# Nitriles and hydroxynitriles

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u15-nitriles-and-hydroxynitriles/

This module covers nitrile and hydroxynitrile naming conventions, nucleophilic addition of HCN to carbonyls, nitrile hydrolysis and reduction, and exam-focused organic synthesis applications aligned to CIE A-Level 9701 A2 requirements.

**Prerequisites:** [Nucleophilic addition to aldehydes and ketones](https://www.owlsprep.com/study/cie-9701-u15-carbonyl-nucleophilic-addition/); [Organic functional group nomenclature rules](https://www.owlsprep.com/study/cie-9701-u12-organic-nomenclature/)

## Learning objectives

- Recall the general formula and naming conventions for nitriles and hydroxynitriles
- Explain the mechanism of nucleophilic addition of HCN to carbonyls to form hydroxynitriles
- Describe hydrolysis and reduction reactions of nitriles to form carboxylic acids and primary amines
- Apply nitrile chemistry to multi-step organic synthesis pathways

## Naming Nitriles and Hydroxynitriles

**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.

*Notation:* CH3CH2CN = Propanenitrile

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.

**Worked example:** 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. $$CH_3CH_2CH_2CN \text{ has 4 total carbons}$$
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

**Check your understanding**

Test your understanding of nitrile naming:

1. What is the correct name for CH₃CH(OH)CN?

   - A. Ethanenitrile
   - B. 2-hydroxyethanenitrile
   - C. 1-hydroxyethanenitrile
   - D. Hydroxyethane nitrile

   *Why:* 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.

## Mechanism of Hydroxynitrile Formation

**Exam command terms**

CIE uses specific command terms for this mechanism question:

- **Draw the mechanism** — You must show all lone pairs, curly arrow movement, intermediate structures, and the role of the cyanide ion catalyst

- **Explain why HCN alone does not react** — You must state that HCN is a weak acid, and the CN⁻ nucleophile is required for the reaction to proceed at a practical rate

**Worked example:** 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. $$CH_3CHO + :CN^- \rightarrow [CH_3CH(O^-)CN]$$
3. Step 2: The negatively charged oxygen intermediate deprotonates a H⁺ ion from the dilute acid catalyst to form the final neutral hydroxynitrile product

> **Common Mechanism Mistake**
>
> Do NOT draw HCN as the attacking nucleophile in the mechanism, you will lose all marks for the mechanism step.

$$CH_3COCH_3 + HCN \xrightarrow{KCN, H^+} CH_3C(OH)(CN)CH_3$$

**Check your understanding**

Confirm you can identify the active nucleophile:

1. What is the active nucleophile in this reaction?

   - A. HCN
   - B. K⁺
   - C. CN⁻
   - D. H₂O

   *Why:* Cyanide ion has a full negative charge on the carbon atom, making it a strong nucleophile that attacks the electrophilic carbonyl carbon.

## Key Synthetic Reactions of Nitriles

**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.

*Notation:* R-C\equiv N \rightarrow R-CH_2NH_2

| 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 |

**Worked example:** 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. $$CH_3CN + 2H_2O + HCl \rightarrow CH_3COOH + NH_4Cl$$
3. Step 2: The products are ethanoic acid (a 2-carbon carboxylic acid) and ammonium chloride as the byproduct

> **mnemonic**
>
> Nitrile reactions: Hydrolysis gives Carboxylic acid, Reduction gives Amine = H C R A to remember the pairings easily.

## Using Nitriles to Extend Carbon Chains

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.

**Worked example:** 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.

## Common pitfalls

- **Wrong:** Counting only alkyl group carbons when naming nitriles
  - Why it fails: Students often forget the nitrile carbon is part of the parent chain, leading to a name one carbon shorter than correct
  - Correct: Always count the C in -C≡N as the first carbon of the parent chain before selecting the root name
- **Wrong:** Drawing HCN as the attacking nucleophile in the hydroxynitrile mechanism
  - Why it fails: 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: Always show CN⁻ as the attacking nucleophile, with the lone pair on the carbon atom of the cyanide ion
- **Wrong:** Stating alkaline hydrolysis of nitriles directly produces pure carboxylic acid
  - Why it fails: Alkaline hydrolysis first produces a carboxylate salt, you need an additional acidification step to get the neutral carboxylic acid
  - Correct: Explicitly mention adding excess dilute strong acid after reflux with NaOH to protonate the carboxylate ion
- **Wrong:** Miscounting carbons in nitrile reduction products
  - Why it fails: Students often draw an amine one carbon shorter than the correct product
  - Correct: Count all carbons in the nitrile starting material, all are retained in the final primary amine product
- **Wrong:** Using HCN directly as a reagent for haloalkane substitution
  - Why it fails: HCN is a very weak acid and does not provide sufficient CN⁻ ions for the substitution reaction to proceed at a fast rate
  - Correct: Use potassium cyanide (KCN) dissolved in ethanol as the source of CN⁻ nucleophile for haloalkane substitution

## 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 |

## 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.

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