Ch 2 · Nitriles and hydroxynitriles · AS & A Level Chemistry 9701
Topic 2 of 2 in Nitrogen compounds — AS & A Level Chemistry 9701 — 44 lessons.
Introduction
Nitriles, mathrmR-C≡N, and hydroxynitriles link the halogenoalkane and carbonyl families to carboxylic acids, and provide the standard AS route for building a longer carbon chain.
Key ideas — Formation of nitriles from halogenoalkanes
Heating a halogenoalkane under reflux with potassium cyanide dissolved in ethanol substitutes the halogen with a -CN group by nucleophilic substitution: CH_3CH_2Br + KCN arrow CH_3CH_2CN + KBr The cyanide ion, :CN^-, attacks the electrophilic carbon using a lone pair on carbon, in the same way a hydroxide ion attacks in the hydrolysis of a halogenoalkane.
🧑🏫 Teacher's explanation
Think of a nitrile as a chemical 'extension pack'. If you have a carbon chain that is too short for the molecule you want to build, you can use a nitrile to add exactly one carbon atom to that chain. It is like adding a single LEGO brick to a pre-built structure to make it longer.
📖 Definitions
🔍 Worked example — Standard
A sample of 0.25 mol of bromoethane ($ ext{CH}_3 ext{CH}_2 ext{Br}$) reacts completely with $ ext{KCN}$ to form propanenitrile ($ ext{CH}_3 ext{CH}_2 ext{CN}$). Calculate the mass of propanenitrile produced. (A_r: extC=12, extH=1, extN=14, extBr=80)
🔍 Worked example — Exam-style
An organic compound with the formula $ ext{C}_3 ext{H}_5 ext{N}$ is formed by reacting $ ext{C}_2 ext{H}_5 ext{Br}$ with $ ext{KCN}$. This nitrile is then hydrolyzed to form a carboxylic acid. Write the balanced equation for the hydrolysis of the nitrile to the carboxylic acid, including state symbols (assume aqueous acid).
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🎯 Exam question
Key ideas — Formation of hydroxynitriles from carbonyl compounds
Hydrogen cyanide adds across the polar mathrmC=O bond of an aldehyde or ketone by nucleophilic addition, forming a hydroxynitrile with both a -OH and a -CN group on the same carbon: CH_3CHO + HCN arrow CH_3CH(OH)CN Because a hydroxynitrile still contains a nitrile group, it can itself be hydrolysed in a further step, giving a hydroxy carboxylic acid; this two-stage sequence, carbonyl compound to hydroxynitrile to hydroxy-acid, is a further one-carbon chain extension.
🧑🏫 Teacher's explanation
Imagine a carbonyl group (C=O) as a target with a high concentration of negative charge around the oxygen. This makes the carbon atom 'electron-hungry' or electrophilic. When we introduce hydrogen cyanide (HCN), the cyanide ion (CN^-) acts like a specialized tool that attacks this carbon, breaking the double bond and attaching itself to the carbon.
📖 Definitions
🔍 Worked example — Easy
Write the balanced equation for the reaction between propanal and hydrogen cyanide to form a hydroxynitrile.
⚠️ Common errors
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🎯 Exam question
(a) Write the equation for this reaction using structural formulas. [2] (b) The hydroxynitrile is then heated under reflux with dilute sulfuric acid. State the functional group present in the resulting organic product. [1] [3 marks]
Key ideas — Hydrolysis of nitriles
Heating a nitrile under reflux with dilute hydrochloric acid (or with aqueous alkali, then acidifying) hydrolyses the -CN group completely to a -COOH group, releasing ammonium ions: CH_3CH_2CN + 2H_2O + HCl arrow CH_3CH_2COOH + NH_4Cl The same conditions hydrolyse a hydroxynitrile to a hydroxy carboxylic acid, the -OH group being unaffected by the acidic hydrolysis.
🧑🏫 Teacher's explanation
Think of a nitrile group (-CN) as a chemical 'extension pack' for a carbon chain. When you react a halogenoalkane with cyanide, you are essentially swapping a halogen atom for a carbon atom, making the chain one unit longer. This is a powerful tool for chemists who need to build larger organic molecules from smaller ones.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of ethanenitrile, CH_3CN.
🔍 Worked example — Standard
A sample of propanenitrile, CH_3CH_2CN, is hydrolyzed with dilute HCl to form propanoic acid, CH_3CH_2COOH. Calculate the mass of propanoic acid produced from 5.00 g of propanenitrile.
🔍 Worked example — Exam-style
Complete the following balanced equation for the hydrolysis of ethanenitrile (CH_3CN) with dilute HCl and determine the mass of NH_4Cl produced from 2.00 g of CH_3CN.
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🎯 Exam question
Key ideas — Nitriles as chain-extending intermediates
Converting a halogenoalkane into a nitrile, then hydrolysing the nitrile to a carboxylic acid, increases the number of carbon atoms in the main chain by exactly one, because the carbon of the cyanide ion becomes part of the product's carbon skeleton. This two-step route, mathrmR-X arrow mathrmR-CN arrow mathrmR-COOH, is the standard method for building a longer-chain carboxylic acid (or, via reduction of the nitrile, a longer-chain primary amine) from a shorter-chain starting material, since no other AS reaction adds a carbon atom to a chain in this way. For example, bromoethane, CH_3CH_2Br (two carbons), is converted via propanenitrile, CH_3CH_2CN (three carbons), into propanoic acid, CH_3CH_2COOH (three carbons) — a route that could not be achieved directly, since no single-step AS reaction adds a carbon atom to an existing chain.
🧑🏫 Teacher's explanation
Think of a carbon chain like a LEGO structure. Most chemical reactions you learn so far either swap one piece for another or add a group to the side. However, if you want to make the actual 'spine' of the molecule longer, you need a way to insert a new carbon atom into the sequence. Nitriles act as the perfect 'bridge' for this.
📖 Definitions
🔍 Worked example — Easy
Calculate the number of carbon atoms in the product formed when bromoethane, CH_3 extCH_2 extBr, reacts with $ ext{KCN}$ followed by hydrolysis.
🔍 Worked example — Standard
A chemist wants to synthesize ethanoic acid, CH_3 extCOOH, from a halogenoalkane. Identify the required starting halogenoalkane and show the two-step reaction sequence.
🔍 Worked example — Exam-style
A sample of propanenitrile, CH_3 extCH_2 extCN, is hydrolyzed using dilute HCl. Calculate the molar mass of the resulting carboxylic acid.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student wants to convert 1-bromoethane, CH_3CH_2Br, into propanoic acid, CH_3CH_2COOH. (a) State the reagents and conditions for the first step, and explain why this step increases the carbon chain length by one. [3] (b) State the reagents and conditions for the second step. [2] (c) Name the organic intermediate formed. [1]
🔍 Worked example — Standard
A student reacts bromoethane, ceCH_3CH_2Br, with KCN in ethanol to form propanenitrile. If 0.10 moles of bromoethane react completely, how many moles of propanenitrile are produced?
🔍 Worked example — Exam-style
An aldehyde, ethanal (ceCH_3CHO), is reacted with hydrogen cyanide (ceHCN) in the presence of a trace amount of KCN. Identify the product and state the type of reaction.
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Practice
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