Account for by-products in a synthetic route
Track atoms removed or released at each step and distinguish organic products from small-molecule by-products.
Section 36 · Organic synthesis
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 36. Routes use reactions listed in the syllabus and state the relevant reagents and conditions. Check every intermediate and possible by-product; no route implies a practical procedure. Written practice is self-checked
Before you begin
- Use the functional-group reactions from Topics 13–19 and 29–35, including their reagents, conditions, products and limitations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the by-product of carboxylic-acid/alcohol esterification?
What you will learn
- Identify by-products in syllabus reactions
- Use atom accounting to check equations
- Separate desired organic products from by-products
Some routes produce a second product
Esterification of a carboxylic acid with an alcohol produces water. Nitrile hydrolysis in acid produces an ammonium salt alongside the carboxylic acid; alkaline hydrolysis first gives a carboxylate and ammonia.
A halogenoalkane reacting with KCN produces a nitrile and a halide salt. A diazonium salt warmed with water to form phenol also releases nitrogen gas and an acid by-product.
Track the source of each atom
For each arrow, mark which atoms remain in the desired organic product and which leave in a small molecule or salt. This makes it easier to spot missing by-products and avoid placing them inside the polymer or target molecule.
Not every route question requires a fully balanced equation. When asked for possible by-products, state those supported by the reagents and the balanced reaction pattern; do not invent a by-product from an unspecified side reaction.
Pause and explain
What gas is released when an aromatic diazonium salt is warmed with water to form phenol?
Worked example
Give the organic product and by-product when ethanoic acid reacts with ethanol to form ethyl ethanoate.
Show the worked solution
- Combine the acid-derived acyl portion with ethanol's oxygen and ethyl group.
- Esterification removes the acid –OH and alcohol H as water.
- Name the product and small molecule separately.
Answer Ethyl ethanoate and water form: CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write the organic product and by-product for propanoic acid reacting with methanol to form an ester.
Give me a hint
Name the alkyl group from the alcohol and the acid-derived part.
Compare my reasoning
- Methanol supplies methyl.
- Propanoic acid supplies propanoate.
- Esterification releases water.
Methyl propanoate and water form: CH₃CH₂COOH + CH₃OH ⇌ CH₃CH₂COOCH₃ + H₂O.
Look for these in your work
- Ester name correct
- Structure assembled correctly
- Water included
During acid hydrolysis of propanenitrile, identify the desired organic product and a nitrogen-containing by-product.
Give me a hint
Hydrolysis changes the nitrile carbon into a carboxyl carbon.
Compare my reasoning
- The organic product is propanoic acid.
- Nitrile nitrogen is protonated in acidic conditions.
- An ammonium salt can form in the acidic mixture.
Propanoic acid forms, with an ammonium salt such as NH₄Cl when hydrochloric acid is used.
Look for these in your work
- Acid product correct
- Nitrogen accounted
- By-product qualified to reagent
A route uses ethanoyl chloride and ethanol to prepare ethyl ethanoate. Which by-product differs from direct acid/alcohol esterification?
Give me a hint
The acid chloride has chlorine as its leaving group.
Compare my reasoning
- Ethanoyl chloride reacts with ethanol to make the ester.
- The acyl chloride route releases HCl.
- The carboxylic acid route releases water.
The acyl-chloride route produces HCl, whereas carboxylic-acid/alcohol esterification produces water.
Look for these in your work
- Correct ester route
- HCl stated
- Routes compared
Common mistakes
- Forgetting water in esterification
- Including HCl when no acyl chloride is used
- Confusing a salt by-product with the main organic product
- Inventing unshown side products
What can you explain now?
What is the by-product of carboxylic-acid/alcohol esterification?
Compare with the explanation
Water
The –OH and alcohol hydrogen leave as H₂O.
After trying it yourself, choose your next review. This is your self-assessment.
Your review choice appears on Today. Sign in to sync it across devices.
Make it stick
How do you identify and account for by-products without confusing them with the target organic product?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.