JC2 H2 Chemistry Tutor | How Cyanide Substitution Adds One Carbon Before Nitrile Hydrolysis

Did you know? Converting bromoethane into propanoic acid through a nitrile increases the carbon skeleton from two to three atoms—but the extra carbon enters during cyanide substitution, not when the nitrile is hydrolysed.

This original Hougang JC2 H2 Chemistry tutor guide follows the 2027 9476 Halogen Derivatives and Carboxylic Acids outcomes. It teaches bromoethane → propanenitrile → propanoic acid with atom ledgers, balanced aqueous models, nomenclature and a sharp distinction between acidic and alkaline hydrolysis.

Cyanide reagents are exceptionally hazardous. This guide uses only official syllabus-level reaction identities and paper transformations; it gives no practical synthesis or handling instructions.

Full chapter index

• 1. The key insight: two stages but only one extra carbon
• 2. Count the atoms in the starting halogenoalkane
• 3. Interpret CN⁻ as the incoming carbon nucleophile
• 4. Write the substitution as a balanced paper equation
• 5. Understand the nitrile functional group before hydrolysis
• 6. Explain acidic nitrile hydrolysis with balanced atoms
• 7. Distinguish basic hydrolysis from the acidic product stage
• 8. Compare chain extension with hydroxide substitution
• 9. Distinguish cyanide substitution from carbonyl cyanide addition
• 10. Check the nomenclature as part of the reaction map
• 11. Audit reaction charges, product masses and incomplete information
• 12. Transfer to an unfamiliar four-carbon halogenoalkane

1. The key insight: two stages but only one extra carbon

In the familiar H2 organic pathway, bromoethane has two carbon atoms. A cyanide nucleophile can replace bromine and form propanenitrile, a compound containing three carbon atoms. The extra carbon comes from the carbon atom of the cyanide ion, CN⁻. A subsequent hydrolysis converts that nitrile into propanoic acid while keeping the same three-carbon skeleton.

Students often attribute the extra carbon to the hydrolysis step because they see the carboxylic acid at the end of a reaction map. That is incorrect. The carbon-chain extension takes place during nucleophilic substitution with cyanide, when the carbon in CN⁻ becomes attached to the original alkyl chain.

This original paper tutorial follows the 2027 H2 syllabus 9476 Section 11.5, which names bromoethane’s formation of nitriles, and Section 11.8, which names carboxylic-acid formation from nitrile hydrolysis. It is a functional-group and carbon-inventory exercise, not a preparation protocol.

Student question: At what stage does the two-carbon bromoethane become a three-carbon compound?

Tutor check: At substitution by carbon-bound cyanide, before hydrolysis.

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2. Count the atoms in the starting halogenoalkane

Bromoethane can be written CH₃CH₂Br, with two carbon atoms and one bromine atom attached to the terminal carbon. The two carbons are already connected in the starting molecule. A reaction that replaces Br with OH yields ethanol, which still contains two carbons.

A separate substitution replacing Br with a carbon-linked cyanide group gives CH₃CH₂CN, a nitrile with three total carbon atoms. The formula contains the original CH₃CH₂ group plus the nitrile carbon. A student should mark that nitrile carbon in a different colour in a paper diagram to show where the chain length changes.

The molecule is named propanenitrile because the nitrile carbon belongs to the three-carbon parent name. Writing ‘ethanenitrile’ merely because bromoethane began with two carbons omits the carbon contributed by the cyanide group.

Student question: How many carbons does CH₃CH₂CN contain?

Tutor check: Three: two from the ethyl group and one from the nitrile C.

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3. Interpret CN⁻ as the incoming carbon nucleophile

A cyanide ion has a carbon and a nitrogen atom and can act as a nucleophile through carbon in the standard alkyl cyanide formation model. Its electron pair participates in forming a new C–C bond at the carbon formerly bonded to the leaving halogen. The result is an alkyl nitrile R–C≡N.

This reaction should not be described as attaching a nitrogen atom directly to the alkyl chain in the intended product. Such a diagram would not represent the named nitrile connectivity. The functional group contains a carbon triple-bonded to nitrogen, and the chain extension is precisely the introduction of that carbon.

The 2027 H2 syllabus names the cyanide substitution as a reaction of bromoethane. Although the official course specifies relevant reagents and conditions for knowledge, cyanide salts are acutely toxic and no independent practical use or detailed execution is appropriate in a parent-facing explanation.

Student question: Which atom of CN⁻ becomes bonded to the original ethyl chain in a nitrile?

Tutor check: Carbon, giving a new carbon–carbon bond and terminal C≡N group.

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4. Write the substitution as a balanced paper equation

The net ionic substitution can be represented as CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻. The reactant and product atom inventories match: the ethyl group remains intact, bromine leaves as bromide and one carbon plus nitrogen from cyanide enters the organic molecule.

The charge balance also matches. The left-hand side has one negative charge on cyanide; the right-hand side has one negative charge on bromide. A student should not write neutral CN on the reactant side while keeping an unbalanced bromide ion on the product side.

This equation describes the named chemical transformation without specifying an apparatus or procedural sequence. A tutor can ask the learner to circle all three product carbons and distinguish the leaving-group change from the carbon-count change. Both facts are necessary to understand the pathway.

Student question: What is the charge-balanced substitution equation?

Tutor check: CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻.

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5. Understand the nitrile functional group before hydrolysis

A nitrile contains a carbon–nitrogen triple bond, commonly represented R–C≡N. The nitrile carbon is attached to the rest of the carbon skeleton. This functional group can undergo transformations including hydrolysis to carboxylic-acid derivatives and reduction to amines under suitable course-specific conditions.

In propanenitrile, CH₃CH₂CN, the terminal nitrile carbon is already part of the organic compound. If the molecule is hydrolysed, that carbon can become the carbonyl carbon of the resulting carboxylic acid. The carbon skeleton is therefore conserved through that step.

An important mistake is to call the hydrolysed product ethanoic acid because the starting haloalkane had two carbons. The product acid must include the nitrile carbon; propanoic acid CH₃CH₂COOH contains three carbons. Students should count atoms at every intermediate rather than infer names only from the first reactant.

Student question: Which carbon becomes the carboxyl carbon on nitrile hydrolysis?

Tutor check: The nitrile C of the –C≡N group.

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6. Explain acidic nitrile hydrolysis with balanced atoms

In a simplified aqueous acid hydrolysis model, a nitrile becomes a carboxylic acid and the nitrogen ends in an ammonium-containing product under acidic conditions. For propanenitrile, an atom- and charge-balanced ionic overall expression is CH₃CH₂CN + 2H₂O + H⁺ → CH₃CH₂COOH + NH₄⁺.

Count the carbon atoms: three on each side. Count the oxygen atoms: two waters supply two oxygens that end in the carboxylic-acid functional group. The nitrogen in the nitrile becomes ammonium. The total hydrogen and positive charge also balance across this overall expression.

The syllabus names hydrolysis with dilute acid and heat as a route to carboxylic acids, but this article is not a laboratory operating guide. The learning objective is to identify the product functional group and account for every atom; detailed acid catalysts or reaction steps depend on the conditions and are not needed for the carbon-count argument.

Student question: What organic product forms from propanenitrile under the acid-hydrolysis model?

Tutor check: Propanoic acid CH₃CH₂COOH, retaining the three carbons.

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7. Distinguish basic hydrolysis from the acidic product stage

With aqueous alkali in the simplified paper model, the immediate organic product of nitrile hydrolysis is a carboxylate salt rather than the protonated free carboxylic acid. A balanced net ionic expression is R–CN + H₂O + OH⁻ → R–COO⁻ + NH₃, with R representing an unchanged organic group.

For the propanenitrile example, CH₃CH₂CN + H₂O + OH⁻ → CH₃CH₂COO⁻ + NH₃. The organic product remains three carbons. Under a separate appropriate acidification step, the propanoate anion can accept a proton, giving propanoic acid.

Students should not automatically name the carboxylic-acid product directly from a question that specifies alkaline hydrolysis but not acidification. The current solution species differs with pH. The 2027 H2 syllabus explicitly allows dilute alkali hydrolysis followed by acidification as a carboxylic-acid formation route.

Student question: What does base hydrolysis yield before acidification?

Tutor check: A carboxylate ion or salt, not immediately the neutral acid.

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8. Compare chain extension with hydroxide substitution

Bromoethane can undergo different nucleophilic substitutions depending on the specified reagent and solvent context. If hydroxide substitutes for bromide, the product is ethanol CH₃CH₂OH, with two carbons. If carbon-bound cyanide substitutes for bromide, the product is propanenitrile CH₃CH₂CN, with three carbons.

The different carbon counts arise from what the attacking nucleophile contributes. Hydroxide contains no carbon; cyanide contributes one carbon. Thus the identity of the nucleophile affects both functional-group product and skeletal chain length, not just a minor change in a substituent’s colour on the page.

An excellent tutor prompt presents two products without reagent names and asks the student to infer which nucleophile’s atoms appear in each. The student should use an atom ledger rather than assume every substitution of a halogenoalkane leaves chain length unchanged.

Student question: Why does substituting with OH⁻ not extend the carbon chain?

Tutor check: OH⁻ adds oxygen and hydrogen but no carbon atom.

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9. Distinguish cyanide substitution from carbonyl cyanide addition

Cyanide can participate in more than one named organic reaction family. In H2 Chemistry, a cyanide-related nucleophile can substitute in a halogenoalkane to make a nitrile, while hydrogen cyanide can add to aldehyde or ketone carbonyl groups to give hydroxynitriles in the relevant syllabus example.

Both may introduce a nitrile carbon into the product, but the starting functional group and bond changes differ. In substitution, a halogen leaving group is replaced. In carbonyl addition, the C=O group is transformed and an OH and CN become associated with the former carbonyl centre.

Students who draw propanenitrile as the product of ethanal plus HCN have confused these mechanisms. The appropriate addition product has hydroxyl and cyanide groups and different connectivity. A pathway map must preserve the identities and bond changes of the actual reaction being described.

Student question: Is cyanide addition to an aldehyde identical to cyanide substitution in bromoethane?

Tutor check: No. Carbonyl addition and halogenoalkane substitution are different mechanisms.

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10. Check the nomenclature as part of the reaction map

The product of bromoethane substitution with cyanide is propanenitrile, not ethanenitrile, because the nitrile C is included in the parent three-carbon structure. Its hydrolysed acid is propanoic acid. The number of carbons should match the names at each stage.

Start with the two-carbon CH₃CH₂ group. Add the nitrile carbon to obtain a C₃ nitrile. Convert the terminal –CN carbon into the –COOH carbon and the result is still C₃. This simple carbon-identity tracking is more robust than memorising a string of three names.

A change in functional group can alter the nomenclature suffix without changing the skeletal carbon count. Conversely adding one carbon during substitution changes the parent length. Tutors should require students to identify which change happened at each step before writing the official name.

Student question: What two product names follow from bromoethane through CN substitution and hydrolysis?

Tutor check: Propanenitrile, then propanoic acid after appropriate hydrolysis/acidification.

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11. Audit reaction charges, product masses and incomplete information

A balanced transformation must conserve both atom counts and overall electrical charge. The cyanide substitution produces bromide as an anionic leaving-group product. Acid hydrolysis can include an ammonium ion product; base hydrolysis can include carboxylate and ammonia. Which form is appropriate depends on the specified medium.

An incomplete question that says only ‘propanenitrile undergoes hydrolysis’ may not establish whether the immediate solution contains carboxylate or free acid. In an acidic environment carboxylic acid is the expected organic form; in a basic environment the carboxylate is expected until acidification.

Do not invent a numerical mass of product if reactant moles, conversion and yield have not been supplied. The correct qualitative prediction can be made from structures without a fictitious laboratory outcome. Scientific answers should state both what is determined and what data are missing.

Student question: What extra information determines whether the organic hydrolysis product is carboxylate or acid?

Tutor check: Whether the medium and final work-up are basic or acidified.

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12. Transfer to an unfamiliar four-carbon halogenoalkane

Imagine a paper model starting with 1-bromopropane, CH₃CH₂CH₂Br, a three-carbon halogenoalkane. Replacing bromide with carbon-bound cyanide gives CH₃CH₂CH₂CN, butanenitrile, with four carbons. Subsequent suitable hydrolysis converts the nitrile group into the four-carbon carboxylic acid butanoic acid.

A student who predicts propanoic acid has likely failed to count the nitrile carbon. Another who predicts pentanoic acid has added two carbons even though only one carbon-bearing CN group entered through the substitution. Both errors are caught by drawing and numbering the carbon positions.

A contrasting transfer task uses 1-bromopropane and hydroxide instead: the organic alcohol product is propan-1-ol, still three carbons. The difference is caused by the attacking group. Parents can ask one precise question: ‘Which reaction actually adds the new carbon?’ The student should point to cyanide substitution.

Student question: What acid results from 1-bromopropane via nitrile formation and later hydrolysis?

Tutor check: Butanoic acid, a four-carbon product.

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Worked evidence: carbon inventory through the pathway

Every row preserves the identity of the carbon skeleton. The cyanide carbon appears only during the substitution stage; subsequent nitrile hydrolysis changes a functional group while retaining the carbon count.

The nitrile carbon eventually becomes the carboxyl carbon. Organic products and protonation states depend on the stated medium; naming the free acid directly after basic hydrolysis without an acidification step overlooks the carboxylate equilibrium.

Four transfer questions

• Bromoethane plus aqueous hydroxide in a syllabus substitution: name ethanol, still two carbons.
• Bromoethane plus carbon-bound cyanide in the named organic transformation: name propanenitrile, now three carbons.
• Propanenitrile under alkaline hydrolysis: report propanoate until acidification is specified.
• 1-Bromopropane through nitrile formation then hydrolysis: predict butanoic acid, not propanoic acid.

Tutor checklist

• Confirm H2 9476 2027 Halogen Derivatives and Carboxylic Acids learning outcomes.
• Draw and count all carbon atoms in the initial halogenoalkane.
• Identify the incoming C of the cyanide group, not just the nitrogen.
• Write a charge-balanced substitution equation without adding procedural chemical instructions.
• Name the nitrile using the full carbon count, including its terminal carbon.
• Distinguish acidic hydrolysis to carboxylic acid from alkaline hydrolysis to carboxylate.
• Keep changes of functional group distinct from changes of chain length.
• Compare hydroxide substitution as a no-carbon-extension counterexample.
• Apply the method to an unfamiliar three-carbon starting halogenoalkane.
• Treat cyanide as hazardous and restrict instruction to paper chemistry.

Frequently asked questions

Connected organic Chemistry learning

• JC2 H2 Chemistry | Halogenoalkane Substitution versus Elimination
• JC2 H2 Chemistry | SN1 versus SN2 Mechanisms
• Hougang Chemistry Tuition and Tutor Library

Bring the student’s course, exam year and attempted reaction-map question when discussing Chemistry tuition. Verify teaching location, group size and actual availability directly. This guide does not establish a vacancy or guarantee a result.

Official syllabus and safety limits

• SEAB 2027 H2 Chemistry 9476 Sections 11.5 and 11.8
• Official SEAB 2027 A-Level syllabus directory
• Immutable eduKateSG Clementi tutorial reference
• Hougang Chemistry Tuition and Tutor Library

The official syllabus names nitrile formation from bromoethane and hydrolysis of nitriles by dilute acid or alkali followed by acidification. This article uses balanced conceptual equations and original carbon-count examples. It intentionally excludes detailed procedures for toxic cyanide reagents and does not describe measurements or experimental results.