C=C π bond and the formation of two new σ bonds.
Electron Pair Donor: The π bond acts as an electron-pair donor in these reactions.
Common Types: Hydrohalogenation, hydration, halogenation, dihydroxylation, and oxidative cleavage.
Example: Addition of HBr to an alkene forms a bromoalkane.

8.2 Alkenes in Nature and Industry

Alkenes are widely found in both natural and industrial contexts, playing crucial roles in biological signaling and chemical manufacturing.

Acyclic Alkenes: Naturally occurring, open-chain alkenes are found in various plant and animal products.
Cyclic and Polycyclic Alkenes: These are present in complex natural molecules, including steroids and pheromones.
Pheromones: Many pheromones contain C=C double bonds, which are essential for their biological activity.
Industrial Precursors: Ethylene and propylene are produced in vast quantities from petroleum cracking and serve as precursors for plastics and other chemicals.

8.3 Alkene Nomenclature

Alkenes are named according to IUPAC rules, with specific modifications to account for the double bond.

Step 1: Identify the parent chain, which must include the C=C double bond. The name ends in -ene.
Step 2: Identify and name substituents.
Step 3: Assign locants to substituents and the double bond, giving the double bond the lowest possible number.
Step 4: List substituents alphabetically before the parent name (ignore prefixes except iso).
Step 5: Place the double bond locant either before the parent name or before the -ene suffix.
Example: (E)-5,5,6-trimethylhept-2-ene indicates the E configuration and the position of the double bond.

8.4 Addition vs. Elimination

Addition and elimination reactions are reversible and their favorability depends on thermodynamic factors.

Enthalpy: Addition reactions are favored by enthalpy because σ bonds are stronger than π bonds.
Equation:
Example Calculation:
Entropy: Addition reactions decrease entropy (two molecules become one).
Temperature Dependence: At low temperatures, enthalpy dominates (favoring addition); at high temperatures, entropy dominates (favoring elimination).

8.5 Hydrohalogenation

Hydrohalogenation involves the addition of HX (HCl, HBr, HI) to an alkene, producing alkyl halides.

Regioselectivity: Follows Markovnikov's rule: H adds to the carbon with more H atoms, X to the more substituted carbon.
Anti-Markovnikov Addition: In the presence of peroxides (ROOR), HBr adds in the opposite regioselectivity.
Mechanism: Two-step process: formation of carbocation (rate-determining), followed by nucleophilic attack.
Stereochemistry: If a chiral center is formed, a racemic mixture of enantiomers results.
Carbocation Rearrangements: Hydride or methyl shifts can occur to form more stable carbocations.

8.6 Acid-Catalyzed Hydration

Acid-catalyzed hydration adds H and OH across the π bond, following Markovnikov regioselectivity.

Catalyst: Sulfuric acid (H2SO4) is commonly used.
Mechanism: Proceeds via carbocation intermediate, followed by nucleophilic attack by water and deprotonation to yield alcohol.
Thermodynamics: Reaction is reversible; Le Chatelier's principle is used to control product formation (excess water favors alcohol).
Stereochemistry: Formation of chiral centers yields racemic mixtures.

8.7 Oxymercuration-Demercuration

This method achieves Markovnikov hydration without carbocation rearrangements.

Lewis Acid: Mercuric cation (Hg2+) acts as the Lewis acid.
Mechanism: Alkene attacks Hg2+, forming a mercurinium ion; nucleophile attacks, then NaBH4 replaces HgOAc with H.
Advantage: No carbocation rearrangements occur.
Comparison: Same product as acid-catalyzed hydration, but more selective.

8.8 Hydroboration-Oxidation

This two-step sequence adds H and OH with anti-Markovnikov regioselectivity and syn stereochemistry.

Reagents: BH3·THF followed by H2O2, NaOH.
Regioselectivity: OH adds to the less substituted carbon.
Stereoselectivity: Syn addition; both H and OH add to the same face.
Mechanism: Boron atom is attacked by the less substituted carbon; hydride shift occurs.
Example: One BH3 reacts with three equivalents of alkene.

8.9 Catalytic Hydrogenation

Hydrogenation reduces alkenes to alkanes by adding H2 across the double bond, requiring a metal catalyst.

Syn Addition: Both H atoms add to the same face of the alkene.
Catalysts: Pt, Pd (heterogeneous); Wilkinson's catalyst (homogeneous).
Asymmetric Hydrogenation: Chiral catalysts can produce a single enantiomer (e.g., L-dopa synthesis).
Example: Syn addition to a symmetrical alkene produces a meso compound.

8.10 Halogenation and Halohydrin Formation

Halogenation adds two halogen atoms (Cl2, Br2) across the double bond, while halohydrin formation occurs in water.

Halogenation: Anti addition; mechanism involves a halonium ion intermediate.
Halohydrin Formation: Water attacks the halonium ion, yielding a halohydrin (halide + OH).
Regioselectivity: Halide adds to less substituted carbon; OH to more substituted carbon.
Stereochemistry: Anti addition; stereospecificity depends on starting alkene.

8.11 Anti Dihydroxylation

Anti dihydroxylation adds two OH groups across the π bond in a two-step process.

Step 1: Alkene is converted to an epoxide using a peroxyacid (RCO3H).
Step 2: Epoxide is opened with water and acid to yield anti diol.
Mechanism: Nucleophile attacks from the side opposite the leaving group (SN2-like).

8.12 Syn Dihydroxylation

Syn dihydroxylation adds two OH groups in a concerted, syn fashion.

Reagents: OsO4 (with NMO or alkyl peroxide as co-oxidant), or KMnO4 under mild conditions.
Mechanism: Both OH groups add to the same face of the alkene.
Example: Useful for synthesizing vicinal diols.

8.13 Oxidative Cleavage (Ozonolysis)

Oxidative cleavage breaks the C=C double bond, forming carbonyl compounds.

Reagents: Ozone (O3), followed by reducing agents like DMS or Zn/H2O.
Products: Aldehydes and/or ketones, depending on alkene substitution.
Example: Ozonolysis of cyclohexene yields adipaldehyde.

8.14 Predicting Products of Addition Reactions

To predict products, analyze reagents, regioselectivity, and stereospecificity.

Key Steps: Identify what groups are added, whether Markovnikov or anti-Markovnikov, and whether syn or anti addition occurs.
Practice: Familiarity with mechanisms and reagents aids in product prediction.

8.15 One-Step Syntheses and Functional Group Manipulation

Planning syntheses involves assessing reactants and products, and choosing appropriate reactions and reagents.

Addition, Substitution, Elimination: Choose based on desired transformation.
Changing Position of Halogen or OH: Often requires a two-step sequence (elimination followed by addition).
Changing Position of π Bond: Combine anti-Markovnikov addition and elimination (e.g., HBr/ROOR, then t-BuOK).
Example: To move a halogen, use elimination with a non-bulky base, then Markovnikov addition.

Review Table: Alkene Addition Reactions

The following table summarizes the main addition reactions of alkenes, their reagents, regioselectivity, and stereochemistry.

Reaction	Reagents	Regioselectivity	Stereochemistry
Hydrohalogenation (Markovnikov)	HX	Markovnikov	Racemic (if chiral center)
Hydrohalogenation (anti-Markovnikov)	HBr, ROOR	Anti-Markovnikov	Racemic
Acid-catalyzed hydration	H2O, H2SO4	Markovnikov	Racemic
Oxymercuration-demercuration	Hg(OAc)2, H2O; NaBH4	Markovnikov	Racemic
Hydroboration-oxidation	BH3·THF; H2O2, NaOH	Anti-Markovnikov	Syn
Hydrogenation	H2, metal catalyst	None	Syn
Bromination/halogenation	Br2 or Cl2	None	Anti
Halohydrin formation	Br2 or Cl2, H2O	OH to more substituted carbon	Anti
Anti dihydroxylation	RCO3H, H2O	None	Anti
Syn dihydroxylation	OsO4, NMO or KMnO4	None	Syn
Ozonolysis	O3, DMS or Zn/H2O	None	Cleavage

Summary

Chapter 8 covers the mechanisms, regioselectivity, and stereochemistry of ten key alkene addition reactions. Understanding these reactions is essential for predicting products and planning organic syntheses.