Distillation and reflux / PAG5
Organic techniques Distillation In general used as separation technique to separate an organic product from its reacting mixture. Need to collect the distillate of the approximate boiling point range of the desired liquid. Classic AS reaction using distillation Reaction: primary alcohol aldehyde Reagent: potassium dichromate (VI) solution and dilute sulphuric acid. Conditions: use a limited amount of dichromate and warm gently and distil out the aldehyde as it forms [This prevents further oxidation to the carboxylic acid] CH3CH2CH2OH + [O] CH3CH2CHO + H2O Observation Orange dichromate solution changes to green colour of Cr3+ ions Reflux Classic AS reaction using reflux Reaction: primary alcohol carboxylic acid Reagent: potassium dichromate(VI) solution and dilute sulphuric acid Conditions: use an excess of dichromate, and heat under reflux: (distill off product after the reaction has finished using distillation set up) CH3CH2CH2OH + 2[O] CH3CH2CO2H + H2O Observation Orange dichromate solution changes to green colour of Cr3+ ions Reflux is used when heating organic reaction mixtures for long periods. The condenser prevents organic vapours from escaping by condensing them back to liquids. Never seal the end of the condenser as the build up of gas pressure could cause the apparatus to explode. This is true of any apparatus where volatile liquids are heated Water in Water out Liebig condenser thermometer Heat Anti-bumping granules are added to the flask in both distillation and reflux to prevent vigorous, uneven boiling.
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4.2.3 Organic synthesis
(a) the techniques and procedures for: (i) use of Quickfit apparatus including for distillation and heating under reflux 5 Synthesis of an organic
liquid
• Heating under reflux1
• Purification using a separating funnel
• Distillation
• Risk assessment
Synthesis of a haloalkane 4.2.3(a)
liquid
• Heating under reflux1
• Purification using a separating funnel
• Distillation
• Risk assessment
Synthesis of a haloalkane 4.2.3(a)
Purifying organic liquid / PAG5
The drying agent should •be insoluble in the organic liquid • not react with the organic liquid • Put the distillate of impure product into a separating funnel • wash product by adding either • sodium hydrogencarbonate solution , shaking and releasing the pressure from CO2 produced. • Saturated sodium chloride solution •Allow the layers to separate in the funnel, and then run and discard the aqueous layer. •Run the organic layer into a clean, dry conical flask and add three spatula loads of drying agent (anhydrous magnesium sulphate or calcium chloride) to dry the organic liquid. • Carefully decant the liquid into the distillation flask •Distill to collect pure product Sodium hydrogencarbonate will neutralise any remaining reactant acid. Sodium chloride will help separate the organic layer from the aqueous layer Purifying an organic liquid Separating funnel
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4.2.3 Organic synthesis
(ii) preparation and purification of an organic liquid including: • use of a separating funnel to remove an organic layer from an aqueous layer • drying with an anhydrous salt (e.g. MgSO4, CaCl 2) • redistillation5 Synthesis of an organic
liquid
• Heating under reflux1
• Purification using a separating funnel
• Distillation
• Risk assessment
Synthesis of a haloalkane 4.2.3(a)
liquid
• Heating under reflux1
• Purification using a separating funnel
• Distillation
• Risk assessment
Synthesis of a haloalkane 4.2.3(a)
Synthetic routes
Br2, Cl2 UV light Free radical Substitution alkane haloalkane alkene alcohol aldehyde ketone carboxylic acid dihalogenoalkane Br2, Cl2 room temp Electrophilic addition HBr , HCl room temp Electrophilic addition If primary Na2Cr2O7 /H+ heat gently and distill partial oxidation If secondary Na2Cr2O7 /H+ heat under reflux oxidation (If primary) Na2Cr2O7 /H+ heat under reflux + excess oxidising agent Oxidation poly(alkene) high pressure Catalyst polymerization KOH aqueous heat under reflux Nucleophilic substitution H2, Nickel Catalyst addition/reduction Conc H2SO4 + sodium halide Heat under reflux substitution Conc H2SO4 or conc H3PO4 S
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4.2.3 Organic synthesis
Synthetic routes (b) for an organic molecule containing several functional groups: (i) identification of individual functional groups (ii) prediction of properties and reactions Learners will be expected to identify the functional groups encountered in Module 4. HSW3 Development of synthetic routes. (c) two-stage synthetic routes for preparing organic compounds. Learners will be expected to be able to devise twostage synthetic routes by applying transformations between all functional groups encountered up to this point of the specification. Extra information may be provided on exam papers to extend the learner’s toolkit of organic reactions. HSW3 Development of synthetic routes.
Credits: Neil Goalby