Introduction

Extending carbon chains is a central skill in advanced organic synthesis, enabling chemists to build more complex molecules by forming new carbon–carbon and nitrogen–carbon bonds.

Extending Carbon Chains in Organic Synthesis

Increasing the length of a carbon chain is essential when designing synthetic routes to larger, more functionalised molecules. OCR emphasises reactions that introduce new C–C or N–C bonds, allowing a small molecule to become the foundation for more advanced structures. These transformations are widely used in laboratory synthesis and industrial chemistry, giving access to pharmaceuticals, polymers, fragrances, and intermediates for multi-stage syntheses.

Importance of Carbon–Chain Extension

Creating new covalent bonds between carbon atoms significantly alters a molecule’s properties by increasing molecular weight, providing new reactive centres, or enabling branching. This process is often the first step in building a target molecule from simple starting materials. Many of these transformations use strong nucleophiles, reactive electrophiles, or reducing agents that modify functional groups after the initial chain extension.

Key C–C Bond-Forming Reactions

A range of reactions from across the specification contribute to carbon-chain extension. OCR focuses on those accessible at A-Level, with clear mechanistic pathways that illustrate nucleophilic attack, heterolytic bond cleavage, and subsequent functional group transformations.

Nucleophilic Substitution with Cyanide Ions

Haloalkanes undergo nucleophilic substitution when reacted with cyanide ions (CN⁻) in ethanol, producing nitriles, which contain an additional carbon atom compared with the starting haloalkane.

This scheme shows how an alkyl halide can be converted into a primary amine with one extra carbon by forming a nitrile intermediate using cyanide ions, followed by reduction. Source

This reaction forms an N–C bond and effectively lengthens the carbon chain by one carbon. Although the detailed mechanism belongs to another subsubtopic, it is important here because nitriles produced by CN⁻ substitution serve as key intermediates in further transformations.

Nucleophilic Addition to Carbonyl Compounds

The addition of hydrogen cyanide (HCN) to aldehydes and ketones also extends carbon chains by one carbon.

This diagram illustrates cyanohydrin formation, where hydrogen cyanide adds to a carbonyl compound to create a hydroxynitrile, extending the carbon chain by one atom. Source

The reaction proceeds by nucleophilic attack of CN⁻ on the partially positive carbonyl carbon, followed by protonation to yield hydroxynitriles. This pathway introduces both a new C–C bond and a reactive hydroxyl group at the same carbon, enabling versatile further modifications.

These functional groups make hydroxynitriles valuable intermediates, as they can be reduced, oxidised, or hydrolysed to generate a wide variety of compounds.

Functional Group Transformations After Chain Extension

Once the carbon chain has been extended using cyanide-based reactions, the resulting nitriles or hydroxynitriles often undergo further transformations to yield amines, carboxylic acids, or other important derivatives. These steps do not directly extend the chain further but convert the newly formed functional groups into those needed for a subsequent synthetic step.

Reduction of Nitriles

Nitriles can be reduced using hydrogen gas with a nickel catalyst to form primary amines, useful intermediates in pharmaceuticals, dyes, and other nitrogen-containing organic compounds.

The product retains the additional carbon introduced during cyanide substitution, preserving the extended chain.

Acid Hydrolysis of Nitriles

Under hot aqueous acidic conditions, nitriles hydrolyse to form carboxylic acids, again conserving the chain extension.

This reaction sequence shows how a nitrile formed during carbon-chain extension can be hydrolysed to a carboxylic acid, retaining the added carbon in the final product. Source

This is particularly valuable in multi-step synthesis because carboxylic acids can be transformed into esters, acyl chlorides, amides, and a variety of other functional groups.

Using Carbonyl Compounds in Chain Extension

Carbonyl compounds such as aldehydes and ketones play a major role in chain-extension strategies due to the electrophilic nature of the carbonyl carbon. Their susceptibility to nucleophilic attack makes them ideal starting materials for forming new C–C bonds.

Reactivity Considerations

Aldehydes are generally more reactive than ketones because of reduced steric hindrance and a greater partial positive charge on the carbonyl carbon. This difference influences reaction conditions and the likely yields of chain-extended products.

Designing Synthetic Routes Involving C–C Bond Formation

When planning a synthesis that requires carbon-chain extension, students must consider:

• Suitable functional group interconversions before chain extension

• Choice of reagents to form nitriles or hydroxynitriles

• Reaction conditions, such as ethanolic medium or acidic conditions

• Subsequent steps, including reduction or hydrolysis

• Presence of other functional groups that may react competitively

• Overall carbon count in the desired final molecule

Building effective synthetic pathways requires understanding both the mechanism of the C–C bond-forming step and the transformations available afterwards.

Typical Strategies for Carbon-Chain Extension

Below are the main strategies emphasised by OCR for extending carbon chains:

• Reaction of haloalkanes with CN⁻ to form nitriles

• Reaction of carbonyl compounds with HCN to form hydroxynitriles

• Transformations of nitriles, including reduction and hydrolysis

• Controlled reaction conditions to prevent unwanted side reactions

• Use of carbonyl groups as electrophiles to generate new stereocentres in hydroxynitriles

Why Nitrile-Based Methods Are Central to A-Level Synthesis

Nitriles are key to chain extension because they:

• introduce exactly one additional carbon

• can be transformed into numerous functional groups

• help build multi-stage routes in a controlled and predictable manner

• appear commonly in exam synthesis pathways

Their versatility makes them essential tools for organic chemists and a central part of many OCR A-Level synthetic questions.

Carbon Count and Reaction Mapping

To successfully extend carbon chains in exam-style synthesis design, it is important to track how many carbons are introduced at each stage. For example:

• Substitution with cyanide introduces +1 carbon

• Nucleophilic addition of HCN introduces +1 carbon

• Transformations following these steps maintain, but do not increase, carbon count

Students should mentally map each step, noting functional groups formed and those required for the next transformation.

Summary of Key Points for OCR

• Reactions must form C–C or N–C bonds to qualify as chain extension

• Cyanide-based mechanisms are the principal focus at A-Level

• Resulting nitriles are crucial intermediates for further synthesis

• Carbonyl compounds provide important electrophilic sites for extension

• Understanding mechanisms helps predict products and plan routes