Electrochemistry – Lecture 4: Mechanisms of Redox Reactions

A modular guide to how oxidation–reduction processes occur at the molecular level

Electrochemistry describes the transfer of electrons between substances, but to truly understand why and how redox reactions occur, we must look deeper into reaction mechanisms. This lecture explores the molecular pathways through which redox reactions proceed from simple electron transfer to bond formation, bond cleavage, and rearrangement processes.

By examining representative mechanisms, reaction coordinates, and energy barriers, we can appreciate how thermodynamics, kinetics, and molecular structure determine whether a redox process is spontaneous, slow, or catalytic.

This lecture forms a bridge between theoretical electrochemistry and real-world chemistry, explaining why metals corrode, why batteries generate current, and how biological systems move electrons through enzymes and cofactors.

1. Understanding Redox Mechanisms

A mechanism in chemistry describes the sequence of elementary steps by which reactants transform into products.

In redox chemistry, these steps involve the transfer of electrons, often accompanied by changes in bonding, oxidation state, or geometry.

The general redox concept can be summarised as:

At the heart of this lies electron flow from the reducing agent (which donates electrons) to the oxidising agent (which accepts them).

Each elementary step must satisfy both mass and charge balance, ensuring the conservation of matter and energy.

Reference: LibreTexts – Oxidation–Reduction Basics

2. Classification of Redox Mechanisms

Redox processes can proceed through different pathways depending on how electrons are exchanged and how bonds change. The main mechanistic categories include:

Direct electron transfer (outer-sphere mechanism)
Bond formation or cleavage (inner-sphere mechanism)
Atom-transfer reactions
Comproportionation and disproportionation
Rearrangement and coupled proton–electron transfer (CPET)

Each mechanism represents a different mode of electron communication between species.

3. Outer-Sphere Electron Transfer

Definition

In an outer-sphere mechanism, electron transfer occurs without the formation of a direct chemical bond between the oxidant and reductant. The two species retain their coordination spheres throughout the process.

Here, the electron moves “through space” or “through solvent” between complexes.

Features

No ligand exchange or bond rearrangement.
Electron transfer depends on reorganisation energy and electronic coupling.
Favoured when coordination shells are rigid and not easily altered.

Marcus Theory

The rate of outer-sphere electron transfer is described by the Marcus theory, developed by Rudolph A. Marcus (Nobel Prize, 1992).

Where:

k_ET = rate constant of electron transfer
λ = reorganisation energy (energy required to rearrange solvent and ligands)
ΔG° = free energy change of the reaction

Marcus theory reveals that fast electron transfer occurs when the system’s reorganisation energy closely matches the driving force (|ΔG°|).

Reference: Nature Chemistry – Electron Transfer and Marcus Theory

4. Inner-Sphere Electron Transfer

Definition

An inner-sphere mechanism involves the formation of a temporary chemical bridge between the redox partners, through which the electron is transferred.

Example reaction:

[Co(NH₃)₅Cl]²⁺ + [Cr(H₂O)₆]²⁺ → [Co(NH₃)₅(H₂O)]²⁺ + [Cr(H₂O)₅Cl]²⁺

Here, chloride (Cl⁻) acts as a bridging ligand, temporarily linking the two metal centres.

Features

Involves ligand exchange.
Electron transfer occurs via the bridging atom or group.
Favoured when one or both reactants have labile (easily exchanged) ligands.
Mechanism is often confirmed by isotopic labelling or ligand tracking.

Mechanistic Steps

Formation of a bridged intermediate.
Electron transfer through the bridge.
Bridge cleavage and product formation.

This mechanism is typical for transition-metal complexes where ligands can act as electron conduits.

Reference: [Inorganic Chemistry (Miessler & Tarr) – Inner-Sphere Reactions]

5. Atom-Transfer Reactions

A specific subclass of redox reactions involves atom transfer, where an atom (often halogen or oxygen) moves from one species to another, accompanied by electron flow.

Example:

Mechanistically, one bromine atom is reduced while the other is oxidised, mediated by partial bond formation during transfer.

These reactions form the basis of many radical halogenation and oxidation processes in organic chemistry.

6. Comproportionation and Disproportionation

Comproportionation

Two species with different oxidation states of the same element react to form an intermediate oxidation state.

Disproportionation

A single species in an intermediate oxidation state splits into two products, one oxidised and one reduced.

These reactions illustrate how redox balance can occur within a single element, driven by thermodynamic stability and potential differences.

Reference: LibreTexts – Disproportionation and Comproportionation

7. Proton-Coupled Electron Transfer (PCET)

In biological and catalytic systems, electron transfer often occurs in tandem with proton transfer, maintaining charge balance and enabling energy-efficient transformations.

Example:

PCET mechanisms underlie processes such as:

Photosynthesis (water oxidation to O₂)
Cellular respiration (proton gradients in mitochondria)
Hydrogen evolution and CO₂ reduction catalysis

Coupling proton and electron motion reduces energetic barriers and allows reactions to proceed under mild conditions.

Reference: Chemical Reviews – Proton-Coupled Electron Transfer

8. Reaction Coordinate and Energy Profile

Redox reactions, like all chemical processes, must overcome an activation energy barrier to proceed.

A simplified energy diagram illustrates the relationship between free energy and reaction progress:

ΔG°: standard Gibbs free energy change (thermodynamic driving force).
ΔG‡: activation energy (kinetic barrier).

The catalysts and electrode surface lower ΔG‡ by stabilising transition states or intermediates, thereby increasing reaction rate without altering thermodynamic outcome.

9. Electron-Transfer in Biological Systems

Nature has perfected redox control through enzyme-mediated electron transfer.

Examples include:

Cytochromes: iron-containing proteins that shuttle electrons in respiration.
Photosystem II: oxidises water to oxygen in photosynthesis.
Iron–sulphur clusters: conduct electrons through multiple redox centres.

These systems use tunnelling pathways and cofactors (e.g. NADH, flavins, quinones) to control redox potentials with extraordinary precision.

Electrons released here are used to drive ATP synthesis, demonstrating that redox reactions sustain life itself.

Reference: Nature Reviews Molecular Cell Biology – Biological Electron Transfer

10. Redox Reactions in Organic Chemistry

Organic redox mechanisms often involve radical or ionic intermediates.

Common patterns:

Hydride transfer (e.g. NADH → NAD⁺ + H⁻)
Oxygenation and dehydrogenation
Single-electron transfer (SET) pathways in radical chemistry

Example:

This reaction produces the superoxide radical, an important species in biological oxidative stress and catalysis.

Reference: Chemistry World – Organic Redox Reactions

11. Solid-State and Electrochemical Redox

In electrochemical cells, redox reactions occur at interfaces where electrons pass between a solid electrode and a liquid electrolyte.

At these boundaries, electron transfer can proceed via:

Adsorbed intermediates (inner-sphere)
Tunnelling through a double layer (outer-sphere)
Sequential charge transfer with ions in solution

Understanding these mechanisms helps design better batteries, fuel cells, and electrocatalysts.

Example: Oxygen reduction reaction (ORR) in fuel cells:

Catalyst materials (Pt, Co–N–C, Fe–N–C) determine the reaction pathway, whether 4e⁻ (complete reduction to H₂O) or 2e⁻ (forming H₂O₂) dominates.

Reference: Nature Energy – Mechanisms of the Oxygen Reduction Reaction

12. Redox Rearrangements and Complex Pathways

Sometimes oxidation and reduction are coupled to rearrangements, leading to structural reorganisation within molecules.

Examples include:

Cannizzaro reaction (disproportionation of aldehydes)
Reductive elimination and oxidative addition in organometallic chemistry

Oxidative Addition

In oxidative addition, a metal in a lower oxidation state inserts into a covalent bond, increasing its oxidation state.

Explanation:

Platinum starts in the 0 oxidation state (Pt⁰).
It reacts with chlorine (Cl₂), breaking the Cl–Cl bond.
Platinum is oxidised to +2 (Pt²⁺) and forms a dichloride complex.
The number of ligands around platinum increases, reflecting coordination number expansion.

Reductive Elimination

Reductive elimination is the reverse process, where a metal in a higher oxidation state eliminates a ligand pair, reducing its oxidation state:

Explanation:

The Pt(II) complex loses a molecule of chlorine.
The metal is reduced back to Pt^0, restoring the original oxidation state.
Reductive elimination is crucial in catalytic cycles, regenerating the active metal species.

Key Points

Oxidative addition increases oxidation state and coordination number.
Reductive elimination decreases oxidation state and coordination number.
Together, they enable catalytic turnover in reactions such as cross-coupling or C–Cl activation.

These steps lie at the heart of catalytic cycles in cross-coupling and C–H activation.

Reference: Royal Society of Chemistry – Organometallic Catalysis

13. The Role of Solvent and Environment

Solvents play a critical role in redox mechanisms by stabilising ions and intermediates through solvation.

Polar protic solvents (e.g. water, ethanol) stabilise ions and favour stepwise electron transfer.
Nonpolar or aprotic solvents can promote radical mechanisms.

The dielectric constant and viscosity of the medium affect reorganisation energy and reaction rate, as predicted by the Marcus theory.

Additionally, ionic strength and pH modulate redox potentials via the Nernst equation:

Thus, understanding solvent effects is crucial for accurate electrochemical measurements.

14. Identifying Mechanisms Experimentally

Chemists use several tools to elucidate redox mechanisms:

Technique	Information Provided
Cyclic voltammetry (CV)	Reversibility and electron count
Spectroelectrochemistry	Intermediate oxidation states
EPR (Electron Paramagnetic Resonance)	Detection of radicals
Kinetic isotope effects	Role of proton transfer
Computational chemistry (DFT)	Energy surfaces and transition states

For example, a reversible redox couple shows mirror-image peaks in a cyclic voltammogram, indicating an outer-sphere mechanism with fast electron exchange.

Reference: Pine Research – Cyclic Voltammetry Explained

15. Environmental and Technological Importance

Redox mechanisms underpin global processes:

Corrosion: spontaneous oxidation of metals (Lecture 14)
Energy conversion: fuel cells, solar-to-chemical conversion
Waste treatment: redox removal of pollutants
Synthetic chemistry: selective oxidation and reduction
Biological metabolism: enzymatic redox cascades

By understanding the underlying mechanisms, we can design safer materials, efficient catalysts, and sustainable energy systems.

16. Summary and Learning Outcomes

By the end of this lecture, you should be able to:

Define redox mechanisms and distinguish between outer-sphere and inner-sphere pathways.
Describe Marcus theory and explain factors affecting electron-transfer rates.
Recognise examples of atom-transfer, comproportionation, and disproportionation reactions.
Explain how proton-coupled electron transfer (PCET) operates in chemical and biological systems.
Understand solvent effects, activation barriers, and energy diagrams.
Apply this mechanistic knowledge to electrochemical and catalytic systems.

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