Chapter 9: Alkenes (Part 2) – Addition Reactions and Stereochemistry

Reactions of Alkenes

Overview of Alkene Addition Reactions

Alkenes undergo a variety of addition reactions, which are among the most common transformations in organic chemistry. These reactions typically involve the addition of atoms or groups across the carbon–carbon double bond, converting the alkene into a more saturated compound.

• Halogenation: Addition of halogens (Br2, Cl2) to form vicinal dihalides.

• Hydrohalogenation: Addition of HX (HBr, HCl) to form alkyl halides.

• Alkoxyhalogenation: Addition of halogen and alcohol or salt to form haloethers or halohydrins.

• Epoxidation: Formation of epoxides using peroxyacids.

• Dihydroxylation: Formation of diols using OsO4 or KMnO4.

• Ozonolysis: Cleavage of double bonds to form carbonyl compounds.

• Catalytic Hydrogenation: Reduction of alkenes to alkanes using H2 and a metal catalyst.

Halogenation

Mechanism and Products

Halogenation involves the addition of halogens (bromine or chlorine) to alkenes in inert solvents, forming vicinal dihalides. The reaction proceeds via a cyclic halonium ion intermediate, which prevents carbocation rearrangements.

• Polarizability: Halogens are polarizable, allowing induced dipoles to facilitate the reaction.

• Oxidation Reaction: The process is considered an oxidation.

• Vicinal: Refers to groups attached to adjacent carbons.

Example Equations:

• $\mathrm{CH_2=CH_2} + \mathrm{Br_2} \rightarrow \mathrm{CH_2Br-CH_2Br}$ (vicinal dibromide)

• $\mathrm{CH_2=CH_2} + \mathrm{Cl_2} \rightarrow \mathrm{CH_2Cl-CH_2Cl}$ (vicinal dichloride)

Mechanism: Formation of a cyclic bromonium ion intermediate, followed by nucleophilic attack by Br- on the opposite side (anti addition).

Key Point: No carbocation rearrangements occur in this reaction.

Hydrohalogenation (Halohydrin Formation)

Competing Nucleophiles and Product Formation

If the solvent is not inert (e.g., water), a competing reaction occurs, forming a halohydrin—a compound containing both a halogen and an alcohol group. This is also an oxidation reaction.

• Major Product: Halohydrin (alcohol and halogen on adjacent carbons)

• Minor Product: Vicinal dihalide

Example Equation:

• $\mathrm{CH_2=CH_2} + \mathrm{Br_2} + \mathrm{H_2O} \rightarrow \mathrm{CH_2Br-CH_2OH}$ (major) $+ \mathrm{CH_2OH-CH_2Br}$ (minor) $+ \mathrm{HBr}$

Mechanism: Cyclic bromonium ion intermediate, followed by nucleophilic attack by water.

Alkoxyhalogenation

Formation of Haloethers and Halohydrins

When other nucleophiles (such as alcohols or salts) are present, they can compete with the halide ion, leading to the formation of new products such as haloethers or halohydrins.

• Example: $\mathrm{CH_3CH=CCH_3} + \mathrm{Cl_2} + \mathrm{CH_3OH} \rightarrow \mathrm{CH_3CHClC(OCH_3)CH_3} + \mathrm{HCl}$

• Example: $\mathrm{CH_2=CHCH_2} + \mathrm{Br_2} + \mathrm{NaCl} \rightarrow \mathrm{CH_2BrCHClCH_2Br} + \mathrm{NaBr}$

Epoxidation

Formation of Epoxides

Epoxidation converts alkenes into epoxides using peroxyacids. An epoxide is a three-membered cyclic ether, and a peroxyacid is a carboxylic acid with an extra oxygen atom.

• Common Reagent: mCPBA (meta-chloroperoxybenzoic acid)

• Mechanism: Concerted addition, no carbocation rearrangement.

Example Equation:

• $\mathrm{RCH=CH_2} + \mathrm{RCO_3H} \rightarrow \mathrm{epoxide} + \mathrm{RCO_2H}$

Dihydroxylation

Formation of Diols

Dihydroxylation adds two hydroxyl groups to an alkene, forming a diol. This can be achieved using osmium tetroxide (OsO4) or cold potassium permanganate (KMnO4).

• Syn Addition: Both OH groups add to the same face of the double bond, forming a cis diol.

• Mechanism: Cyclic osmate ester intermediate.

Example Equation:

• $\mathrm{alkene} + \mathrm{OsO_4} + \mathrm{NaHSO_3} \rightarrow \mathrm{cis-diol}$

• $\mathrm{alkene} + \mathrm{KMnO_4} + \mathrm{H_2O} \rightarrow \mathrm{cis-diol}$

Trans Diol Formation: Epoxidation followed by acid-catalyzed hydrolysis yields trans diols.

Ozonolysis

Cleavage of Alkenes to Carbonyl Compounds

Ozonolysis is a reaction where alkenes are cleaved by ozone (O3), forming carbonyl compounds such as aldehydes and ketones.

• Mechanism: Formation of molozonide and ozonide intermediates, followed by reductive workup (e.g., Zn or DMSO).

Example Equation:

• $\mathrm{RCH=CHR'} + \mathrm{O_3} \rightarrow \mathrm{RCHO} + \mathrm{R'CHO}$

Catalytic Hydrogenation

Reduction of Alkenes to Alkanes

Catalytic hydrogenation reduces alkenes to alkanes by adding hydrogen across the double bond. The reaction requires a metal catalyst, typically palladium on carbon (Pd/C).

• Example Equation:

• $\mathrm{alkene} + \mathrm{H_2} \xrightarrow{\mathrm{Pd/C}} \mathrm{alkane}$

• Mechanism: Not fully known; involves adsorption of H2 and alkene onto the metal surface.

Stereochemistry of Alkene Addition Reactions

Key Concepts

Stereochemistry refers to the spatial arrangement of atoms in molecules. Addition reactions to alkenes can be stereoselective (favoring one stereoisomer) or stereospecific (where the stereochemistry of the reactant determines the product).

• Chiral Centers: Formation of new chiral centers can lead to stereoisomers (enantiomers, diastereomers).

• Anti Addition: Groups add to opposite faces of the double bond (e.g., halogenation).

• Syn Addition: Groups add to the same face of the double bond (e.g., dihydroxylation, catalytic hydrogenation).

Carbocation-Based Stereochemistry

Reactions that proceed via carbocation intermediates (e.g., hydrohalogenation) can yield racemic mixtures due to nucleophilic attack from either face of the planar carbocation.

• Enantiomers: Formed when nucleophile attacks from top or bottom.

• Diastereomers: Possible if multiple chiral centers are formed.

Summary Table: Stereochemistry of Alkene Addition Reactions

Reaction	Type of Addition	Stereoisomers Formed
Addition creating one asymmetric center	syn or anti	If no asymmetric center, racemic mixture; if one, pair of diastereomers
Addition creating two asymmetric centers (carbocation intermediate)	syn and anti	Four stereoisomers (two pairs of enantiomers)
Addition of Br2	anti	cis—erythro or trans—threo enantiomers
Addition of peroxyacid	syn	cis—erythro or cis enantiomers
Addition of OsO4	syn	cis—erythro or cis enantiomers
Addition of Br2 + H2O	anti	cis—threo or trans—erythro enantiomers

Practice Examples

Predicting Products and Mechanisms

Students are encouraged to predict the products of alkene addition reactions and to draw the mechanisms, considering both regiochemistry and stereochemistry.

• Example: What is the product of cyclohexene + Br2 in CH2Cl2? Answer: trans-1,2-dibromocyclohexane (anti addition)

• Example: What is the product of 1-butene + mCPBA? Answer: 1,2-epoxybutane (syn addition)

Additional info:

• These notes cover the content of Chapter 9 (Alkenes II: Oxidation and Reduction) and integrate stereochemical concepts from Chapter 6.

• Mechanistic details and stereochemical outcomes are essential for understanding organic reaction pathways and predicting products.