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

Reactivity of the Carbon–Carbon Double Bond

Stability and Reactivity of Alkenes

The carbon–carbon double bond in alkenes consists of a sigma (σ) bond and a pi (π) bond. The π bond is less stable than the σ bond, making it more reactive and susceptible to chemical transformations. Most reactions of alkenes involve converting the π bond into a σ bond, typically through addition reactions.

• Addition Reaction: A reaction where atoms or groups are added to the carbons of a double bond, converting it into a single bond.

• Example: Catalytic hydrogenation of an alkene:

$\ce{C=C} + \ce{H2} \xrightarrow{\text{catalyst}} \ce{CH-CH}$

• This process releases energy and forms two new C–H σ bonds.

Electrophilic Addition

General Mechanism

Electrophilic addition is the most common reaction of alkenes, involving two main steps:

1. Attack of π Electrons: The π electrons of the alkene attack an electrophile (E+), forming a carbocation intermediate.

2. Nucleophilic Attack: A nucleophile (Nuc−) attacks the carbocation, yielding the addition product.

$\ce{C=C} + \ce{E^+} \rightarrow \ce{C^+–C} \rightarrow \ce{C–C}$

• The overall result is the addition of E and Nuc to the double-bonded carbons.

Types of Additions to Alkenes

Overview Table

The following table summarizes the main types of addition reactions for alkenes:

Type of Addition [Elements Added]	Product
Hydration [H, OH]	Alcohol
Hydrogenation [H, H]	Alkane
Dihydroxylation [OH, OH]	Glycol (1,2-diol)
Oxidative Cleavage [O]	Aldehyde/Ketone/Carboxylic Acid
Halogenation [X, X]	Vicinal Dihalide
Halohydrin Formation [X, OH]	Halohydrin
HX Addition [H, X]	Alkyl Halide
Epoxidation [O]	Epoxide
Cyclopropanation [CH2]	Cyclopropane Derivative

Additional info: Table adapted from slide; only main types shown.

Addition of HX to Alkenes

Mechanism and Regioselectivity

• Step 1: Protonation of the double bond forms the most stable carbocation possible.

• Step 2: The halide ion (X−) attacks the carbocation, forming an alkyl halide.

• Applicable for HBr, HCl, and HI.

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

• Markovnikov's Rule: The proton adds to the carbon with more hydrogens, forming the most stable carbocation intermediate.

Orientation of Addition: Markovnikov's Rule

Regioselectivity in Electrophilic Addition

• When an unsymmetrical alkene reacts with HX, two possible products can form, but the major product results from the most stable carbocation intermediate.

• The halide adds to the more substituted carbon (the one with fewer hydrogens).

• Markovnikov's Rule (Extended): In electrophilic addition, the electrophile adds to generate the most stable carbocation intermediate.

The Reaction-Energy Diagram

Energy Profile of Electrophilic Addition

• The first step (carbocation formation) is rate-determining.

• More stable carbocations have lower activation energies, leading to faster reactions and major products.

Free-Radical Addition of HBr (Anti-Markovnikov Addition)

Mechanism and Peroxide Effect

• In the presence of peroxides (ROOR), HBr adds to alkenes to give the anti-Markovnikov product.

• Peroxides generate free radicals, initiating a chain reaction.

• Only HBr undergoes this reaction efficiently; HCl and HI do not due to unfavorable energetics.

Mechanism:

1. Peroxide bond breaks homolytically, forming radicals.

2. Radical abstracts H from HBr, generating Br•.

3. Br• adds to the alkene, forming a new alkyl radical.

4. Alkyl radical abstracts H from HBr, forming the product and regenerating Br•.

• Overall, H adds to the more substituted carbon, Br to the less substituted (anti-Markovnikov orientation).

Addition of Water: Hydration of Alkenes

Acid-Catalyzed Hydration

• Alkenes react with water in the presence of a strong acid (e.g., H2SO4) to form alcohols.

• This is the reverse of alcohol dehydration.

• Follows Markovnikov's rule; the OH group attaches to the more substituted carbon.

Mechanism:

1. Protonation of the double bond forms a carbocation.

2. Water attacks the carbocation, forming a protonated alcohol.

3. Deprotonation yields the neutral alcohol.

• Carbocation rearrangements (methyl or hydride shifts) may occur to form more stable intermediates.

Oxymercuration–Demercuration Reaction

Markovnikov Hydration Without Rearrangement

• Converts alkenes to alcohols with Markovnikov orientation using mercuric acetate (Hg(OAc)2), water, and sodium borohydride (NaBH4).

• No free carbocation is formed, so rearrangements do not occur.

Mechanism:

1. Electrophilic attack by Hg(OAc)2 forms a three-membered mercurinium ion.

2. Water opens the ring (anti addition), forming an organomercurial alcohol.

3. NaBH4 reduces the organomercurial intermediate to the alcohol.

• Alkoxymercuration–demercuration uses an alcohol instead of water, yielding ethers.

Hydroboration of Alkenes

Anti-Markovnikov Hydration

• Diborane (B2H6) adds to alkenes with anti-Markovnikov orientation, followed by oxidation to give alcohols.

• Commonly performed with BH3·THF and H2O2, NaOH.

Mechanism:

1. Borane adds to the less substituted carbon in a concerted, syn addition.

2. Oxidation replaces boron with OH, retaining stereochemistry.

• Product is an alcohol with anti-Markovnikov orientation and syn stereochemistry.

Addition of Halogens

Halogenation and Stereochemistry

• Cl2, Br2, and sometimes I2 add to alkenes to form vicinal dihalides (halogens on adjacent carbons).

• Reaction proceeds via a three-membered halonium ion intermediate.

• Back-side attack by the halide ion leads to anti addition (opposite sides of the double bond).

Example: Addition of Br2 to cyclohexene yields trans-1,2-dibromocyclohexane (enantiomers).

Bromine Test for Unsaturation

Qualitative Test for Double Bonds

• Addition of Br2 in CCl4 to an alkene causes the red-brown color to disappear as bromine adds to the double bond.

• If no double bond is present, the color remains.

• This is a classic test for unsaturation in organic compounds.

Formation of Halohydrins

Mechanism and Regioselectivity

• Halohydrins are compounds with OH and a halogen on adjacent carbons.

• Formed by adding a halogen to an alkene in the presence of water.

• Water acts as the nucleophile, attacking the more substituted carbon of the halonium ion (Markovnikov orientation).

• Addition is anti (opposite sides) due to the mechanism.

Example: Addition of Br2 in water to cyclohexene yields trans-2-bromocyclohexanol.

Summary Table: Types of Alkene Additions

Reaction Type	Reagents	Orientation	Stereochemistry	Product
Hydrohalogenation	HX	Markovnikov	Not stereospecific	Alkyl halide
Hydration	H2O, acid	Markovnikov	Not stereospecific	Alcohol
Oxymercuration–Demercuration	Hg(OAc)2, H2O; NaBH4	Markovnikov	Anti addition	Alcohol
Hydroboration–Oxidation	BH3·THF; H2O2, NaOH	Anti-Markovnikov	Syn addition	Alcohol
Halogenation	X2	—	Anti addition	Vicinal dihalide
Halohydrin Formation	X2, H2O	Markovnikov (OH to more substituted C)	Anti addition	Halohydrin

Additional info: Table summarizes key reactions, orientation, and stereochemistry for exam review.