Nitrogen compounds, polymers and synthesis - 6.2.5 Organic synthesis — OCR A-Level Chemistry

This row is a practical-skills heading within organic synthesis, not a separate examinable fact. Read it as a signpost: the assessed practical content sits in the numbered statements that follow, especially preparation and purification of organic solids and the linked techniques in 4.2.3 a. When revising, work through each named technique as a procedure you could carry out and describe: choosing apparatus, heating under reflux, filtration under reduced pressure, recrystallisation and drying. Link each step to its purpose, such as maximising yield or removing impurities, and to safety. Treat the heading as course guidance telling you where practical skills are assessed, and check the specification for the specific techniques listed rather than inventing content from the heading alone.

This statement requires knowledge of the techniques and procedures for preparing and purifying organic solids, using a range of methods and cross-referencing 4.2.3 a. Preparation typically involves heating under reflux to complete the reaction, then cooling and filtering the solid. Purification commonly uses recrystallisation: dissolve the crude solid in the minimum of hot solvent, cool slowly to form crystals, filter under reduced pressure and wash with cold solvent, then dry. Choosing a suitable solvent matters because the solid should be soluble when hot but only sparingly soluble when cold. Each step has a purpose, such as removing soluble impurities or maximising crystal yield, and links to the techniques in 4.2.3 a.

This clause concerns the practical preparation of an organic compound in the laboratory, not planning a route on paper. A learner carries out a preparation by choosing suitable apparatus, assembling it correctly, heating or stirring as the method requires, then isolating the product by techniques such as filtration under reduced pressure, solvent extraction, drying with a drying agent and distillation. Purity is checked by measuring a melting point and comparing it with a data value, and yield is calculated from the mass obtained and the theoretical maximum. Assessment looks at whether the learner selects and uses equipment safely and records observations and results accurately.

Quickfit apparatus is a set of interchangeable glassware with ground-glass joints of standard sizes, so a round-bottomed flask, condenser, still head, receiver adaptor and thermometer can be joined without corks or rubber bungs. The ground joints give a good seal and resist attack by organic vapours. For heating under reflux the flask carries a vertical condenser only, with water entering at the lower inlet and leaving at the upper outlet. A still head and thermometer belong to distillation, where vapour is led away and condensed into a receiver. Learners should be able to select the correct combination of pieces for reflux or for distillation and explain why each piece is used.

Distillation and heating under reflux are the two standard techniques for heating a reaction mixture, and the choice depends on whether a volatile product must be removed as it forms. In simple distillation the reaction flask is fitted with a still head and condenser, and the vapour of the lower-boiling component leaves the flask, condenses and is collected, so an equilibrium such as an esterification is driven towards the product. Under reflux the flask carries a vertical condenser: vapour rises, condenses on the cool inner surface and drips back, so the mixture can be boiled for a long period without losing solvent or volatile reactants. Reflux suits slow reactions needing prolonged heating, for example oxidising a primary alcohol to a carboxylic acid with acidified dichromate(VI). Distillation suits separating a product from the reaction mixture by boiling point.

An organic solid made in the laboratory is usually contaminated with soluble impurities and with some of the solvent used to crystallise it. Purification typically uses recrystallisation: the impure solid is dissolved in the minimum volume of hot solvent, the hot solution is filtered to remove insoluble impurities, and the filtrate is cooled so the purified solid crystallises while soluble impurities stay in solution. The crystals are then filtered off under reduced pressure using a Buchner funnel, washed with a small volume of cold solvent and dried. Choosing a solvent in which the solid is very soluble when hot but only slightly soluble when cold is essential. Purity is checked by measuring the melting point and comparing it with the literature value; a sharp melting point close to the accepted value indicates a pure sample, while a depressed or broad range indicates impurities.

Filtration under reduced pressure (suction filtration) separates a solid from a liquid quickly by pulling air out of the receiving flask with a water pump or vacuum line, so the pressure below the filter is lower than atmospheric pressure and liquid is drawn through faster than under gravity. The solid is collected on a sintered-glass or Hirsch funnel, washed with a little cold solvent, and air is drawn through it to help drying. The technique suits organic preparations where a crude solid must be isolated before recrystallisation. Learners should recognise the apparatus, the reason for the reduced pressure, and why the solid is washed and dried by suction rather than left wet with mother liquor.

Recrystallisation purifies a crude solid by dissolving it in the minimum volume of hot solvent, then cooling so the purified compound crystallises while impurities stay dissolved or are removed by filtration. The solvent is chosen so the compound is very soluble when hot but only slightly soluble when cold. The hot solution is filtered to remove insoluble impurities, cooled slowly to form crystals, and the crystals are collected by suction filtration, washed with cold solvent and dried. Learners should recognise the purpose of each step, why the minimum volume of hot solvent is used, and why slow cooling gives purer, larger crystals.

Melting-point measurement identifies a solid organic product and tests its purity. Pack a thin, dry sample into a capillary tube, tap it down, and heat slowly near the expected value so the rise is gradual. Record the range from the first sign of melting to the point where the solid has just liquefied. A pure solid melts sharply over a narrow range close to its literature value; an impure solid melts lower and over a wider range because impurities disrupt the crystal lattice. Compare your measured range with a data book value for the compound you synthesised. A melting point well below the literature figure, or a broad range, signals impurities that recrystallisation should remove.

A synthetic route is a planned sequence of reactions converting a starting material into a target molecule. Work backwards from the target: identify the functional group needed and the reaction that installs it, then repeat until you reach an available starting material. Justify each step with a named reagent, a condition and the functional-group change, for example converting a halogenoalkane to an amine by heating with excess ammonia in ethanol, or reducing a nitrile to a primary amine. Check that each intermediate can actually be made and that competing reactions are controlled. Consider yield, atom economy and safety when choosing between routes, and state how you would confirm the product, such as by melting point or infrared spectroscopy.

This sub-statement sets the scene for working with an organic molecule that carries more than one functional group. The skill is to read a displayed structure and treat each group on its own merits rather than assuming one group dominates. For example, a molecule might contain both a hydroxyl group and a carbonyl group, or an amine alongside a carboxylic acid. You identify each group separately, then decide how the molecule would behave in a chosen reaction. Because several groups are present, a reagent may attack one site while leaving another untouched, so you must justify which group reacts and why. The following sub-statement (i) narrows this to identifying the individual functional groups.

This sub-statement asks you to identify each functional group in an organic molecule individually. You examine the displayed structure and pick out every group, naming each one correctly: for instance an alcohol, an aldehyde, a ketone, a carboxylic acid, an amine, an amide, an ester or a halogenoalkane. Identification means recognising the arrangement of atoms that defines the group, not just spotting a single atom. For example, a carbon double-bonded to an oxygen with a hydrogen attached is an aldehyde, whereas the same carbonyl with two carbon groups attached is a ketone. You then report each group separately so that later synthesis or reaction questions can build on an accurate list.

Synthesis planning depends on predicting how a chosen functional group will behave before you commit to a route. In this sub-topic you use the reaction chemistry of nitrogen compounds and related groups to forecast properties and reactions. For example, you predict that a primary amine such as ethylamine acts as a nucleophile and a base, forming an amide with an acyl chloride, whereas a phenylamine-type aromatic amine is far less basic because its lone pair is delocalised into the ring. You also predict solubility from hydrogen bonding and chain length, and predict that an amide hydrolyses under acid or alkaline conditions to give a carboxylic acid or its salt plus an ammonium ion or amine. Prediction means naming the functional group, recalling the general reaction of that group, then applying it to the specific molecule in the question.

A multi-stage route converts a stated starting material into a target molecule through two or more sequential reactions. You plan it backwards: compare the target with the start, identify the functional group change needed, and select a reaction that delivers it. Each stage must specify reagents and conditions, and you must check that earlier stages do not destroy groups needed later. For example, converting a halogenoalkane into a carboxylic acid can proceed via a nitrile: react with KCN in ethanol under reflux to form the nitrile, then hydrolyse with dilute acid to give the acid, adding a carbon atom at the first stage. Where a group must be protected, say so and justify the order. Present the route as numbered stages with named intermediates, and explain why each reagent is chosen.