Chapter 8: Reactions of Alkenes – Mechanisms, Regioselectivity, and Stereochemistry

Chapter 8: Reactions of Alkenes

Introduction to Alkene Reactivity

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. The double bond consists of a sigma (σ) and a pi (π) bond, with the π bond being more reactive due to its higher electron density and accessibility. This chapter explores the various reactions alkenes undergo, focusing on electrophilic addition mechanisms, regioselectivity, and stereochemistry.

• π bonds are more reactive than σ bonds because π electrons are more exposed and can interact with electrophiles.

• Electrophilic addition is the most common reaction type for alkenes, where an electrophile attacks the π bond, forming a carbocation intermediate.

• After addition, rearrangements may occur to yield more stable carbocations, affecting the final product distribution.

General Mechanism of Electrophilic Addition

• The π electrons of the alkene attack an electrophile (E+), generating a carbocation intermediate.

• A nucleophile (Nu-) then attacks the carbocation, forming the addition product.

Example: Addition of HBr to an alkene:

$\ce{CH2=CH2} + \ce{HBr} \rightarrow \ce{CH3-CH2Br}$

Regioselectivity: Markovnikov's Rule

Markovnikov's Rule predicts the orientation of addition in unsymmetrical alkenes:

• The electrophile adds to the carbon with more hydrogens (less substituted), while the nucleophile adds to the more substituted carbon (more stable carbocation).

Example:

$\ce{CH3-CH=CH2} + \ce{HBr} \rightarrow \ce{CH3-CHBr-CH3}$ (major product)

Carbocation Rearrangements

• Carbocations may rearrange via hydride or alkyl shifts to form more stable intermediates, leading to unexpected products.

Addition of Hydrogen Halides (HX)

• Follows Markovnikov's rule.

• Possible rearrangements if a more stable carbocation can form.

Example:

$\ce{CH3-CH=CH2} + \ce{HCl} \rightarrow \ce{CH3-CHCl-CH3}$

Hydration: Addition of Water (H2O, Acid-Catalyzed)

• Requires a strong acid catalyst (e.g., H2SO4).

• Follows Markovnikov's rule; rearrangements possible.

• Forms alcohols from alkenes.

Example:

$\ce{CH3-CH=CH2} + \ce{H2O} \xrightarrow{H2SO4} \ce{CH3-CH(OH)-CH3}$

Oxymercuration-Demercuration

• Adds H and OH across the double bond without carbocation rearrangement.

• Reagents: 1) Hg(OAc)2, H2O, THF; 2) NaBH4.

• Markovnikov addition of water.

Hydroboration-Oxidation

• Anti-Markovnikov addition of H and OH (OH adds to less substituted carbon).

• Reagents: 1) BH3·THF; 2) H2O2, NaOH.

• Syn addition (H and OH add to the same side).

Stereochemistry of Addition Reactions

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

• Anti addition: Groups add to opposite faces.

• Some reactions yield racemic mixtures if new stereocenters are formed.

Addition of Halogens (X2)

• Halogens (Cl2, Br2) add anti across the double bond via a halonium ion intermediate.

• Produces vicinal dihalides (halogens on adjacent carbons).

• In the presence of water, forms halohydrins (one halogen, one OH group).

Hydrogenation (Addition of H2)

• Adds H2 across the double bond, saturating the molecule to form alkanes.

• Requires a metal catalyst (Pt, Pd, Ni).

• Syn addition; produces racemic mixtures if stereocenters are formed.

Epoxidation

• Forms a three-membered cyclic ether (epoxide) from an alkene using a peracid (e.g., mCPBA).

• Epoxides are useful synthetic intermediates.

• Syn addition; stereochemistry of the alkene is preserved in the epoxide.

Acid-Catalyzed Ring Opening of Epoxides

• Epoxides react with acids (H3O+, H2SO4, H2O) to form trans-1,2-diols (anti dihydroxylation).

• Mechanism involves nucleophilic attack on the more substituted carbon.

Syn Dihydroxylation

• Adds two OH groups to the same side of the alkene (syn addition).

• Reagents: OsO4 or cold, dilute KMnO4.

• Forms cis-1,2-diols.

Oxidative Cleavage of Alkenes

• Strong oxidants (hot, concentrated KMnO4) cleave double bonds, forming ketones, carboxylic acids, or CO2 depending on substitution.

• Ozonolysis (O3) is a milder method, cleaving alkenes to aldehydes and/or ketones.

Alkene Type	KMnO4 Product	Ozonolysis Product
Disubstituted	Ketone	Ketone
Monosubstituted	Carboxylic Acid	Aldehyde
Unsubstituted	CO2	CO2

Example:

$\ce{RCH=CHR'} \xrightarrow{O_3/(CH_3)_2S} \ce{RCHO + R'CHO}$

Additional info: Ozonolysis is preferred when aldehydes are desired, as KMnO4 oxidizes aldehydes further to carboxylic acids.