Electrophilic Addition Mechanisms to Alkenes: Organic Chemistry Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Reactions of Alkenes

Overview of Alkene Reactivity

Alkenes are unsaturated hydrocarbons characterized by the presence of a carbon-carbon double bond. This double bond is electron-rich and acts as a nucleophile, making alkenes highly reactive toward electrophiles. Electrophilic addition reactions are fundamental transformations in organic chemistry, leading to a variety of useful products.

Key Products: Halohydrins, alcohols, 1,2-diols, 1,2-dihalides, halides, epoxides, cyclopropanes, and carbonyl compounds.
General Mechanism: Electrophile attacks the π electrons of the alkene, followed by nucleophilic addition.
Applications: Synthesis of pharmaceuticals, polymers, and fine chemicals.

Addition of Halogens to Alkenes

Halogenation Reaction

Bromine (Br2) and chlorine (Cl2) add to alkenes to form vicinal dihalides (1,2-dihalides). Fluorine is too reactive, and iodine does not add under normal conditions.

Example Reaction:
- Ethylene + Cl2 → 1,2-Dichloroethane (Ethylene dichloride)
Stereochemistry: Addition is typically anti (trans) due to the mechanism.

Addition of Br2 to Cyclopentene

Stereospecificity of Halogen Addition

When Br2 is added to cyclopentene, the product is exclusively trans-1,2-dibromocyclopentane. The cis isomer is not formed due to the anti addition mechanism.

Mechanistic Reason: Formation of a bromonium ion intermediate enforces trans addition.

Mechanism of Bromine Addition

Bromonium Ion Formation

Bromine adds to an alkene, producing a cyclic bromonium ion. This intermediate is stabilized by charge sharing between bromine and carbon atoms.

Step 1: π electrons of alkene attack Br2, forming a bromonium ion and Br-.
Step 2: Br- attacks the bromonium ion from the side opposite the C–Br bond (anti addition).
Result: Trans stereochemistry in the product.

Bromonium Ion Mechanism

Electrophilic Addition and Stereochemistry

The bromonium ion is a reactive electrophile, and the bromide ion is a good nucleophile. The reaction is stereospecific, resulting in anti addition.

Key Feature: Attack occurs on the side opposite the bromonium ion, leading to trans products.
Example: Cyclopentene + Br2 → trans-1,2-dibromocyclopentane

Formation of Bromonium Ion

Electron Flow and Polarization

Mutual polarization of electron distributions between Br2 and the alkene initiates the reaction. Electrons flow from the alkene toward Br2, and π electrons of the alkene displace Br- from Br2.

Mechanistic Step:

Stereochemistry of Halogen Addition

Anti Addition and Product Formation

Attack of Br- from the side opposite the C–Br bond of the bromonium ion gives anti addition. This leads to trans stereochemistry in vicinal dibromides.

Example: Cyclohexene + Br2 → trans-1,2-dibromocyclohexane (racemic mixture)
Note: Syn addition does not occur in this reaction.

Mechanistic Steps

Stepwise Formation of Products

Step 1: Formation of cyclic bromonium ion (major contributing structures retain geometry).
Step 2: Anti (coplanar) orientation of added bromine atoms, leading to trans products.

Regioselectivity in Halogen Addition

Influence of Substituents and Nucleophiles

If Br2 is added to propene, there is no regioselectivity issue. In the presence of alternative nucleophiles (e.g., CH3OH), regioselectivity becomes important.

Major Product: Nucleophile attacks the more carbocation-like carbon in the bromonium ion.

Bromination of Substituted Cyclohexene

Conformational Effects

The presence of bulky groups (e.g., tert-butyl) locks the ring conformation, affecting the stereochemistry of bromine addition. Only one stereoisomer is formed due to restricted ring flipping and steric effects.

Observed Diastereomer: Bromines are kept anti to each other (axial positions).

Addition of Hypohalous Acids to Alkenes: Halohydrin Formation

Halohydrin Synthesis

Formally, the addition of HO–X to an alkene gives a 1,2-halo alcohol (halohydrin). The actual reagent is a dihalogen (Br2 or Cl2) in water or an organic solvent.

General Reaction:
Mechanism: Bromonium ion forms, then water acts as nucleophile and attacks the more substituted carbon.

Mechanism of Halohydrin Formation

Markovnikov Orientation

For unsymmetrical alkenes, halohydrin formation follows Markovnikov-like orientation. Water adds to the carbon with the most δ+ charge, predicted by carbocation stability.

Step 1: Br2 adds to the double bond, forming a bromonium ion.
Step 2: H2O attacks the more substituted carbon, leading to halohydrin formation.

Addition of Water to Alkenes: Oxymercuration

Oxymercuration-Demercuration

Hydration of an alkene is the addition of H–OH to give an alcohol. Oxymercuration uses mercuric acetate in THF, followed by sodium borohydride, to achieve Markovnikov orientation via a mercurinium ion intermediate.

General Reaction: (then NaBH4 replaces HgOAc with H)
Key Intermediates: Mercurinium ion, organomercury compound

Addition of Water to Alkenes: Hydroboration

Hydroboration-Oxidation

Borane (BH3) is electron deficient and acts as a Lewis acid. It adds to alkenes to give organoboranes, which are then oxidized to alcohols. The reaction is anti-Markovnikov and stereospecific (syn addition).

General Reaction: (then gives alcohol)
Regiochemistry: OH is added to the less substituted carbon.
Stereochemistry: H and OH add to the same face of the alkene (syn addition).

Borane Chemistry

Structure and Reactivity

Borane: , often exists as diborane ()
Complexes: Borane forms adducts with tetrahydrofuran (THF)
Isoelectronic: Borane is isoelectronic with carbocations

Mechanism of Hydroboration

Transition State and Product Formation

Transition State: Involves anionic development on boron
Product: More stable carbocation is consistent with observed regioselectivity

Catalytic Hydrogenation Mechanism

Syn Addition of Hydrogen

Catalytic hydrogenation is a heterogeneous reaction where H2 is added to alkenes in the presence of a metal catalyst. The addition is syn, meaning both hydrogens add to the same face of the alkene.

Key Feature: Used industrially for the production of saturated hydrocarbons.

Oxidation of Alkenes: Epoxidation and Hydroxylation

Epoxidation and Diol Formation

Oxidation involves the addition of oxygen or loss of hydrogen. Epoxidation produces cyclic ethers (epoxides), while hydroxylation yields 1,2-diols.

Epoxidation: MCPBA in CH2Cl2 is commonly used.
Stereochemistry: Syn addition for both epoxidation and hydroxylation.
Hydroxylation: Osmium tetroxide catalyzes syn-diol formation.

Osmium Tetroxide Catalyzed Formation of Diols

Syn-Diol Synthesis

Osmium tetroxide (OsO4) converts alkenes to syn-diols via cyclic osmate di-ester intermediates. Sodium bisulfate is used to cleave the intermediate and release the diol.

Key Feature: Osmium is toxic; catalytic amounts and NMO (N-methylmorpholine N-oxide) are used.