Corey-House Synthesis: Unsymmetrical Alkanes
The Corey-House Synthesis is a versatile organometallic reaction used to couple two different alkyl halides to form an unsymmetrical alkane. It overcomes the limitations of the Wurtz reaction by using a lithium dialkylcuprate, also known as the Gilman Reagent.
1. General Reaction
The reaction involves the coupling of a lithium dialkylcuprate ($R_2CuLi$) with an alkyl halide ($R'X$).
Key Components:
- Gilman Reagent: $R_2CuLi$ (Source of nucleophilic alkyl group $R^-$).
- Substrate: $R'-X$ (Alkyl Halide).
- Product: Alkane ($R-R'$).
2. Preparation of Gilman Reagent
The reagent is prepared in situ from an alkyl halide in two steps.
Step 1: Formation of Alkyllithium
An alkyl halide ($R-X$) reacts with Lithium metal.
Step 2: Formation of Dialkylcuprate
Alkyllithium reacts with Cuprous Iodide ($CuI$).
3. Mechanism and Constraints
The coupling step proceeds via a nucleophilic substitution mechanism similar to $S_N2$.
Rules for $R$ and $R'$
- Group R in Reagent ($R_2CuLi$): Can be Methyl, Primary ($1^\circ$), Secondary ($2^\circ$), Tertiary ($3^\circ$), Aryl, or Vinyl. It is versatile.
- Group R' in Substrate ($R'X$): Must be Methyl or Primary ($1^\circ$).
- Reason: Since the mechanism is $S_N2$-like, steric hindrance in secondary or tertiary substrates ($R'X$) leads to Elimination (forming alkenes) instead of coupling.
4. Comparison with Wurtz Reaction
| Feature | Wurtz Reaction | Corey-House Synthesis |
|---|---|---|
| Best for | Symmetrical Alkanes (Even Carbon #) | Unsymmetrical Alkanes (Odd Carbon #) |
| Product Mixture | Mixture of 3 products if reactants different | Selective formation of cross-product |
| Mechanism | Free Radical / Organometallic | Nucleophilic Substitution ($S_N2$) |
5. Example: Synthesis of Propane
To synthesize Propane ($CH_3-CH_2-CH_3$), we need to couple a methyl group and an ethyl group.
Correct Combination:
$$ (CH_3)_2CuLi + CH_3CH_2-Br \rightarrow CH_3-CH_2-CH_3 $$(Or $(CH_3CH_2)_2CuLi + CH_3-Br$)
Corey-House Quiz
Test your concepts on Alkane synthesis. 10 MCQs with explanations.
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