Alkanes form a fundamental class of organic molecules whose behaviour is governed by the nature of the σ-bonds between carbon and hydrogen atoms. Understanding σ-bond formation and the resulting free rotation around C–C bonds provides an essential basis for explaining the structural features, conformational flexibility and relatively low reactivity of alkanes across the wider organic chemistry specification.

σ-Bond Formation in Alkanes

Alkanes are defined as saturated hydrocarbons, containing only single covalent bonds between carbon atoms. Each of these C–C and C–H bonds is a σ-bond, created by direct (end-to-end) overlap of orbitals.

The diagram illustrates σ-bonding in ethane, with sp³–sp³ overlap forming the C–C σ-bond and sp³–1s overlaps forming C–H σ-bonds. Additional labelled angles and distances extend slightly beyond this subsubtopic but remain consistent with later learning. Source

σ-bonds originate from overlapping orbitals such as sp³–sp³ (for C–C bonds) or sp³–1s (for C–H bonds). In alkanes, each carbon typically undergoes sp³ hybridisation, producing four equivalent hybrid orbitals arranged tetrahedrally to minimise electron-pair repulsion.

A key consequence of σ-bond formation is the resulting cylindrical symmetry of the electron density around the bond axis. This symmetry is responsible for the characteristic flexibility of alkanes.

Characteristics of σ-Bonds in Alkanes

Important features that arise from σ-bonding include:

• High bond enthalpy, giving alkanes notable thermal and chemical stability.

• Electron density concentrated directly between nuclei, creating strong, localised bonding interactions.

• Lack of π-bonding, meaning no regions of electron density above or below the bond plane.

• A rigid bond axis but a freely rotating bond orientation.

These properties contribute to the macroscopic inertness of alkanes, which depend heavily on the robustness of their σ-bonding framework.

Free Rotation Around C–C σ-Bonds

The OCR specification highlights that σ-bonds allow free rotation around the C–C bond. Because the electron density in a σ-bond is symmetrically distributed around the internuclear axis, rotation does not significantly disrupt orbital overlap.

The image shows staggered and eclipsed conformations of ethane viewed along the C–C σ-bond axis. Rotation about this axis interconverts conformations; energetic differences are beyond this subsubtopic but link to later material. Source

This free rotation is a defining feature of alkane conformations, permitting molecules to adopt numerous spatial arrangements. Although alkanes can rotate freely, some conformations are energetically more favourable than others due to repulsion between electron clouds of neighbouring bonds.

Consequences of Free Rotation

Free rotation affects both the physical and conceptual understanding of alkanes:

1. Conformational Diversity

Because rotation around C–C σ-bonds is unrestricted, alkanes possess an array of conformations — different spatial orientations of atoms caused solely by rotation.

Key points:

• Conformations exist on a continuous spectrum, not as discrete isomers.

• They interconvert rapidly under ambient conditions.

• Conformations cannot be isolated in simple alkanes because rotation barriers are low.

2. No Stereochemical Fixedness

Unlike alkenes, which contain π-bonds that restrict rotation, alkanes cannot show E/Z stereoisomerism. The structural fluidity of σ-bonded frameworks prevents stable spatial restrictions around single bonds.

3. Influence on Physical Properties

Conformational flexibility plays a role in:

• Intermolecular contact, and thus boiling points.

• Packing efficiency in the solid state.

• Shape and chain folding, particularly in longer alkanes.

Even though the specification does not require quantitative treatments of conformational energies, understanding how free rotation affects shape is essential to interpreting alkane properties more broadly.

Bonding Description at the Orbital Level

In alkanes, σ-bonds arise predominantly from sp³ hybrid orbitals. Each sp³-hybridised carbon forms four σ-bonds arranged approximately 109.5° apart.

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Visualising σ-Bonds and Rotation in Structural Representations

OCR expects students to understand how σ-bonds and free rotation are represented across different formula types. Recognition of these conventions is crucial for correctly interpreting exam questions involving structures and conformations.

Structural and Displayed Formulae

When drawing alkanes:

• Displayed formulae show each σ-bond explicitly.

• Structural formulae may condense long chains but still reflect single-bond connectivity.

• Skeletal formulae, typically used for more complex molecules, imply σ-bonds at line ends and vertices.

Rotation around C–C bonds is not usually depicted directly but is inferred from the single-bond notation.

Three-Dimensional Representations

Because σ-bonds derive from sp³ hybrid orbitals, each carbon in an alkane adopts a tethedral arrangement. Common 3D representations include:

• Wedge–dash diagrams, highlighting bonds coming out of or going behind the plane.

• Newman projections, useful for visualising rotation around C–C bonds.

• Sawhorse projections, offering an angled perspective of a bond axis.

These tools allow students to examine how rotation leads to different conformations and how the relative positions of substituents shift in space.

Summary of Key Points for OCR Study

Students must be confident with:

• The nature of σ-bonds in alkanes and their formation through direct orbital overlap.

• The consequence of cylindrical symmetry in σ-bonds leading to free rotation.

• The relationship between σ-bond properties, conformational flexibility and alkane shape.

• The difference between rotation-permitting σ-bonds and rotation-restricting π-bonds.

• Recognising σ-bonds and conformational implications in different structural formula conventions.

These principles form the essential bonding framework upon which more advanced organic reactivity and mechanistic topics are built within the A-Level Chemistry course.