Prepare nitriles by substitution
Use ethanolic KCN and heat to replace a halogen with –CN, adding the nitrile carbon to the chain.
Section 19 · Nitrogen compounds
AS Level; Cambridge International Chemistry 9701, syllabus for 2028–2030. Classification of amines is not tested at AS Level. Cyanides, hydrogen cyanide and pressurised reactions are hazardous: these examples teach exam chemistry, not practical instructions. Written practice is self-checked
Before you begin
- Use nucleophilic substitution and addition, carbonyl compounds, functional-group names and balanced equations from Topics 13–18.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which reagent is used to make a nitrile from a halogenoalkane?
What you will learn
- Recall KCN in ethanol and heat
- Predict the nitrile formed by substitution
- Track the additional carbon supplied by CN⁻
Cyanide adds one carbon to the chain
A halogenoalkane reacts with KCN in ethanol and heat to form a nitrile. Cyanide ion substitutes for the halide; the carbon atom in –C≡N becomes part of the organic chain.
This means the product has one more carbon than the starting halogenoalkane. For example, bromoethane forms propanenitrile, CH₃CH₂C≡N, not ethanenitrile. The nitrile carbon counts when naming the product.
Distinguish the nitrile route from amine preparation
Both routes start from a halogenoalkane and use ethanolic conditions, but KCN gives a nitrile while NH₃ gives an amine. State the nucleophile clearly so the product can be predicted.
Cyanide reagents are highly hazardous. The reaction conditions here are syllabus facts, not a procedure for unsupervised laboratory work.
Pause and explain
What happens to the carbon count when a halogenoalkane forms a nitrile?
Worked example
Give the product and conditions when 1-bromopropane reacts with KCN.
Show the worked solution
- KCN supplies CN⁻.
- Use ethanol and heat.
- Replace Br with –C≡N and count the nitrile carbon.
Answer CH₃CH₂CH₂C≡N, butanenitrile; use KCN in ethanol and heat.
Try a topic test
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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.
Predict the product when 1-bromobutane is heated with KCN in ethanol.
Give me a hint
Replace Br with –C≡N, then count every carbon including the nitrile carbon.
Compare my reasoning
- The alkyl chain has four carbons.
- CN contributes one more carbon.
- Name the five-carbon nitrile.
CH₃CH₂CH₂CH₂C≡N, pentanenitrile.
Look for these in your work
- KCN route predicts nitrile
- One carbon added
- Pentanenitrile named
Which product forms from 1-chloropropane: an amine or nitrile, when the reagent is KCN in ethanol and heat? Explain.
Give me a hint
Identify the nucleophile supplied by the stated reagent.
Compare my reasoning
- KCN supplies CN⁻.
- CN⁻ replaces Cl.
- The product contains –C≡N.
A nitrile forms: butanenitrile, because CN⁻ replaces chlorine and contributes a fourth carbon.
Look for these in your work
- Correct functional group
- Substitution stated
- Product chain length correct
A proposed product from bromoethane and ethanolic KCN is ethanenitrile. Find the error and correct the product.
Give me a hint
Count the –CN carbon as part of the nitrile chain.
Compare my reasoning
- Bromoethane contains two carbons.
- The cyanide ion supplies a third carbon.
- The product must therefore have three carbons.
The product is propanenitrile, CH₃CH₂C≡N; the nitrile carbon is included in the name.
Look for these in your work
- Starting chain counted
- CN carbon included
- Correct product named
Common mistakes
- Omitting the nitrile carbon when naming the product
- Using NH₃ and predicting an amine
- Using aqueous KCN instead of ethanolic KCN
- Confusing a nitrile with a hydroxynitrile
What can you explain now?
Which reagent is used to make a nitrile from a halogenoalkane?
Compare with the explanation
KCN
Cyanide ion replaces the halide and supplies the nitrile carbon.
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Make it stick
Why does cyanide substitution increase the carbon count by one, and what conditions must you state?
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.