Organic Synthesis (OCR A Level Chemistry A): Flashcards
Exam code: H432
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What is recrystallisation?
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What is recrystallisation?
Recrystallisation is a purification technique in which an impure solid is dissolved in the minimum volume of hot solvent, then allowed to cool so that the pure product crystallises out while impurities remain in solution.
Define target molecule
A target molecule is the desired final product of an organic synthesis, from which chemists work backwards to identify suitable starting materials and the reagents and conditions needed at each step.
Why is it important to use the minimum amount of solvent during recrystallisation?
Using too much solvent dissolves a greater proportion of the product as it cools, reducing the yield of pure crystals recovered.
What is the first step when designing a multi-stage synthetic route?
Draw the structures of both the starting molecule and the target molecule, then determine whether they contain the same number of carbon atoms before planning the functional group interconversions.
After recrystallisation, the product crystals are recovered by .......... and washed with fresh .......... solvent.
After recrystallisation, the product crystals are recovered by filtration and washed with fresh cold solvent.
Why is nucleophilic substitution with KCN used when the target molecule has more carbon atoms than the starting material?
Reaction with KCN adds a nitrile group, which introduces one extra carbon atom, allowing the carbon chain to be extended when no suitable longer-chain starting material is available.
What does a broad melting point range indicate about the purity of a solid?
A broad melting point range indicates the solid is impure, as impurities lower the melting point and cause it to melt gradually over a wide temperature range rather than sharply.
To design a synthetic route, chemists identify all compounds reachable from the .......... molecule and all compounds that can be converted into the .......... molecule, then match the common .......... groups.
To design a synthetic route, chemists identify all compounds reachable from the starting molecule and all compounds that can be converted into the target molecule, then match the common functional groups.
True or False?
A pure solid has a sharp, well-defined melting point close to its data book value.
True.
Pure solids melt over a narrow temperature range, and a melting point close to the data book value confirms both purity and identity.
True or False?
A multi-stage synthetic route typically involves no more than four steps at A-level.
True.
Past papers generally require up to four steps in a multi-step synthesis, although the specification does not state a fixed upper limit.
Why should the heating rate be very slow when determining a melting point?
Heating too quickly causes the temperature of the apparatus to overshoot the actual melting point, giving an inaccurate and misleadingly high reading.
What two categories of reaction pathway must a student know when planning a synthetic route at A-level?
Students must know aliphatic reaction pathways (interconversions between alkanes, alkenes, haloalkanes, amines, alcohols, carbonyls, nitriles, carboxylic acids, esters and amides) and aromatic reaction pathways involving benzene.
True or False?
Buchner filtration uses reduced pressure to speed up the recovery of crystals.
True.
Buchner filtration applies reduced pressure beneath the filter, drawing liquid through faster than gravity filtration and speeding up crystal recovery.
True or False?
Knowing the functional groups of organic molecules is sufficient for planning a synthesis; reagents and conditions do not need to be specified.
False.
A complete synthetic route requires both the correct functional group interconversions and the specific reagents and conditions for each step.
Why must a water bath or electric heater be used instead of a Bunsen burner when preparing organic solids?
Many organic solvents are flammable, so a Bunsen burner creates a fire risk. A water bath, electric heater, or sand bath provides controlled heating without a naked flame.
Name three organic reactions used to demonstrate the preparation and purification of an organic solid.
Preparation of aspirin
Preparation of benzoic acid
Nitration of methyl benzoate (to give methyl 3-nitrobenzoate)
In the preparation of an organic solid, suitable heating methods include a .........., .........., or .......... — but not a Bunsen burner.
In the preparation of an organic solid, suitable heating methods include a water bath, electric heater, or sand bath — but not a Bunsen burner.
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