Alcohols, haloalkanes and analysis - 4.2.3 Organic synthesis — OCR A-Level Chemistry
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Alcohols, haloalkanes and analysis - 4.2.3 Organic synthesis explained
Alcohols, haloalkanes and analysis - 4.2.3 Organic synthesis covers 12 specification points, including Practical skills and (a) the techniques and procedures for:.
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Practical skills
This row signals that practical skills are assessed through organic synthesis work in section 4.2.3, not as a separate body of facts. When you read it, connect the synthesis reactions you study (for example oxidation of alcohols, substitution of haloalkanes and preparation of esters) to the practical techniques used to carry them out. Focus on choosing apparatus, heating under reflux, distillation, purification and safe handling, and on recording observations and yields. Treat the label as a pointer to where practical skills are developed and assessed, and check your specification for the exact techniques and apparatus named there rather than inventing detail.
(a) the techniques and procedures for:
This heading introduces the practical techniques and procedures that underpin organic synthesis in this section. Read it as a signpost: the detail that follows sets out the apparatus and manipulations you must be able to describe, select and explain, including Quickfit apparatus for distillation and for heating under reflux. Your job as a learner is to connect each named technique to the purpose it serves in a synthesis, such as separating a volatile product by distillation or driving a slow reaction to completion under reflux. Study the procedures as sequences of steps with reasons, and be ready to explain why each piece of apparatus or control is used rather than merely list it.
(i) use of Quickfit apparatus including for distillation and heating under reflux
Quickfit apparatus is a set of glassware with ground-glass joints that fit together to give a closed, leak-free assembly. For distillation, the assembly lets you boil a mixture and condense the vapour so a more volatile component is collected separately; a thermometer at the still head monitors the temperature of the vapour so you can collect the fraction boiling over a chosen range. For heating under reflux, a vertical condenser returns condensed vapour to the flask, so a reaction can be heated for a long time without losing volatile reactants or solvent. In both cases the joints must be greased and clamped securely, and the condenser water enters at the lower end and leaves at the upper end.
(ii) preparation and purification of an organic liquid including:
This sub-statement introduces the practical sequence for making an organic liquid in the laboratory and then purifying it. Preparation typically means heating a reaction mixture under reflux so volatile reactants and products are returned and the reaction can proceed without loss, then cooling and transferring the mixture. Purification then removes impurities and water: a separating funnel isolates the organic layer, a drying agent such as anhydrous sodium sulfate or calcium chloride removes residual water, and distillation collects a fraction boiling over a narrow range. The MCQ tests whether you can order these steps and match each technique to its purpose, for example why reflux is used rather than simple heating, or why the drying agent is added before the final distillation.
• use of a separating funnel to remove an organic layer from an aqueous layer
A separating funnel exploits the immiscibility of an organic liquid and water so the two layers can be drawn off separately. After the reaction mixture is cooled and poured into the funnel, the layers settle: the denser layer collects at the bottom and the less dense layer floats above. The stopper is removed or the tap opened carefully, and the lower layer is run out through the tap into a clean vessel while the upper layer remains. You identify which layer is organic from density data or by adding a little water and seeing which layer grows. The MCQ tests the practical reasoning: why the stopper is removed, which layer is run off first, and how repeated extraction with fresh solvent improves recovery.
• drying with an anhydrous salt (e.g. MgSO₄, CaCl₂)
After a liquid organic product has been separated and purified, traces of water often remain. An anhydrous salt such as MgSO₄ or CaCl₂ is added to the crude liquid in a small conical flask, where it hydrates by taking up water and clumps together. The mixture is swirled, then left to stand so the hydrated solid settles, and the dried liquid is decanted or filtered off. Enough drying agent is used to leave some free-flowing solid; if all of it clumps, more is added. This step removes water only, not other impurities, so it follows separation and precedes redistillation.
• redistillation
Redistillation means distilling a liquid product a second time to raise its purity. In synthesis, the crude product from a separating funnel often carries a volatile impurity with a boiling point close to the target, plus traces of water or solvent. A first distillation removes non-volatile impurities, which stay in the flask while the more volatile product distils over. A second distillation then separates the product from volatile impurities whose boiling points differ enough from the product's. Each distillation collects a fraction boiling over a narrow range, so redistillation sharpens that range and improves purity. Assess purity by comparing the boiling point with a data-book value and by checking the range is narrow. Redistillation cannot remove an impurity that boils at the same temperature as the product; that needs another method such as solvent extraction or a drying agent.
Synthetic routes
A synthetic route is a planned sequence of reactions that converts a named starting material into a target molecule. To design one, identify the functional group in the target, work backwards (retrosynthesis) to a precursor, and repeat until you reach the given starting material. Each step must use a reagent and condition you can justify: for example converting a primary alcohol to a carboxylic acid uses acidified potassium dichromate(VI) under reflux, whereas converting a haloalkane to an alcohol uses aqueous potassium hydroxide under reflux. Check carbon skeleton, functional group interconversions and any change in chain length, and state reagents and conditions for every step.
(b) for an organic molecule containing several functional groups:
When a molecule carries several functional groups, each group can react independently, so a route must account for all of them. Identify every functional group present, decide which one must change at each step, and choose reagents that act on the intended group without unwanted side reactions on the others. For example, an oxidising agent may convert an alcohol to a carbonyl while leaving a halogen substituent intact, but a reagent that also attacks the halogen would spoil the route. Working backwards from the target and checking each group at every stage keeps the plan consistent.
(i) identification of individual functional groups
Functional-group identification means reading a displayed or condensed structure and naming each group it contains, one at a time, rather than labelling the whole molecule with a single class. Work systematically: locate the carbon skeleton, then check for an –OH attached to an sp³ carbon (alcohol), an –OH attached directly to a benzene ring (phenol), a C=O with H on the carbon (aldehyde), a C=O between two carbons (ketone), a C=O with an O neighbour (ester or carboxylic acid), a C–X bond (haloalkane), C=C (alkene) and N–H or C≡N groups. A molecule may carry several groups, so a correct answer names all of them and does not stop at the first. Distinguish aldehyde from ketone by what is bonded to the carbonyl carbon, and alcohol from phenol by whether the –OH sits on the ring. In an MCQ, eliminate options that misplace the group or merge two groups into one name.
(ii) prediction of properties and reactions
Predicting properties and reactions means using the functional groups you have identified to say how a molecule will behave. Link each group to its characteristic chemistry: alcohols undergo oxidation to aldehydes, ketones or carboxylic acids depending on their class and the conditions, and react with sodium to give hydrogen; haloalkanes undergo nucleophilic substitution and elimination; alkenes undergo electrophilic addition and decolourise bromine water; aldehydes are oxidised by Tollens' or Fehling's reagents while ketones are not; carboxylic acids react with carbonates to release carbon dioxide; esters hydrolyse. Physical properties follow structure too: hydrogen bonding raises boiling point in alcohols and carboxylic acids, and larger chains raise boiling point through stronger induced dipole–dipole forces.
(c) two-stage synthetic routes for preparing organic compounds.
A two-stage route converts a stated starting material into a stated target by naming two reactions in order, each with its reagent and condition. Stage 1 changes the functional group; stage 2 changes it again. For example, ethene to ethanoic acid runs through ethanol: steam hydration with H₃PO₄ catalyst gives ethanol, then heating under reflux with acidified potassium dichromate(VI) oxidises the primary alcohol to the carboxylic acid. Alternatively, bromoethane to ethanoic acid goes via ethanol using aqueous NaOH hydrolysis, then oxidation. Check carbon skeleton, functional group and carbon number at each stage, and reject routes that skip a step or use a reagent that cannot deliver the change.
Your focus
- Link named organic synthesis reactions to the practical techniques used to carry them out.
- Describe apparatus and procedures such as heating under reflux and distillation.
- Explain purification steps and safe handling in a synthesis.
Show all 36 objectives
- Identify the techniques and procedures covered under this heading in organic synthesis.
- Explain the purpose of each named technique within a synthesis.
- Describe a procedure as an ordered sequence of steps with justified controls.
- Describe the use of Quickfit apparatus for distillation and for heating under reflux.
- Explain the function of the condenser in each technique.
- Identify correct assembly details such as water flow direction and secure clamping.
- Order the main stages of preparing and purifying an organic liquid.
- Match reflux, separating funnel, drying agent and distillation to their purposes.
- Explain why each purification step is needed before the final distillation.
- Explain how a separating funnel separates immiscible layers.
- Describe the safe procedure for running off the lower layer.
- Justify which layer is organic using density or a water test.
- Describe how an anhydrous salt such as MgSO₄ or CaCl₂ is used to dry a liquid organic product
- Explain why the salt must be anhydrous and how the hydrated solid is removed
- Place the drying step correctly within a purification sequence that ends with redistillation
- Explain how redistillation purifies a liquid product by separating it from impurities with different boiling points
- Describe how non-volatile impurities are left behind in the flask during distillation
- Evaluate the boiling-point evidence used to judge the purity of a redistilled product
- Plan a multi-step synthetic route from a given starting material to a target molecule.
- Select and justify a reagent and condition for each functional group interconversion.
- Present a route as an ordered sequence that conserves the carbon skeleton unless chain length is deliberately changed.
- Identify all functional groups in a polyfunctional organic molecule.
- Select reagents that convert one group while leaving others unchanged.
- Order reaction steps so that required functional groups survive to the target molecule.
- Identify each functional group present in a given organic structure
- Distinguish aldehyde from ketone and alcohol from phenol using bonding evidence
- Select the option that lists all functional groups in a polyfunctional molecule
- Predict the reaction a functional group undergoes given a reagent and condition
- Predict oxidation products of alcohols and distinguish aldehyde from ketone behaviour
- Explain a physical property difference using intermolecular forces
- Construct a two-stage route from a given starting material to a given target.
- Justify the reagent and condition chosen for each stage.
- Identify and correct an invalid step in a proposed route.
Alcohols, haloalkanes and analysis - 4.2.3 Organic synthesis exam tips
Marking Points
- Identifies that Quickfit apparatus uses ground-glass joints to give a closed, leak-free assembly.
- Explains that in distillation the vapour is condensed and a more volatile component is collected separately.
- States that under reflux a vertical condenser returns condensed vapour to the flask so heating can continue without loss of volatile material.
- Describes that condenser water enters at the lower end and leaves at the upper end.
- Recognises that joints are greased and apparatus clamped securely to keep the assembly safe and leak-free.
- Identifies reflux as the technique that returns volatile vapours to the reaction flask during heating.
- Places separation of the organic layer in a separating funnel before drying and distillation.
- Selects an anhydrous drying agent to remove residual water from the organic layer.
- Recognises that distillation purifies by collecting a fraction over a narrow boiling range.
- Distinguishes the purpose of each step so preparation is not confused with purification.
- Explains that the separating funnel works because the organic and aqueous layers are immiscible and settle by density.
- States that the lower layer is run out through the tap first while the upper layer is retained.
- Identifies that the stopper is removed or the tap opened to let air in and allow smooth flow.
- Describes shaking with a venting step to relieve pressure from volatile solvents.
- Recognises that the organic layer can be identified by density data or by adding water to see which layer increases.
- Identifies an anhydrous salt such as MgSO₄ or CaCl₂ as the drying agent added to the crude liquid organic product
- Explains that the anhydrous salt removes residual water by hydrating and forming a hydrated solid
- States that the dried liquid is separated from the solid by decanting or filtration after the mixture stands
- Recognises that drying removes water only and does not replace redistillation for removing other volatile impurities
- States that redistillation distils a liquid product a second time to increase its purity
- Explains that non-volatile impurities remain in the distillation flask and so are removed from the distillate
- Explains that volatile impurities with sufficiently different boiling points are separated from the product during redistillation
- Identifies a narrow boiling-point range matching a data-book value as evidence of improved purity
- Recognises that an impurity with a boiling point identical to the product cannot be removed by redistillation
- Identifies the functional groups in the starting material and target and states the interconversions needed.
- Works backwards from the target through logical precursor molecules to reach the given starting material.
- Names a specific reagent for each step, such as acidified potassium dichromate(VI) or aqueous potassium hydroxide.
- States appropriate conditions for each step, such as reflux or heating under reflux.
- Checks that the number of carbon atoms is conserved unless a step deliberately changes chain length.
- Presents the route as an ordered sequence of numbered steps with reagents and conditions for each.
- Identifies every functional group present in the molecule rather than only one.
- States which functional group is being converted at each step of the route.
- Selects a reagent that reacts with the intended group and explains why other groups are unaffected.
- Recognises that some reagents may react with more than one functional group and adjusts the route accordingly.
- Checks that the target molecule's functional groups are all produced by the chosen sequence.
- Names each functional group present in the displayed structure separately rather than assigning one class to the whole molecule
- Distinguishes an aldehyde from a ketone by identifying whether the carbonyl carbon carries a hydrogen atom
- Distinguishes an alcohol from a phenol by identifying whether the hydroxyl group is bonded directly to an aromatic ring
- Identifies a haloalkane by locating a carbon–halogen bond and states which halogen is present
- Recognises an ester by the carbonyl carbon bonded to an oxygen that links to a further carbon chain
- Selects the option that accounts for every functional group present when a molecule contains more than one
- Links each identified functional group to a characteristic reaction and states the reagent or condition needed
- Predicts the oxidation product of a primary alcohol as an aldehyde or carboxylic acid according to the conditions given
- States that a ketone resists oxidation by Tollens' reagent while an aldehyde reduces it
- Predicts nucleophilic substitution when a haloalkane reacts with aqueous hydroxide and elimination when ethanolic hydroxide is heated
- Explains a difference in boiling point between two molecules by reference to hydrogen bonding or chain length
- Selects the option whose predicted product contains the functional group consistent with the stated reaction type
- Names two reactions in the correct order, each with a reagent and a condition that can deliver the required functional-group change.
- Checks that the carbon skeleton and carbon number are preserved or deliberately changed at each stage of the route.
- Selects acidified potassium dichromate(VI) under reflux to oxidise a primary alcohol to a carboxylic acid.
- Uses aqueous sodium hydroxide and heat to hydrolyse a haloalkane to an alcohol before a later oxidation step.
- Rejects a one-step proposal when the target functional group cannot be reached directly from the starting material.
Examiner Tips
- 💡Map each synthesis reaction to the practical technique it needs, such as reflux for oxidation.
- 💡Read the specification wording for the named apparatus and techniques before revising.
- 💡Practise describing observations and explaining purification steps in your own words.
- 💡Read the heading together with the sub-statements beneath it so you see the full scope of techniques required.
- 💡For each technique, write a short purpose statement explaining what it achieves in a synthesis.
- 💡Practise describing procedures as ordered steps with a reason for each control, so explanations are ready when asked.
- 💡Link each piece of apparatus to its function: still head and condenser for distillation, vertical condenser for reflux.
- 💡When asked about safety or setup, mention greasing joints and clamping securely.
- 💡Check the direction of condenser water flow before committing to an answer.
- 💡Read the whole stem to see whether the question asks about preparation or purification before choosing an option.
- 💡Link each named technique to a single purpose: reflux for heating without loss, funnel for layer separation, drying agent for water removal, distillation for final purification.
- 💡Eliminate options that reverse the order of steps or give a technique the wrong purpose.
- 💡Check that any named drying agent is described as anhydrous and does not react with the product.
- 💡Use density information in the stem to decide which layer is organic before answering.
- 💡Remember the practical rule that the lower layer leaves through the tap first.
- 💡Link venting during shaking to the volatility of organic solvents and pressure build-up.
- 💡Reject options that keep the stopper sealed or that assume the organic layer is always uppermost.
- 💡Name a specific anhydrous drying agent rather than saying 'a drying agent'
- 💡Explain the observable sign that enough drying agent has been added, namely some solid remaining free-flowing
- 💡Link the drying step to its position in the sequence: after separation, before redistillation
- 💡Link each purification step to the physical property it exploits, here differences in boiling point
- 💡Quote a boiling-point range and compare it with a data-book value when asked how purity is checked
- 💡Distinguish clearly between removing non-volatile and volatile impurities in your answer
- 💡Write each step on its own line with the reagent and condition, so the examiner can credit each conversion separately.
- 💡Name reagents precisely, for example acidified potassium dichromate(VI) rather than just 'dichromate'.
- 💡Check the final step actually produces the target functional group before finishing.
- 💡State conditions such as reflux or heat where they are needed for the reaction to occur.
- 💡List all functional groups in the starting material and target before choosing any reagent.
- 💡For each step, state which group changes and why the others are left unchanged.
- 💡Order the steps so a group needed later is not removed early.
- 💡Re-read the target structure to confirm every functional group has been produced.
- 💡Redraw or annotate the structure and tick each group as you find it, so none is missed
- 💡Check the atoms bonded to each carbonyl carbon before deciding aldehyde or ketone
- 💡Compare each option against the structure group by group and reject any option that invents a group
- 💡Watch for condensed formulae where the –OH or C=O is easy to overlook
- 💡Write the functional group beside the reagent before choosing an option
- 💡Check the oxidation state of the carbon bearing the –OH to decide whether oxidation is possible
- 💡Use the product's functional group to test whether the proposed reaction makes chemical sense
- 💡Recall that hydrogen bonding requires an O–H or N–H bond, so check for one before claiming it
- 💡Write the route as numbered stages, each with starting compound, reagent, condition and product.
- 💡Check the oxidation state of the carbon centre at each stage to confirm the change is feasible.
- 💡Underline the functional group in the start and target before choosing reactions.
Common Mistakes
- Treats 'Practical skills' as a list of facts to memorise; correct this by linking each skill to a named synthesis reaction and the apparatus it needs.
- Assumes a fixed number of marks attaches to practical work; correct this by checking the specification and assessment materials for how practical skills are assessed.
- Ignores safety and purification steps when planning a synthesis; correct this by including reflux, distillation and purification where the reaction requires them.
- Treats the heading as a stand-alone fact to memorise; instead read it as a signpost and study the techniques listed beneath it.
- Lists apparatus without linking it to a purpose; instead state what each technique achieves in a synthesis, such as separating a volatile product.
- Skips the reasons behind each procedural step; instead learn procedures as ordered steps each with a justification.
- Assumes reflux collects a product; correct this by stating that reflux returns condensed vapour to the flask to keep heating without loss.
- Connects the condenser the wrong way round for water flow; correct this by stating water enters at the lower end and leaves at the upper end.
- Treats Quickfit joints as ordinary corks or bungs; correct this by stating the joints are ground-glass and fit together to give a closed assembly.
- Treats distillation as the first step of preparation; place reflux heating first and distillation later in purification.
- Assumes any drying agent is suitable regardless of whether it reacts with the product; choose an anhydrous agent that does not react with the organic liquid.
- Confuses the separating funnel with a distillation apparatus; use the funnel only to separate immiscible layers.
- Believes reflux increases yield by removing product; state that reflux returns volatile vapours to prevent loss of reactants and solvent.
- Leaves the stopper firmly in place while running off liquid; remove the stopper so pressure does not stop the flow.
- Assumes the organic layer is always on top; check densities, since halogenated organic liquids can be denser than water.
- Shakes the funnel without venting; open the tap periodically to release built-up pressure.
- Discards the wrong layer without checking which is organic; confirm identity before disposing of either layer.
- Adds water or a dilute solution to the product; use a dry, anhydrous salt instead
- Treats drying as the final purification step; follow it with redistillation to remove other impurities
- Uses a hydrated salt such as the blue copper(II) sulfate crystals; choose the anhydrous form so it can absorb water
- Claims that distillation cannot remove non-volatile impurities; correct this by stating that non-volatile impurities are left behind in the flask while the product distils over
- Treats redistillation as able to separate any impurity; correct this by noting that an impurity boiling at the same temperature as the product distils with it
- Describes redistillation as a chemical reaction that purifies the product; correct this by describing it as a physical separation by boiling point
- Lists reactions in a random order without a logical link from starting material to target; plan the sequence backwards from the target.
- Names a reagent without its conditions, such as omitting reflux; state both reagent and conditions for every step.
- Uses a reagent that cannot achieve the stated conversion, such as an oxidising agent to reduce a ketone; match the reagent to the functional group change.
- Ignores changes in carbon chain length; confirm the carbon count is conserved or justify any step that alters it.
- Considers only one functional group and ignores the others; list every group before planning.
- Assumes a reagent affects only the intended group without checking; state why the other groups are unaffected.
- Chooses a step that destroys a group needed later in the route; plan the order so protected groups survive.
- Stops the route before all target functional groups are formed; confirm each group is present at the end.
- Labels a molecule containing both an alkene and a hydroxyl group with only one group name; list every group the structure contains
- Calls a ketone an aldehyde because a carbonyl is present; check whether the carbonyl carbon is bonded to hydrogen
- Describes a phenol as an alcohol; check whether the –OH is attached directly to the benzene ring
- Treats a C–X bond as a substituent with no functional significance; identify it as a haloalkane group and name the halogen
- Predicts that a tertiary alcohol is oxidised to a ketone; recognise that tertiary alcohols resist oxidation under these conditions
- Assumes all alcohols react with Tollens' reagent; only aldehydes give a positive result
- Predicts addition across a C=O bond when the reagent attacks a C=C bond; match the reagent to the correct functional group
- Ignores the solvent or condition when predicting haloalkane reactions; aqueous hydroxide favours substitution and ethanolic hydroxide with heat favours elimination
- Omits the reagent or condition for one stage; state both the reagent and the condition for every stage.
- Uses distillation instead of reflux when a carboxylic acid is required; choose reflux for full oxidation of a primary alcohol.
- Proposes oxidation of a haloalkane directly; hydrolyse the haloalkane to an alcohol first, then oxidise.