Chlorination
Alcohols can be converted into chloroalkanes to create a better leaving group for downstream substitution.
- Reagents
- PCl5
- Conditions
- room temperature
- Reaction class
- halogenation
- Equation
- R-OH + PCl5 -> R-Cl + POCl3 + HCl
Overview
Alcohols can be converted into chloroalkanes to create a better leaving group for downstream substitution.
Transformation
- Equation
- R-OH + PCl5 -> R-Cl + POCl3 + HCl
- Reagents
- PCl5
- Environment
- room temperature
- Reaction class
- halogenation
- Mechanism
- halogenation
- Evidence level
- textbook core
Reference procedure
2-Chloro-2-methylpropane preparation
2-Methylpropan-2-ol → 2-chloro-2-methylpropane
Materials and quantities
- 2-Methylpropan-2-ol and hydrochloric acid
- Hydrogencarbonate wash and suitable drying agent; quantities from the cited method
Apparatus
- Separating funnel
- Dry flask
- Distillation apparatus
Procedure
- Follow the cited mixing stage; formation of a second layer provides a visible separation, not proof of purity.
- Separate the organic layer, wash as directed and release pressure during the gas-forming hydrogencarbonate wash.
Work-up and isolation
- Dry, remove the drying agent and collect the stated distillation fraction.
- Compare the boiling range and supporting identity evidence with the intended product.
Critical controls
- Hydrogen chloride fumes require appropriate extraction; the organic liquids are flammable.
- This tertiary-alcohol example must not be treated as evidence that every primary alcohol reacts under identical conditions.
Practical techniques
Liquid–liquid work-up
Separate the organic product layer from the aqueous reaction mixture, then wash and dry it before final purification.
Setup
- Identify which layer contains the product from density data or a small drop test; do not assume the organic layer is always on top.
- Invert gently during washing and vent frequently away from people, especially when carbonate wash produces carbon dioxide.
- Drain and retain both layers until product identity has been confirmed.
- 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
- Remove visible aqueous liquid before treating the organic phase.
- 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.
Distillation setup
Rearrange the apparatus to collect a volatile product or fraction instead of returning condensate to the reaction flask.
Setup
- Position the thermometer bulb at the entrance to the condenser so it measures vapour entering the condenser.
- Run cooling water into the lower condenser connection and out through the upper connection.
- 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.
Scope and limitations
- Scope
- Alcohols react with phosphorus(V) chloride to form chloroalkanes.
- Limitations
- The route records the PCl5 transformation; the mechanism diagram uses a primary-alcohol exemplar and is not generalized to secondary/tertiary alcohols or SOCl2 conditions.
Related reactions
- Chlorination with thionyl chloride: Alcohols → Chloroalkanes
Thionyl chloride converts alcohols into chloroalkanes with sulfur dioxide and hydrogen chloride as by-products.
- Bromination: Alcohols → Bromoalkanes
Bromide and acid conditions convert alcohols into bromoalkanes.
- Iodination: Alcohols → Iodoalkanes
Iodine and red phosphorus conditions convert alcohols into iodoalkanes.
- Hydrochlorination: Alkenes → Chloroalkanes
Addition of hydrogen chloride across an alkene can form a chloroalkane.
- Hydrolysis: Chloroalkanes → Alcohols
The C-Cl bond can be displaced by hydroxide to form an alcohol.
- Cyanation: Chloroalkanes → Nitriles
Cyanide substitution forms a nitrile and extends the carbon chain by one carbon.
- Amination: Chloroalkanes → Amines
Ammonia substitutes chloride to form a primary amine; excess ammonia limits further alkylation.
- Elimination: Chloroalkanes → Alkenes
Base-promoted elimination removes HCl to form an alkene.
- Hydrolysis: Bromoalkanes → Alcohols
The C-Br bond can be displaced by hydroxide to form an alcohol.
- Hydrolysis: Iodoalkanes → Alcohols
The C-I bond can be displaced by hydroxide to form an alcohol.
- Oxidation: Alcohols → Aldehydes
Controlled oxidation of a primary alcohol gives an aldehyde.
- Oxidation: Alcohols → Ketones
Oxidation of a secondary alcohol gives a ketone.
- Oxidation: Alcohols → Carboxylic acids
Full oxidation of a primary alcohol gives a carboxylic acid.
- Free radical chlorination: Alkanes → Chloroalkanes
Photochemical chlorination can convert alkanes into chloroalkanes.
- Hydration: Alkenes → Alcohols
Catalytic hydration converts alkenes into alcohols.
- Dehydration: Alcohols → Alkenes
Dehydration converts alcohols into alkenes.
References
- 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.
- Core practical 6: Investigating chlorination of 2-methylpropan-2-olPearson Education Limited · Pearson qualifications
Tertiary chloroalkane preparation, washing, drying and distillation.
- 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.
- 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.