Question
Show three different methods to prepare bromoethane (): (a) from ethanol, (b) from ethane, and (c) from ethylene. Which method is best for lab preparation and why?
(CBSE Class 12 / JEE Main pattern)
Solution — Step by Step
flowchart TD
A["Alkyl Halide\n(R-X)"] --- B["From Alcohol"]
A --- C["From Alkane"]
A --- D["From Alkene"]
B -->|"R-OH + HBr\nor R-OH + PBr₃\nor R-OH + SOCl₂"| A
C -->|"R-H + X₂\nhv (free radical)"| A
D -->|"R₂C=CR₂ + HBr\n(Markovnikov or\nanti-Markovnikov)"| AMethod 1:
Method 2 (better yield):
Method 3:
SOCl₂ (thionyl chloride) with pyridine is the best lab method because both by-products (SO₂ and HCl) are gases and escape, making purification easy. This is called the Darzen’s process.
This is a free radical chain reaction with three stages: initiation (Br₂ → 2Br· by UV light), propagation (Br· + C₂H₆ → C₂H₅· + HBr, then C₂H₅· + Br₂ → C₂H₅Br + Br·), and termination.
Limitation: gives a mixture of mono-, di-, and poly-substituted products. Not suitable for selective preparation.
This follows Markovnikov addition — H goes to the carbon with more H atoms. For ethylene (symmetrical alkene), there is no selectivity issue.
For anti-Markovnikov addition (peroxide effect): HBr in the presence of benzoyl peroxide gives the opposite orientation. This works only for HBr, not HCl or HI.
Why This Works
Each method breaks different bonds. The alcohol route replaces O-H/C-O with C-X (substitution). The alkane route replaces C-H with C-X (free radical). The alkene route adds H and X across C=C (addition). The choice depends on the starting material available and the selectivity needed.
The SOCl₂ method is preferred in labs because it gives the cleanest product. PBr₃ is good for bromides specifically. HX directly works but may give rearranged products with secondary and tertiary alcohols via carbocation intermediates.
Alternative Method — Finkelstein and Swarts Reactions
For interconversion of halides:
- Finkelstein: (works because NaCl precipitates in acetone, driving equilibrium)
- Swarts:
JEE Main frequently asks about the order of reactivity of alcohols with HBr: 3° > 2° > 1°. This is because tertiary carbocations are more stable (SN1 mechanism). Primary alcohols react through SN2, which is slower with HBr but works well with PBr₃.
Common Mistake
A classic error: writing SOCl₂ as the reagent for preparing bromides. SOCl₂ (thionyl chloride) gives chlorides only. For bromides, use PBr₃ or HBr. Similarly, PCl₅ gives chlorides, not bromides. Always match the halide in the reagent with the halide in the product.