How do chemists plan a route to make a target organic molecule?
The reactions of the main functional groups including nucleophilic substitution, elimination, oxidation, reduction, condensation and hydrolysis, the use of these reactions to design multi-step synthetic routes, and the assessment of a route by percentage yield, atom economy and hazards.
An SQA Advanced Higher Chemistry answer on synthesis, covering the reactions of the main functional groups (nucleophilic substitution, elimination, oxidation, reduction, condensation and hydrolysis), how these reactions are combined into multi-step synthetic routes, and how a route is assessed by percentage yield, atom economy and hazards.
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What this key area is asking
The SQA wants you to know the reactions of the main functional groups (substitution, elimination, oxidation, reduction, condensation and hydrolysis), to combine them into multi-step synthetic routes, and to assess a route using percentage yield, atom economy and hazards. Designing a route from one functional group to another and calculating yield and atom economy are reliable exam earners.
The toolkit of functional-group reactions
The core reactions are:
- Nucleophilic substitution. A nucleophile replaces a leaving group, for example converting a haloalkane to an alcohol with aqueous alkali.
- Elimination. A small molecule (usually water from an alcohol, or a hydrogen halide from a haloalkane) is removed to form a carbon-to-carbon double bond.
- Oxidation. A primary alcohol is oxidised to an aldehyde and then to a carboxylic acid; a secondary alcohol gives a ketone.
- Reduction. Aldehydes, ketones and carboxylic acids are reduced back to alcohols.
- Condensation. Two molecules join with the loss of water, forming the ester link or the amide (peptide) link.
- Hydrolysis. Water breaks an ester or amide link to regenerate the smaller molecules.
Designing a multi-step route
To plan a route, work backwards from the target, asking which reaction could form its functional group, then which molecule that reaction needs as its starting point. Repeat until you reach the available starting material. For example, to make ethyl ethanoate from ethene: hydrate ethene to ethanol (addition), oxidise some ethanol to ethanoic acid (oxidation), then react the acid with the remaining ethanol (condensation).
Assessing a route
A good synthesis is judged not just on whether it works but on its efficiency and safety.
Addition reactions have a high atom economy because there is only one product, whereas substitution and elimination reactions have a lower atom economy because they produce by-products.
Examples in context
Synthetic planning is the heart of the chemical and pharmaceutical industries. A drug such as aspirin is made by esterifying salicylic acid, and chemists choose reagents to maximise yield while keeping the process safe and cheap. Green chemistry pushes manufacturers towards high-atom-economy addition reactions and catalysts that reduce waste, energy and hazardous solvents. The same route-design skill lets a chemist convert cheap feedstocks such as crude-oil fractions into high-value products: cracking gives alkenes, which are hydrated to alcohols, oxidised to acids, and condensed into the esters used as solvents, flavourings and plasticisers. Assessing percentage yield against atom economy is exactly the analysis carried out in the Researching Chemistry project.
Try this
Q1. Name the type of reaction that converts a haloalkane into an alcohol. [1 mark]
- Cue. Nucleophilic substitution (with aqueous alkali).
Q2. State the formula for percentage yield. [1 mark]
- Cue. Actual mass of product divided by theoretical mass of product, times 100.
Q3. State one advantage of a synthetic route with a high atom economy. [1 mark]
- Cue. Less waste (also lower cost and a more sustainable process).
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH 20193 marksPropan-1-ol is to be converted into propanoic acid. (a) Name the type of reaction and a suitable reagent. (b) Name the intermediate formed. (c) Suggest a reagent that would convert propan-1-ol into propene instead.Show worked answer →
Markers reward the oxidation route, the intermediate aldehyde, and the dehydration reagent.
(a) The conversion is an oxidation. A suitable oxidising agent is acidified potassium dichromate(VI), .
(b) The intermediate is the aldehyde propanal; further oxidation gives propanoic acid (so the dichromate is used in excess and the mixture heated under reflux).
(c) To make propene instead, dehydrate (eliminate water from) the alcohol using a catalyst such as concentrated sulfuric acid or aluminium oxide; this is an elimination reaction.
SQA AH specimen3 marksA reaction has the equation . (a) Calculate the atom economy of this reaction. (b) State one advantage of a reaction with a high atom economy. (Use , , .)Show worked answer →
The answer must use the atom economy formula and give an advantage.
(a) Atom economy is the mass of the desired product divided by the total mass of reactants, as a percentage:
This is an addition reaction with a single product, so all the reactant atoms end up in the product.
(b) A high atom economy means less waste, lower raw-material cost and a more sustainable (greener) process.
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Sources & how we know this
- SQA Advanced Higher Chemistry Course Specification — SQA (2019)