Organic compounds can be represented using several formula types, each providing different structural information. Understanding these representations is essential for interpreting reactivity, structure, and relationships between organic molecules across the OCR course.

Representing Organic Formulae

Organic chemistry relies on multiple formula representations that convey varying levels of structural detail. OCR expects students to interpret and use these forms confidently, selecting the most appropriate representation for the context. This subsubtopic focuses on how to read and write general, structural, displayed, and skeletal formulae, and how standard conventions such as COOH, COO, and ring symbols are used to avoid ambiguity.

General Formulae

General formulae describe an entire homologous series, indicating the relationship between carbon, hydrogen, and other atoms. They do not show bonding but give a pattern applicable to all members of the series.

General formulae are most useful when predicting the composition of related molecules or identifying trends across a series.

Structural Formulae

Structural formulae show how atoms are arranged in a molecule. OCR requires students to recognise both condensed and semi-condensed versions.

These formulae provide clarity about connectivity while remaining compact, making them widely used for mid-detail representations.

Displayed Formulae

Displayed formulae show every atom and every bond, including lone pairs when required. They are used when bond-by-bond clarity is essential for discussing mechanisms or identifying features such as bond angles.

Displayed formulae are especially helpful for visualising bonding and identifying functional groups.

Skeletal Formulae

Skeletal formulae are highly simplified representations used extensively in organic chemistry. OCR expects students to read and draw skeletal structures confidently.

Skeletal formulae highlight functional groups and carbon frameworks efficiently and are the preferred style for complex structures.

Skeletal formula of hexane, showing a six‑carbon chain as a zig‑zag line with selected carbons numbered. Each vertex represents a carbon atom with implied C–H bonds. This diagram focuses only on skeletal conventions and does not show other representation types. Source

Normal alkanes, aromatic compounds, and cyclic structures are often shown in skeletal form to reduce visual clutter and emphasise key structural features.

Conventions for COOH and COO Groups

OCR requires recognition of the shorthand conventions COOH (carboxylic acids) and COO (esters). These condensed forms avoid lengthy repeated structural detail.

• COOH denotes a carboxylic acid containing a carbonyl and hydroxyl directly bonded to the same carbon.

• COO is commonly used for an ester linkage, representing the carbonyl–oxygen–alkyl arrangement.

These conventions appear frequently in equations, mechanisms, and structural diagrams.

Representing Rings: Cyclohexane and Benzene

Students must recognise common ring symbols, particularly cyclohexane and benzene, which are often drawn in shorthand without explicit hydrogens.

• Cyclohexane is typically shown as a hexagon with alternating single bonds implied.

• Benzene is represented either with a hexagon containing a circle to indicate a delocalised π-system or using alternating double bonds.

A benzene ring is usually drawn as a hexagon with a circle inside or with alternating double bonds, representing aromatic π-delocalisation.

Displayed structural formula of benzene, showing a six‑membered carbon ring with alternating C=C double bonds and one hydrogen attached to each carbon. This reinforces how aromatic rings can be represented explicitly, not just as a simple hexagon symbol. The image includes more detail than needed for skeletal shorthand but remains fully aligned with recognising benzene ring structures. Source

These conventions allow rapid identification of alicyclic and aromatic features.

Choosing the Appropriate Representation

The OCR specification emphasises selecting the appropriate representation depending on the task. Students should understand the advantages and limitations of each format.

When to Use Each Type

• General formulae:

  • Identifying trends in a homologous series

  • Predicting molecular composition

• Structural formulae:

  • Showing atom connectivity clearly

  • Distinguishing between structural isomers

• Displayed formulae:

  • Demonstrating electron-pair movements in mechanisms

  • Visualising bonding and geometry

• Skeletal formulae:

  • Presenting large or complex molecules concisely

  • Emphasising functional groups

Bullet-Point Summary of Key Skills Required by OCR

Students should be able to:

• Interpret general, structural, displayed, and skeletal formulae in unfamiliar contexts.

• Convert between different formula types accurately.

• Recognise and reproduce COOH and COO conventions.

• Identify cyclic structures such as cyclohexane and benzene in symbolic shorthand.

• Use skeletal formulae to emphasise carbon frameworks and functional groups.

• Distinguish clearly between hydrogens attached to carbon and those attached to heteroatoms.

• Apply systematic representation choices to reduce ambiguity in organic structures.

Additional Important Terminology

Introducing key terminology early supports fluency when interpreting organic representations.

A brief understanding of functional groups is essential to interpreting formulae correctly, as their shapes and arrangements guide reactivity patterns.

Organic formula representations form a foundational language for the remainder of A-Level organic chemistry. Recognising, interpreting, and selecting representations accurately ensures that mechanisms, naming, and structural reasoning remain clear and consistent across the specification.