23 1 Electrophilic addition of bromine and chlorine to alkenes
Addition of Bromine and Chlorine Across Double Bonds
Overview of Halogenation
Halogenation is the addition of bromine (Br2) or chlorine (Cl2) across a carbon-carbon double bond.
The process often involves using a chlorinated solvent to facilitate the reaction.
Resulting compound: dibromide or dichloride depending on the halogen used.
Mechanistic Details
The addition of bromine is characterized by stereospecific control — the reaction follows an anti-addition pathway.
Stereochemistry is significant as the products can exhibit different configurations leading to enantiomers or meso compounds.
Example: Cyclohexene Bromination
When Br2 is added to cyclohexene:
Two bromine atoms add to the double bond in an anti configuration.
Possible configurations:
One bromine appears above the plane, the other below.
Opposite scenario with bromines positioned inversely.
Resulting configurations are enantiomers, resulting in a racemic mixture.
Stereocenter Assignment for Cyclohexene Products
For the first product:
Highest to lowest priority: 1. Br, 2. C, 3. H (hydrogens facing away).
Configuration: R (clockwise).
For the second product:
Similar highest to lowest priority as above, leading to S configuration (counterclockwise with H coming towards).
Conclusion: The two products are enantiomers; every stereocenter has been inverted, creating a racemic mixture.
Example: Chlorination of Trans-Substituted Ethylene
Chlorine addition to trans-substituted ethylene can yield different stereochemical outcomes:
Comparison between different configurations leads to one stereocenter being S and another being R.
There exists a mirror plane bisecting the molecule, indicating meso compound formation.
Final product: only one compound due to the symmetry of the molecule.
Mechanism of Bromonium Ion Formation
The mechanism involves the formation of a bromonium ion:
Bromine polarizes in the presence of an alkene to create a partial positive and negative charge.
The alkene's electrons attack the bromine, generating a bromonium ion signature (Br^+ with associated lone pairs).
This is a key divergence from the carbocation mechanism seen in other addition reactions.
The bromonium ion structure is formed when one bromine forms a bond with both carbons of the double bond.
Stereochemistry of Bromonium Ion Reaction
Nucleophilic attack occurs at the bromonium ion, influencing the stereochemistry:
Attack direction determines whether the resulting product has bromine or hydrogen positioned above or below the plane.
This leads to products that can be assigned R or S configurations based on the three-dimensional arrangement of groups around the stereocenters.
Conclusion of Reaction Outcomes
The outcome of halogenation of alkenes is dependent on the substrate and the stereochemistry of addition.
Different attack directions lead to different stereo configurations and product outcomes.
It is encouraged to practice drawing product structures to reinforce understanding of stereochemistry and reaction pathways.
Addition of Bromine and Chlorine Across Double Bonds
Overview of Halogenation
Halogenation is a chemical process that involves the addition of halogens, specifically bromine (Br2) or chlorine (Cl2), across carbon-carbon double bonds (alkenes). This reaction is significant in organic chemistry as it modifies the properties of alkenes, turning them into dibromides or dichlorides, depending on the halogen utilized. The halogenation typically requires a chlorinated solvent preferably chloroform (CHCl3) or dichloromethane (DCM), which aids in the solubility of the reactants and promotes the reaction's progress.
Mechanistic Details
The addition of bromine follows a stereospecific control mechanism characterized by anti-addition. This stereochemistry is crucial because it results in the formation of products with varying configurations, which can include enantiomers or meso compounds. The anti-addition pathway ensures that the two bromine atoms or chlorine atoms add to opposite sides of the double bond, resulting in distinct spatial arrangements of the atoms in the product.
Example: Cyclohexene Bromination
When Br2 is reacted with cyclohexene, the following occurs:
Two bromine atoms are added to the double bond of cyclohexene in an anti configuration.
**Possible Configurations: **
One bromine appears above the plane (wedge), and the other below (dash).
Conversely, one bromine appears below while the other is above.
These two configurations formed are geometric isomers known as enantiomers, which leads to a racemic mixture due to the equal formation of both configurations.
Stereocenter Assignment for Cyclohexene Products
For the formation of the two products from cyclohexene:
First Product:
Priority assignment (Highest to Lowest): 1. Br, 2. C, 3. H (where hydrogens are positioned so that they face away from the viewer).
Configuration is designated as R (clockwise).
Second Product:
Similar priority assignment as above, leading to an S configuration (counterclockwise, with H coming towards the viewer).
Thus, the formed products are enantiomers as each stereocenter is inverted, culminating in a racemic mixture (equal amounts of both enantiomers).
Example: Chlorination of Trans-Substituted Ethylene
The chlorination of trans-substituted ethylene presents additional stereochemical outcomes:
The addition leads to different configurations resulting in one stereocenter being assigned S and another being assigned R.
A significant feature is the presence of a mirror plane bisecting the molecule, indicative of a meso compound formation.
The final product comprises only a single compound due to the inherent symmetry of the trans-substituted ethylene molecule, influencing the overall stereochemical outcome.
Mechanism of Bromonium Ion Formation
The mechanism of bromination revolves around the formation of a reactive bromonium ion intermediate:
Initially, bromine becomes polarized in the presence of the alkene, resulting in a partial positive charge on one bromine atom and a negative charge on the other, leading to the bromonium ion formation.
The alkene’s π electrons attack the polarized bromine, which creates a bromonium ion (Br^+), where one bromine atom forms bonds to both carbons of the double bond.
This mechanism distinctly contrasts with the carbocation mechanism observed in many other addition reactions, marking a unique path for bromination.
Stereochemistry of Bromonium Ion Reaction
The stereochemical outcomes of the bromonium ion reaction are influenced by the direction of nucleophilic attack on the bromonium ion:
The direction from which the nucleophile attacks determines whether bromine or hydrogen ends up positioned above or below the reaction plane.
This stereochemical influence allows for products to be designated with R or S configurations based on the three-dimensional arrangement of substituents around the stereocenters formed during the reaction.
Conclusion of Reaction Outcomes
The outcome of halogenation reactions of alkenes is intricately dependent on:
The structure of the substrate (alkene),
The stereochemistry inherent in the addition process.
Different nucleophilic attack directions can lead to diverse stereo configurations and product types.
It is beneficial for learners to practice drawing the product structures to cement their understanding of stereochemistry and reaction pathways in halogenation reactions.