Prepare a nitrile from a halogenoalkane
Use ethanolic KCN and heat to replace X with CN and extend the carbon chain by one atom.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the nucleophile in KCN substitution?
What you will learn
- Identify CN⁻ as the nucleophile
- Use ethanolic KCN and heat
- Count the extra nitrile carbon
Cyanide substitutes for the halogen
A halogenoalkane heated with KCN in ethanol undergoes nucleophilic substitution. CN⁻ forms a new C–C bond at the carbon that carried X.
Bromoethane gives propanenitrile: C₂H₅Br + KCN → C₂H₅CN + KBr. The nitrile carbon comes from CN⁻, so the carbon chain increases by one.
Name and count the product carefully
When naming a nitrile, the carbon in –C≡N is included in the parent carbon count. Thus C₂H₅CN has three carbons and is propanenitrile.
Do not use aqueous KCN if the question specifies ethanolic KCN and heat. Do not forget that the cyanide carbon becomes part of the organic product.
Pause and explain
How many carbon atoms are in the nitrile product from bromoethane + KCN?
Worked example
Predict and name the product of 1-bromopropane with KCN in ethanol and heat.
Show the worked solution
- CN⁻ replaces Br by forming a bond at the C–Br carbon.
- Include the cyanide carbon in the product chain count.
- Three original carbons plus one nitrile carbon give four total.
Answer CH₃CH₂CH₂Br + KCN → CH₃CH₂CH₂CN + KBr; butanenitrile.
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 and name the nitrile formed from 1-bromobutane and ethanolic KCN on heating.
Give me a hint
Replace Br with CN, then include its carbon in the chain count.
Compare my reasoning
- The butyl skeleton contains four carbons.
- CN adds one more carbon at the substitution site.
- The five-carbon nitrile is named pentanenitrile.
CH₃CH₂CH₂CH₂Br + KCN → CH₃CH₂CH₂CH₂CN + KBr; pentanenitrile.
Look for these in your work
- Br replaced
- Nitrile carbon counted
- Product named
A student names CH₃CH₂CN as ethanenitrile. Correct the parent-chain count.
Give me a hint
Count the carbon within the –C≡N group.
Compare my reasoning
- CH₃CH₂ contributes two carbons.
- The nitrile carbon contributes one more.
- The full parent contains three carbons.
It is propanenitrile because the –C≡N carbon is included in the three-carbon parent.
Look for these in your work
- Alkyl carbons counted
- Nitrile carbon included
- Name corrected
Why does cyanide substitution increase the carbon count while hydroxide substitution does not?
Give me a hint
Compare the atom groups entering the molecule.
Compare my reasoning
- OH⁻ contributes oxygen and hydrogen but no carbon.
- CN⁻ contributes a carbon atom that bonds to the organic skeleton.
- The nitrile product therefore has one more carbon than the starting halogenoalkane.
CN⁻ adds its carbon to the organic chain; OH⁻ adds no carbon, so hydroxide substitution preserves the carbon count.
Look for these in your work
- Both incoming groups compared
- Cyanide carbon located
- Chain change explained
Common mistakes
- Omitting the carbon in –CN from the name
- Using aqueous KCN instead of ethanolic conditions
- Writing nitrile formation as elimination
What can you explain now?
What is the nucleophile in KCN substitution?
Compare with the explanation
CN⁻
Cyanide donates an electron pair to the carbon bearing X.
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Make it stick
Why does a nitrile made from a halogenoalkane contain one more carbon than the starting skeleton?
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.