Alkenes react readily with chlorine through an electrophilic addition reaction, where the carbon-carbon double bond opens up, leading to the formation of a dichloroalkane.
The Addition Reaction Explained
When alkenes react with chlorine, the unsaturated carbon-carbon double bond (C=C) undergoes an addition reaction. This process involves the breaking of the weaker pi (π) bond within the double bond, allowing new single bonds to form with the chlorine atoms. As a result, the C=C bond opens, and each carbon atom that was part of the original double bond becomes bonded to a chlorine atom. This transforms the alkene from an unsaturated molecule into a saturated molecule, where there is a single bond between the carbon atoms and each carbon is bonded to a halogen atom (chlorine, in this case). The general reaction can be represented as:
R-CH=CH-R' + Cl₂ → R-CHCl-CHCl-R'
Mechanism of Reaction
The reaction of alkenes with chlorine proceeds via an electrophilic addition mechanism. This mechanism is characteristic of reactions involving alkenes, where the electron-rich double bond acts as a nucleophile, attacking an electron-deficient species (an electrophile).
Key Steps
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Step 1: Electrophilic Attack and Formation of a Cyclic Chloronium Ion.
- The electron cloud of the alkene's pi bond is attracted to the electrophilic chlorine molecule.
- One of the chlorine atoms in the Cl₂ molecule accepts a pair of electrons from the alkene, forming a bond with both carbons of the original double bond simultaneously. This displaces the other chlorine atom as a chloride ion (Cl⁻).
- A three-membered ring intermediate, known as a cyclic chloronium ion, is formed.
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Step 2: Nucleophilic Attack by the Chloride Ion.
- The chloride ion (Cl⁻), which was released in the first step, acts as a nucleophile.
- It attacks one of the carbon atoms of the cyclic chloronium ion from the side opposite to the existing chlorine atom (this is known as anti-addition).
- This attack opens the three-membered ring, resulting in the formation of the final dichloroalkane product, where the two chlorine atoms are attached to adjacent carbon atoms on opposite faces of the original double bond.
Products of the Reaction
The product of the reaction between an alkene and chlorine is a vicinal dichloroalkane, which means that the two chlorine atoms are attached to adjacent carbon atoms. These products are saturated hydrocarbons, as all carbon-carbon bonds are now single bonds.
Example: Ethene with Chlorine
When ethene (C₂H₄), the simplest alkene, reacts with chlorine, it forms 1,2-dichloroethane.
CH₂=CH₂ + Cl₂ → CH₂Cl-CH₂Cl
(Ethene + Chlorine → 1,2-dichloroethane)
Example: Propene with Chlorine
Propene (CH₃CH=CH₂) reacts with chlorine to yield 1,2-dichloropropane.
CH₃-CH=CH₂ + Cl₂ → CH₃-CHCl-CH₂Cl
(Propene + Chlorine → 1,2-dichloropropane)
Practical Aspects and Uses
- Conditions: This reaction is typically very fast and occurs readily at room temperature without the need for a catalyst or light. It can be carried out by bubbling chlorine gas through the alkene or by mixing the alkene with a solution of chlorine in an inert solvent like tetrachloromethane (CCl₄).
- Test for Unsaturation: The reaction with chlorine (or other halogens like bromine) is a classic qualitative test for the presence of carbon-carbon double or triple bonds (unsaturation). Chlorine gas is pale yellow-green, and chlorine water is yellowish. When an alkene is present, the characteristic color of the chlorine rapidly disappears as it is consumed in the addition reaction, indicating unsaturation.
- Industrial Significance: The halogenation of alkenes is an important industrial process for synthesizing various organic compounds. For example, 1,2-dichloroethane is a crucial precursor in the production of vinyl chloride, which is then polymerized to make polyvinyl chloride (PVC), a widely used plastic.
Summary Table
| Reaction Type | Reactant | Reagent | Product Type | Conditions |
|---|---|---|---|---|
| Electrophilic Addition | Alkene | Chlorine (Cl₂) | Vicinal Dichloroalkane | Room temperature, no catalyst/light |
Further Resources
For a deeper dive into electrophilic addition mechanisms, you can refer to organic chemistry textbooks on addition reactions.
Learn more about the industrial applications of halogenated alkanes at chemical engineering resources.
