Nitriles and hydroxynitriles
Chemistry· 9701 A2 Unit 15: Carbonyl compounds and carboxylic acids· 12 min read
1. Naming Nitriles and Hydroxynitriles★★☆☆☆⏱ 3 min
Nitrile Naming Rule
The carbon of the -C≡N group is counted as the first carbon of the parent alkane chain, so the total number of carbons (including the nitrile carbon) gives the root name, followed by the suffix -nitrile.
For hydroxynitriles, the parent chain is numbered starting from the nitrile carbon, and the position of the hydroxyl group is specified with a locant number before the hydroxy prefix.
Name the nitrile with condensed formula CH₃CH₂CH₂CN, and the hydroxynitrile formed from propanone and HCN
- 1
Step 1: Count total carbons in the nitrile, including the C in -C≡N
- 2
- 3
Step 2: Match the 4-carbon root to butane, add suffix -nitrile to get butanenitrile
- 4
Step 3: For the propanone-derived hydroxynitrile, the parent 3-carbon chain has -OH and -CN on carbon 2
- 5
Final names: Butanenitrile, 2-hydroxy-2-methylpropanenitrile
Test your understanding of nitrile naming:
What is the correct name for CH₃CH(OH)CN?
A. Ethanenitrile
B. 2-hydroxyethanenitrile
C. 1-hydroxyethanenitrile
D. Hydroxyethane nitrile
Reveal answer
B —Total carbons = 2, -CN is the end group, so the -OH is on carbon 2 adjacent to the nitrile carbon.
Exam tip:
CIE examiners regularly mark down answers that forget to count the nitrile carbon in the parent chain length, leading to one-carbon shorter root names.
2. Mechanism of Hydroxynitrile Formation★★★★☆⏱ 4 min
Draw the full mechanism for the reaction of ethanal with KCN/H₂SO₄ to form 2-hydroxypropanenitrile
- 1
Step 1: The cyanide nucleophile (CN⁻ from KCN) attacks the partially positive carbonyl carbon of ethanal, pushing the pi bond electrons up to the oxygen atom
- 2
- 3
Step 2: The negatively charged oxygen intermediate deprotonates a H⁺ ion from the dilute acid catalyst to form the final neutral hydroxynitrile product
Confirm you can identify the active nucleophile:
What is the active nucleophile in this reaction?
A. HCN
B. K⁺
C. CN⁻
D. H₂O
Reveal answer
C —Cyanide ion has a full negative charge on the carbon atom, making it a strong nucleophile that attacks the electrophilic carbonyl carbon.
3. Key Synthetic Reactions of Nitriles★★★☆☆⏱ 3 min
Nitrile Reduction
Catalytic hydrogenation of nitriles using H₂ and a nickel catalyst produces a primary amine with one extra carbon atom compared to the starting alkyl halide if nitrile was formed via substitution.
Reaction Type | Reagents and Conditions | Product | Carbon Chain Change |
|---|---|---|---|
Acid Hydrolysis | Dilute HCl, heat under reflux | Carboxylic acid + ammonium salt | No change, nitrile carbon becomes the carboxyl carbon |
Alkaline Hydrolysis | Aqueous NaOH, heat under reflux | Carboxylate salt + ammonia | No change, followed by acidification to get free carboxylic acid |
Reduction | H₂ gas, Ni catalyst, high pressure | Primary aliphatic amine | No change, nitrile triple bond fully hydrogenated |
Show the full reaction pathway for converting ethanenitrile to ethanoic acid under acidic conditions, and name the products
- 1
Step 1: Combine ethanenitrile with dilute hydrochloric acid and heat under reflux
- 2
- 3
Step 2: The products are ethanoic acid (a 2-carbon carboxylic acid) and ammonium chloride as the byproduct
4. Using Nitriles to Extend Carbon Chains★★★★☆⏱ 2 min
Nitriles are one of the most reliable methods in the CIE specification to increase the length of an organic carbon chain by one carbon atom, via nucleophilic substitution of a haloalkane with CN⁻ ion, followed by subsequent transformation to carboxylic acid or amine.
Design a 2-step synthesis to convert ethanol to propanoic acid, using a nitrile intermediate
- 1
Step 1: First convert ethanol to chloroethane via nucleophilic substitution with PCl₅
- 2
Step 2: Reflux chloroethane with KCN in ethanol to form propanenitrile, adding one extra carbon to the chain
- 3
Step 3: Heat propanenitrile under reflux with dilute HCl to hydrolyze it to propanoic acid
Exam tip:
This 2-step carbon chain extension is a standard 4-6 mark CIE synthesis question, you will almost always be asked to state the reagents for each step.
5. Common Pitfalls
Wrong move:
Counting only alkyl group carbons when naming nitriles
Why:
Students often forget the nitrile carbon is part of the parent chain, leading to a name one carbon shorter than correct
Correct move:
Always count the C in -C≡N as the first carbon of the parent chain before selecting the root name
Wrong move:
Drawing HCN as the attacking nucleophile in the hydroxynitrile mechanism
Why:
HCN is a weak acid and does not have a full negative charge on the carbon atom, so it is not the active nucleophile
Correct move:
Always show CN⁻ as the attacking nucleophile, with the lone pair on the carbon atom of the cyanide ion
Wrong move:
Stating alkaline hydrolysis of nitriles directly produces pure carboxylic acid
Why:
Alkaline hydrolysis first produces a carboxylate salt, you need an additional acidification step to get the neutral carboxylic acid
Correct move:
Explicitly mention adding excess dilute strong acid after reflux with NaOH to protonate the carboxylate ion
Wrong move:
Miscounting carbons in nitrile reduction products
Why:
Students often draw an amine one carbon shorter than the correct product
Correct move:
Count all carbons in the nitrile starting material, all are retained in the final primary amine product
Wrong move:
Using HCN directly as a reagent for haloalkane substitution
Why:
HCN is a very weak acid and does not provide sufficient CN⁻ ions for the substitution reaction to proceed at a fast rate
Correct move:
Use potassium cyanide (KCN) dissolved in ethanol as the source of CN⁻ nucleophile for haloalkane substitution
6. Quick Reference Cheatsheet
Starting Material | Reagents | Product | Key Exam Note |
|---|---|---|---|
Aldehyde/Ketone | KCN / dilute H₂SO₄ | Hydroxynitrile | CN⁻ is the nucleophile, not HCN |
Nitrile | Dilute HCl, heat reflux | Carboxylic acid + NH₄Cl | No extra acidification step required |
Nitrile | Aqueous NaOH, heat reflux | Carboxylate salt + NH₃ | Must acidify to get free carboxylic acid |
Nitrile | H₂ / Ni catalyst, high pressure | Primary amine | Used for 1-carbon chain extension |
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2024 · 42
Hydroxynitrile formation mechanism
- 2023 · 41
Nitrile hydrolysis reaction pathway
- 2022 · 43
Synthesis of amine via nitrile intermediate
What's Next
Mastering nitrile and hydroxynitrile chemistry is a critical milestone for CIE A-Level organic synthesis, as this is one of the only spec-defined methods to add a single carbon atom to your target molecule. You will see these reactions appear repeatedly in multi-step synthesis questions that combine carbonyl, halogenoalkane, and amine functional group transformations. This knowledge directly supports your preparation for the full A2 organic paper section, where 15-20 mark synthesis and mechanism questions are standard. Build on this foundation by exploring the related core topics below.