Carbon–Carbon Bond Formation
A concise revision guide to lengthening a carbon chain in synthesis: forming C–C≡N by nucleophilic substitution of a halogenoalkane with cyanide and by nucleophilic addition of HCN to a carbonyl compound, converting the nitrile to an amine or a carboxylic acid, and planning the extra carbon into a route.
Before You Start
Every reaction on this page is one you have met before. This page puts them to a new use: making the carbon chain longer.
- A halogenoalkane has a δ+ carbon that nucleophiles attack: nucleophilic substitution. See Nucleophilic Substitution Reactions.
- A carbonyl carbon is δ+ too, so HCN adds across C=O: nucleophilic addition. See Reduction and Nucleophilic Addition.
- Nitriles are reduced to primary amines, R–C≡N + 4[H] → R–CH₂NH₂: see Preparing Amines.
- A benzene ring gains a carbon group by Friedel–Crafts alkylation or acylation: see Friedel–Crafts Alkylation and Acylation.
- The functional group tests and the spectra that identify the products: Identifying Functional Groups by Chemical Tests and Deducing Formulae from Analytical Data.
Why Chain Length Matters
Most reactions in organic chemistry change the functional group but keep the carbon skeleton. A target molecule often has more carbons than the starting material, so one step must make a new C–C bond.
- Converting a group (alcohol → aldehyde → acid, or halide → alcohol → aldehyde) never changes the number of carbons.
- Adding one carbon needs a carbon nucleophile: the cyanide ion, CN⁻.
- Adding a carbon chain to a benzene ring needs an electrophilic carbon: Friedel–Crafts alkylation or acylation with an AlCl₃ catalyst.
- Planning when to add the carbon is part of every route: see Planning a Synthetic Route.
The three ways to add a carbon atom, the conditions for each and what the new carbon can be turned into afterwards.
Exam focus: Count the carbons in the starting material and in the target before you plan anything. A difference of one carbon points to a nitrile step.
The Nitrile Route from a Halogenoalkane
The cyanide ion is a nucleophile with its lone pair on carbon. It substitutes the halogen of a halogenoalkane, and the new carbon arrives already bonded to the chain.
| Detail | |
|---|---|
| reagent | potassium cyanide, KCN |
| conditions | dissolved in ethanol, heated under reflux |
| equation | CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻ (bromoethane → propanenitrile) |
| type of reaction | nucleophilic substitution |
| carbons | two in bromoethane, three in propanenitrile: one added |
- Ethanol is the solvent: in water the hydroxide ions present would compete and give the alcohol.
- Name the nitrile from the whole chain including the C≡N carbon: CH₃CH₂CN is propanenitrile, not ethyl cyanide.
- The C≡N group is then a branching point: it can become an amine or a carboxylic acid (below).
Remember: KCN in ethanol under reflux adds one carbon. The same halogenoalkane with ammonia or hydroxide keeps the chain length.
Nitriles from Aldehydes and Ketones
Hydrogen cyanide adds across the C=O bond of an aldehyde or ketone. The product is a hydroxynitrile (cyanohydrin), again with one carbon more.
| Detail | |
|---|---|
| reagent | HCN with KCN as catalyst (the CN⁻ ion is the nucleophile) |
| conditions | heat, in solution; HCN is toxic, so work in a fume cupboard |
| equation | CH₃CHO + HCN → CH₃CH(OH)CN (ethanal → 2-hydroxypropanenitrile) |
| with a ketone | CH₃COCH₃ + HCN → (CH₃)₂C(OH)CN (propanone → 2-hydroxy-2-methylpropanenitrile) |
| type of reaction | nucleophilic addition |
- The OH and the C≡N finish on the same carbon, the old carbonyl carbon.
- Hydrolysing the nitrile group gives a 2-hydroxycarboxylic acid: CH₃CH(OH)CN → CH₃CH(OH)COOH, 2-hydroxypropanoic acid.
- The mechanism is on the carbonyl page, Reduction and Nucleophilic Addition.
Quick Check: Hydroxynitriles from New Carbonyl Compounds
Pick the one accurate statement in each set.
What the Nitrile Becomes
A nitrile is rarely the target. Its C≡N group is converted in one more step into an amine or a carboxylic acid, each keeping the extra carbon.
| Conversion | Reagents and conditions | Equation |
|---|---|---|
| reduction to a primary amine | LiAlH₄ in dry ether, or H₂ with a nickel catalyst | CH₃CH₂CN + 4[H] → CH₃CH₂CH₂NH₂ (propylamine) |
| acid hydrolysis to a carboxylic acid | dilute hydrochloric acid, heat under reflux | CH₃CH₂CN + 2H₂O + HCl → CH₃CH₂COOH + NH₄Cl (propanoic acid) |
| alkaline hydrolysis to the carboxylate | sodium hydroxide solution, heat under reflux, then add dilute acid | CH₃CH₂CN + H₂O + NaOH → CH₃CH₂COONa + NH₃; then CH₃CH₂COO⁻ + H⁺ → CH₃CH₂COOH |
- The nitrile carbon becomes the CH₂ next to nitrogen in the amine, or the COOH carbon in the acid.
- Acid hydrolysis gives the acid and an ammonium salt; alkaline hydrolysis gives the salt of the acid and ammonia gas.
- Both hydrolyses are expected: heat under reflux with dilute acid, or with aqueous alkali and then acidify to free the acid.
Both nitrile routes as one scheme: bromoethane to propanenitrile and on to propylamine or propanoic acid; ethanal to 2-hydroxypropanenitrile and on to 2-hydroxypropanoic acid.
The nitrile as a branching point: reduce it to the amine, or hydrolyse it to the acid with acid or with alkali.
Exam focus: Two steps add one carbon and set the group: halogenoalkane → nitrile (KCN, ethanol, reflux) → acid (dilute HCl, reflux) or amine (LiAlH₄ or H₂/Ni). Give both reagents and both conditions.
Check: The Nitrile Routes
Give the reagents and conditions for turning a halogenoalkane or a carbonyl compound into a nitrile, count the carbons, and choose the step that converts the nitrile into an amine or an acid.
Quick Check: Choosing the First Step
Count the carbons at both ends of each route, then choose the first step.
Adding Carbons to a Benzene Ring
A benzene ring is attacked by electrophiles, not nucleophiles, so a carbon chain is added to it with a δ+ carbon: Friedel–Crafts alkylation and acylation (Friedel–Crafts Alkylation and Acylation).
| Reaction | Reagents and conditions | Product |
|---|---|---|
| alkylation | a halogenoalkane such as chloroethane with an AlCl₃ catalyst, heat under reflux | ethylbenzene, C₆H₅CH₂CH₃ |
| acylation | an acyl chloride such as ethanoyl chloride with an AlCl₃ catalyst, heat under reflux | phenylethanone, C₆H₅COCH₃ |
- The catalyst AlCl₃ polarises the C–Cl bond so the carbon becomes a strong enough electrophile for the ring.
- Acylation adds one acyl group cleanly; the ketone made can be reduced to an alcohol or used in a nitrile step.
- The mechanism and the reasons for choosing acylation are on Friedel–Crafts Alkylation and Acylation; this page only places the step in a route.
Remember: Chain: cyanide adds a carbon by nucleophilic attack. Ring: Friedel–Crafts adds a carbon by electrophilic substitution with an AlCl₃ catalyst.
Check: Choosing the Carbon-Adding Step
Choose the reagent that adds a carbon to a chain or a ring, give its conditions, and work out the product from the starting material and the step that follows.
Common Exam Points
- Halogenoalkane + KCN in ethanol under reflux → nitrile, one carbon longer: CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻ (nucleophilic substitution).
- Aldehyde or ketone + HCN (with KCN) → hydroxynitrile by nucleophilic addition: CH₃CHO + HCN → CH₃CH(OH)CN.
- Nitrile + 4[H] (LiAlH₄ in dry ether, or H₂ with a nickel catalyst) → primary amine; nitrile + dilute HCl under reflux → carboxylic acid + NH₄⁺.
- Alkaline hydrolysis of a nitrile gives the carboxylate salt and ammonia; acidify to obtain the acid.
- Friedel–Crafts alkylation or acylation adds a carbon group to a benzene ring with an AlCl₃ catalyst: electrophilic substitution.
- Always count carbons: a target with one more carbon than the start needs a C–C bond-forming step somewhere in the route.
FAQs
Use these quick answers to check why a synthesis sometimes needs an extra carbon, how the nitrile route adds one from a halogenoalkane or a carbonyl compound, and what the nitrile is turned into afterwards.
Why does chain length matter when I plan a synthesis?
Most reactions keep the carbon skeleton and only change the functional group. If the target has more carbon atoms than the starting material, one step must form a new carbon–carbon bond, and the nitrile route is the simplest way to add exactly one carbon.
How does the nitrile route add one carbon to a halogenoalkane?
Heat the halogenoalkane under reflux with potassium cyanide in ethanol: bromoethane gives propanenitrile, CH₃CH₂CN, by nucleophilic substitution. Reduce the nitrile with hydrogen and a nickel catalyst (or LiAlH₄) to propylamine, or heat it with dilute hydrochloric acid to hydrolyse it to propanoic acid.
How does HCN add a carbon to an aldehyde or ketone?
The cyanide ion attacks the δ+ carbonyl carbon in a nucleophilic addition, using HCN with KCN as catalyst, and heat. Ethanal gives 2-hydroxypropanenitrile, CH₃CH(OH)CN, and hydrolysis of the nitrile with dilute acid then gives the 2-hydroxy acid, 2-hydroxypropanoic acid (lactic acid).
What happens when a nitrile is hydrolysed?
Heating under reflux with dilute hydrochloric acid gives a carboxylic acid and an ammonium ion: CH₃CH₂CN + 2H₂O + H⁺ → CH₃CH₂COOH + NH₄⁺.
Hot aqueous sodium hydroxide gives the carboxylate salt and ammonia instead, so the mixture is then acidified to release the acid.
Why is the potassium cyanide dissolved in ethanol rather than water?
Ethanol dissolves both the halogenoalkane and the cyanide salt, so the two meet and react. In water the halogenoalkane is almost insoluble and hydroxide ions, present because cyanide is a weak-acid anion, would compete and substitute to give the alcohol instead of the nitrile.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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