How do you synthesize cis-1,2-dibromocyclohexane from the starting product of 1-methylcyclohexane
The correct answer and explanation is :
To synthesize cis-1,2-dibromocyclohexane from 1-methylcyclohexane, follow these steps:
Reaction Steps:
- Allylic Bromination (Optional): Since 1-methylcyclohexane has a methyl group, it can undergo allylic bromination with NBS (N-Bromosuccinimide) and light (hv) to form 1-bromomethylcyclohexane. However, this step is unnecessary if we proceed with direct bromination.
- Hydrohalogenation to Form Cyclohexene:
- Treat 1-methylcyclohexane with concentrated sulfuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) under heat.
- This causes E1 elimination of H₂O to form 1-methylcyclohexene (a more substituted, stable alkene).
- Bromination of Alkene:
- React 1-methylcyclohexene with Br₂ in an inert solvent like CCl₄ or CH₂Cl₂.
- This results in an anti-addition of bromine via a bromonium ion intermediate, forming trans-1-bromo-2-methylcyclohexane.
- However, we need the cis isomer, so this pathway isn’t ideal.
- Catalytic Hydrogenation of Bromocyclohexene (Preferred Route for cis):
- Instead of Br₂, use Br₂ in water (Br₂/H₂O) to form 1-bromo-2-hydroxycyclohexane (via anti-addition).
- Convert the alcohol (-OH) into a bromide using PBr₃ or HBr to get cis-1,2-dibromocyclohexane.
Why This Works for cis?
- The halohydrin intermediate undergoes SN2 substitution with inversion, which compensates for the original anti-addition of Br₂, yielding the cis product.
Summary:
- Convert 1-methylcyclohexane → 1-methylcyclohexene via acidic elimination.
- Brominate using Br₂/H₂O to get 1-bromo-2-hydroxycyclohexane.
- Convert -OH to Br using PBr₃, ensuring a final cis configuration.
This pathway efficiently produces cis-1,2-dibromocyclohexane while avoiding the undesired trans product.