Bromination

Bromide and acid conditions convert alcohols into bromoalkanes.

Reagents
KBr and 50% H2SO4(aq)
Conditions
heat under reflux, then distil the bromoalkane
Reaction class
halogenation
Equation
R-OH + HBr -> R-Br + H2O

Overview

Bromide and acid conditions convert alcohols into bromoalkanes.

Transformation

Alcohols → Bromoalkanes

Equation
R-OH + HBr -> R-Br + H2O
Reagents
KBr and 50% H2SO4(aq)
Environment
heat under reflux, then distil the bromoalkane
Reaction class
halogenation
Mechanism
halogenation
Evidence level
textbook core

Reference procedure

Reference preparation: 1-bromobutane

Butan-1-ol → 1-bromobutane

Materials and quantities

  • Sodium bromide — 10 g
  • Water — 10 cm³
  • Butan-1-ol — 7.5 cm³
  • Concentrated sulfuric acid — 10 cm³, added slowly
  • Concentrated hydrochloric acid — wash
  • Dilute sodium carbonate solution — wash

Apparatus

  • Round-bottom flask
  • Large cold-water bath
  • Vertical condenser
  • Tap funnel
  • Distillation head and receiver
  • Separating funnel
  • Electric heat source

Procedure

  1. Place sodium bromide, water and butan-1-ol in the flask and partially immerse the flask in a cold-water bath.
  2. Fit a vertical condenser and tap funnel. Add the concentrated sulfuric acid slowly through the tap funnel while gently shaking and maintaining external cooling.
  3. Remove the tap funnel and cold-water bath, then heat the mixture gently under reflux for about 45 minutes.
  4. Stop heating, allow the apparatus to cool, then rearrange it for distillation and collect the 1-bromobutane/water distillate.

Work-up and isolation

  1. Separate the 1-bromobutane layer from water.
  2. Wash the organic layer with concentrated hydrochloric acid to remove unreacted butan-1-ol.
  3. Wash with dilute sodium carbonate solution, venting frequently because carbon dioxide is produced.
  4. Dry the organic layer and redistil using an appropriate boiling-range cut if a purified sample is required.

Critical controls

  • The acid addition is exothermic: add slowly with cooling; do not charge all acid at once.
  • Use a fume cupboard, eye protection and an electric heat source; the organic materials are volatile and flammable.
  • Confirm which separating-funnel layer contains 1-bromobutane from density data rather than assuming it is the upper layer.

Practical techniques

Reflux setup

Boil the reaction while returning condensed vapour to the same flask, so volatile material is not continuously lost.

Setup

  1. Charge the flask before fitting the condenser; do not fill the flask more than about half full.
  2. Run cooling water into the lower condenser inlet and out from the upper outlet.
  3. Keep the top of an ordinary reflux condenser open to the atmosphere unless the cited procedure specifies a pressure-rated closed system.
  4. Heat to a gentle, steady boil so the condensation line remains within the condenser and liquid returns continuously to the flask.

Operating checks

  • Use an electric bath or mantle for flammable organic liquids; do not use a naked flame.
  • Start condenser water before heating and confirm that hoses are secure.
  • If addition is exothermic, complete the controlled addition and cooling stage before sustained reflux.

Distillation setup

Rearrange the apparatus to collect a volatile product or fraction instead of returning condensate to the reaction flask.

Setup

  1. Position the thermometer bulb at the entrance to the condenser so it measures vapour entering the condenser.
  2. Run cooling water into the lower condenser connection and out through the upper connection.
  3. Heat gradually and collect only the fraction specified by the cited boiling range or procedure.

Operating checks

  • Do not distil a flask to dryness.
  • Use an electric heat source for flammable mixtures.
  • Change from reflux to distillation only after heating has stopped and the apparatus has cooled enough to handle safely.

Liquid–liquid work-up

Separate the organic product layer from the aqueous reaction mixture, then wash and dry it before final purification.

Setup

  1. Identify which layer contains the product from density data or a small drop test; do not assume the organic layer is always on top.
  2. Invert gently during washing and vent frequently away from people, especially when carbonate wash produces carbon dioxide.
  3. Drain and retain both layers until product identity has been confirmed.
  4. Dry the isolated organic layer until the drying agent remains free-flowing, then decant or filter before final distillation.

Operating checks

  • Remove the stopper before draining through the tap.
  • Release pressure after each inversion during gas-forming washes.
  • Record which layer was retained at every wash.

Drying an organic liquid

Remove residual water from a separated organic layer with a compatible anhydrous drying agent.

Setup

  1. Remove visible aqueous liquid before treating the organic phase.
  2. Mix with small portions of drying agent, allow contact and separate the clear liquid from the solid before final purification.

Operating checks

  • Choose the agent for the product: a drying agent must not react with or strongly retain it.
  • Drying removes water, not dissolved organic impurities; boiling-range or spectroscopic checks are still needed.

Scope and limitations

Scope
Alcohols can be converted into bromoalkanes under bromide and acid conditions that generate hydrogen bromide in situ.
Limitations
Hydrogen bromide is generated in situ. The sulfuric acid is kept at about 50% because concentrated sulfuric acid oxidises some bromide ions to bromine; downstream substitution and elimination are tracked separately.

Related reactions

References

  1. Pearson Edexcel Level 3 Advanced GCE in Chemistry specificationPearson Education Limited · Pearson qualifications · 2024

    Supports the textbook-core organic reaction routes used for displayed route and mechanism content.

  2. Pearson International Advanced Level Chemistry Unit 3B: preparation of 1-bromobutanePearson Education Limited · Pearson qualifications · 2015

    Provides the stated quantities, addition order, reflux time, distillation and wash sequence for the 1-bromobutane reference preparation.

  3. Pearson Edexcel International Advanced Level Chemistry Student Practical GuidePearson Education Limited · Pearson qualifications · 2018

    Supports the practical distinctions between reflux, distillation, liquid-liquid separation, washing and drying in organic preparations.

  4. Reflux and distillation – practical videosRoyal Society of Chemistry · RSC Education

    Explains the different purposes and apparatus arrangements for reflux and distillation, including organic preparation and work-up.