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How do we plan a multi-step synthesis and check the product?

Reaction pathways linking functional groups, planning multi-step syntheses, practical techniques of preparation and purification, and assessing purity.

A focused answer to WJEC A-Level Chemistry Unit 4, covering reaction pathways linking functional groups, planning multi-step syntheses, the practical techniques of preparation and purification, and assessing the purity of a product.

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What this dot point is asking

WJEC wants you to link functional groups through reaction pathways, plan multi-step syntheses with reagents and conditions, carry out the practical techniques of preparation and purification, and assess the purity of a product.

The answer

Reaction pathways

Planning a multi-step synthesis

Preparation and purification

Practical techniques include reflux (to react without losing volatile material), distillation (to separate by boiling point), separation with a separating funnel, drying with an anhydrous salt, and recrystallisation (for solids).

The functional-group conversion map

A workable mental map of the common one-step conversions is: alkene to alcohol (steam, acid catalyst); alcohol to halogenoalkane (HX or PCl5\text{PCl}_5); halogenoalkane to nitrile (KCN, lengthens the chain) or to amine (excess ammonia); nitrile to amine (LiAlH4\text{LiAlH}_4) or to carboxylic acid (hydrolysis); primary alcohol to aldehyde (distil) to carboxylic acid (reflux); secondary alcohol to ketone; carbonyl to hydroxynitrile (HCN); acid to ester (alcohol plus acid catalyst) or to acyl chloride (SOCl2\text{SOCl}_2). Choosing the right path means matching the carbon skeleton and the oxidation level of the target.

Choosing techniques for a preparation

Aim Technique
React without losing volatile reagents Reflux
Separate by boiling point Fractional distillation
Separate immiscible liquids Separating funnel
Remove water from an organic layer Anhydrous drying agent
Purify a solid Recrystallisation
Check purity Melting point, or single TLC spot

Selecting the correct technique, and saying why, is as important in the exam as the reaction chemistry itself.

Examples in context

Industrial synthesis of aspirin. A multi-step route from phenol to salicylic acid to aspirin, followed by recrystallisation and a melting-point check, mirrors the planning and purity assessment tested here. Pharmaceutical purity control. Drug manufacturers combine recrystallisation, chromatography and melting-point determination to certify that an active ingredient meets strict purity standards before release.

Try this

Q1. State the technique used to purify an impure organic solid. [1 mark]

  • Cue. Recrystallisation.

Q2. State what a sharp melting point at the literature value indicates. [1 mark]

  • Cue. The compound is pure.

Q3. Calculate the percentage yield if 3.03.0 g of product is made from a theoretical maximum of 5.05.0 g. [1 mark]

  • Cue. (3.0/5.0)×100=60(3.0/5.0) \times 100 = 60 percent.

Q4. State the reagent that lengthens a carbon chain by one carbon when reacted with a halogenoalkane. [1 mark]

  • Cue. Potassium cyanide, KCN (forming a nitrile).

Q5. State how an impurity affects the melting point of a solid. [1 mark]

  • Cue. It lowers and broadens the melting point.

Exam-style practice questions

Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

WJEC 20205 marksOutline a two-step synthesis of propanoic acid starting from 1-bromopropane, giving the reagents and conditions for each step and an equation.
Show worked answer →

Step 1: hydrolyse 1-bromopropane by refluxing with aqueous sodium hydroxide to give propan-1-ol: CH3CH2CH2Br+NaOH→CH3CH2CH2OH+NaBr\text{CH}_3\text{CH}_2\text{CH}_2\text{Br} + \text{NaOH} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + \text{NaBr}.

Step 2: oxidise propan-1-ol by refluxing with excess acidified potassium dichromate(VI) to give propanoic acid: CH3CH2CH2OH+2[O]→CH3CH2COOH+H2O\text{CH}_3\text{CH}_2\text{CH}_2\text{OH} + 2[\text{O}] \rightarrow \text{CH}_3\text{CH}_2\text{COOH} + \text{H}_2\text{O}.

The orange dichromate turns green, confirming oxidation.

Markers reward the hydrolysis step with conditions, the oxidation step with reflux and excess oxidant, the colour change, and correct equations.

WJEC 20184 marksA student prepares an ester and obtains 6.60 g from a reaction with a theoretical yield of 8.80 g. Calculate the percentage yield, and explain how recrystallisation followed by a melting point measurement would confirm the product is pure.
Show worked answer →

Percentage yield =actual masstheoretical mass×100=6.608.80×100=75.0= \dfrac{\text{actual mass}}{\text{theoretical mass}} \times 100 = \dfrac{6.60}{8.80} \times 100 = 75.0 percent.

Recrystallisation dissolves the product in hot solvent, then on cooling pure crystals form while soluble impurities stay in solution and are removed by filtration.

A pure compound melts sharply at its true melting point; impurities lower and broaden the melting point, so a sharp melting point at the literature value confirms purity.

Markers reward the percentage yield, the principle of recrystallisation, and the sharp melting point as a purity check.

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