Lecture on Alkyl Halides: Bromoethane and Bromohexane Preparation

Introduction

• Alkyl Halides: Compounds with an alkane carbon bonded to a halogen (e.g., fluorine, chlorine, bromine, iodine).
  • Applications: Pesticides, flame retardants, drugs, lubricants, solvents, refrigerants.
• Purpose: To make grignard reagents.
• Known Reactions:
  • Made chloroform via the haloform reaction.
  • Produced chlorobenzyl chloride through photochemical halogenation.

Safety Precautions

• Corrosive Substances: Hydrobromic and sulfuric acids.
• Carcinogenic Compounds: Bromoethane and bromohexane.
• Volatility: Bromoethane.
• Safety Measures: Wear gloves, work in a fume hood or outside.

Bromoethane Production

Setup

• Equipment: Distillation apparatus.
  • Boiling flask: 1L, Receiver flask: 500mL.
• Cooling: Pack boiling flask in ice water initially.

Procedure

1. Hydrobromic Acid: Add 350g of 48% hydrobromic acid.
2. Cooling Period: Wait 10 minutes.
3. Sulfuric Acid: Add 55mL concentrated sulfuric acid slowly.
4. Ethanol Addition: Add 120mL water-free or 95% ethanol.
5. Temperature Control: Allow mixture to cool, then remove ice bath.
6. Additional Sulfuric Acid: Drip 90mL concentrated sulfuric acid slowly.

Reaction

• Chemical Reaction: Hydrobromic acid reacts with ethanol to create bromoethane.
  • Reaction Type: Sn2 nucleophilic substitution.
  • Role of Sulfuric Acid: Absorbs water, aiding reaction continuation.
• Distillation: Heat using a water bath (40°C to 70°C). Gradually ramp up temperature.

Purification

• Contaminants: Bromine and diethyl ether.
• Washing:
  1. Mix with 300mL of water, decant supernatant.
  2. Stir in 5% sodium bicarbonate solution, decant again.
  3. Additional water wash.
• Drying: Use anhydrous magnesium sulfate to remove trace water.
• Yield: 116g (~50% yield).

Bromohexane Production

Alternative Method

• Sodium Bromide: 150g as bromide source.
• Water and 1-Hexanol: Add 150mL water and 102g of 1-hexanol.

Procedure

1. Sulfuric Acid Addition: Use addition funnel for 100mL of sulfuric acid.
2. Reflux: Reflux mixture for 6 hours.

Reaction

• Formation of Hydrobromic Acid: Sodium bromide and sulfuric acid form hydrobromic acid.
• Bromohexane Formation: Similar Sn2 reaction mechanism.

Distillation

• Initial Distillate: Collect azeotrope of bromohexane and impurities.
• Fractional Distillation: Collect bromohexane at 150°C to 154°C.

Purification

• Washing: Water and sodium bicarbonate wash.
• Drying: Use magnesium sulfate or distill to remove volatiles.
• Yield: 155g (~93% yield).

Conclusion

• Applications: Both products to be used for grignard reagents.
• Acknowledgements: Thanks to supporters and patrons.