Wurtz Reaction: Mechanism, Examples & Limitations
The Wurtz Reaction is a fundamental coupling reaction in organic chemistry used to synthesize symmetrical alkanes. Named after Charles Adolphe Wurtz, this reaction involves the treatment of alkyl halides with sodium metal in the presence of dry ether to form higher alkanes.
1. General Reaction
In this reaction, two molecules of an alkyl halide ($R-X$) react with two atoms of sodium ($Na$) to form an alkane ($R-R$) and sodium halide ($NaX$).
Where:
- $R$ = Alkyl group (e.g., Methyl, Ethyl)
- $X$ = Halogen (Cl, Br, I)
- Dry Ether: Used as a solvent to ensure moisture does not react with Sodium.
2. Mechanism of Wurtz Reaction
There are two primary proposed mechanisms for the Wurtz reaction: the Free Radical Mechanism and the Ionic (Organometallic) Mechanism.
A. Free Radical Mechanism
In this pathway, the sodium atom transfers an electron to the alkyl halide, generating an alkyl free radical.
Step 1: Formation of radical
$$ R-X + Na \rightarrow R^\bullet + Na^+X^- $$Step 2: Dimerization
$$ R^\bullet + R^\bullet \rightarrow R-R $$B. Ionic Mechanism
This involves the formation of an organometallic intermediate (organosodium).
Step 1: Formation of carbanion
$$ R-X + 2Na \rightarrow R^-Na^+ + Na^+X^- $$Step 2: Nucleophilic attack ($S_N2$)
$$ R^-Na^+ + R-X \rightarrow R-R + Na^+X^- $$3. Examples
Example 1: Synthesis of Ethane
Reaction of methyl bromide with sodium.
$$ 2CH_3Br + 2Na \xrightarrow{\text{dry ether}} CH_3-CH_3 + 2NaBr $$Example 2: Synthesis of n-Butane
Reaction of ethyl chloride with sodium.
$$ 2C_2H_5Cl + 2Na \xrightarrow{\text{dry ether}} C_4H_{10} + 2NaCl $$4. Limitations of Wurtz Reaction
Crucial Exam Points
- Cannot prepare Methane ($CH_4$).
- Poor yield for unsymmetrical alkanes.
- $3^\circ$ Alkyl halides undergo elimination.
- Methane cannot be prepared: The smallest product formed is ethane ($n=2$), as it involves the coupling of two carbon units.
- Mixture of Products: If two different alkyl halides ($R-X$ and $R'-X$) are used, a mixture of three alkanes ($R-R$, $R'-R'$, and $R-R'$) is obtained, which is difficult to separate.
- Side Reactions: Tertiary alkyl halides usually undergo dehydrohalogenation (elimination) in the presence of sodium to form alkenes instead of alkanes.
5. Related Reactions
- Fittig Reaction: Coupling of two aryl halides to form bi-phenyls.
- Wurtz-Fittig Reaction: Coupling of an alkyl halide and an aryl halide to form alkylarenes.
Wurtz Reaction Mastery Quiz
Test your understanding with these 10 MCQs. Explanations provided upon submission.
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