Liquid water, much more than a solvent: an accelerator of chemical reactions

Water: an essential mineral but not a nutritional source

Water, an inorganic mineral

From a physical and chemical standpoint, water is classified as a mineral, that is, an inorganic, simple substance composed of \(\mathrm{H_2O}\) molecules. Unlike organic nutrients (carbohydrates, lipids, proteins), water provides neither chemical energy nor structural elements directly usable by cells.

An essential vector for life

Its main function is therefore not to feed cells, but to constitute the intracellular and extracellular medium indispensable to life. It plays the role of a universal solvent, facilitating nutrient transport, waste elimination, thermal regulation, and the mediation of biochemical reactions.

Water does, however, supply dissolved ions (minerals, electrolytes), which themselves participate in physiological functions, but these come from the dissolution of other minerals, not from water itself. In summary, water is an essential vector for life, but does not constitute a direct nutritional intake.

Water and biological reactions: a catalytic interface

The role of water in enzymatic catalysis

In biological systems, water plays a fundamental role in enzymatic catalysis. Enzyme active sites are often partially hydrated: water is confined, oriented, and functionalized there. It enables charge transfers, stabilizes excited states, and sometimes participates as a reactant or cofactor.

Water as an actor in energy metabolism

In the photolysis of water in photosynthesis, or in cellular respiration (electron transport chain), water is far more than a backdrop: it is a main actor in energy metabolism.

Water: the molecular catalyst par excellence

Water is much more than a simple universal solvent. It plays an active role in the chemistry of living systems and aqueous media. Thanks to its polarity, its perpetually rearranging hydrogen bond network (10¹¹ reorganizations per second), and its ability to stabilize transition states, liquid water acts as a true accelerator of chemical reactions.

Water is not a simple passive solvent

Unlike many organic solvents (such as hexane or toluene), water interacts directly with reactants and modifies the course of chemical reactions.

Mechanisms of water's intervention

Water can influence reactions in three main ways:

• Proton transfer (acid-base reactions): Water (H₂O) can act as an acid (by donating a proton H⁺) or as a base (by accepting a proton), facilitating reactions such as hydrolysis.
• Stabilization of ionic intermediates: The partial charges of water (Oδ⁻, Hδ⁺) stabilize charged species (ions, polar transition states), which accelerates certain reactions.
• Modification of the energy landscape: Water changes the energy barriers of transition states (via electrostatic interactions or hydrogen bonds), which can make certain reaction pathways more favorable.

Consequences for the reaction

Water is not just a "spectator": it is an active participant that shapes chemical reactions at several levels.

• Kinetics: reaction rate (e.g., faster hydrolysis in aqueous media).
• Selectivity: orientation toward one product rather than another (e.g., preferential formation of one isomer).
• Thermodynamics: relative stability of products/reactants (e.g., solvation of ions modifies free enthalpy).

In summary: Unlike inert solvents, water can participate directly in reaction mechanisms: through proton transfer (acid-base reactions), through stabilization of ionic intermediates, or by modifying the energy landscape of transition states. It influences not only kinetics but also the selectivity and thermodynamics of reactions.

A structured and dynamic medium

A hydrogen bond network in perpetual rearrangement

At the microscopic scale, liquid water forms a three-dimensional network of hydrogen bonds, constantly reorganizing. This intermediate structure between crystal order and gaseous disorder creates reaction microenvironments that facilitate effective collisions between reactants.

Examples of reactions in aqueous media

Example 1: Self-ionization of water
The self-ionization of water relies on the transfer of a proton between two molecules via a transient hydrogen bridge:

$$2 \, \mathrm{H_2O} \ \rightleftharpoons \ \mathrm{H_3O^+} + \mathrm{OH^-}$$

This transfer is facilitated by the Grotthuss mechanism, in which the proton propagates through the network without itself moving, thanks to the rapid reconfiguration of hydrogen bonds. This phenomenon exists only in a highly structured and mobile liquid medium such as water.

Example 2: Diels-Alder reaction in aqueous media
The Diels-Alder reaction between a diene and a dienophile is significantly accelerated in water compared to organic solvents. This rate gain is due to the hydrophobic effect: nonpolar reactants are brought together by water, which tends to exclude hydrophobic compounds from its network. This aggregation pressure increases the probability of effective collision and directs the reaction toward specific products, demonstrating the structuring effect of water on chemical reactivity.

Comparative table: impact of solvent on reaction rate

The following table compares the effect of different solvents on the rate of several chemical reactions, illustrating the active role of water.

Analysis of comparative data

• Source: Data from J. Phys. Chem. B (ACS), Angew. Chemie Int. Ed., and F. Franks, *Water: A Matrix of Life*.

An irreplaceable molecule? Reflection on a possible substitute for water

Unique physicochemical properties of water

Water possesses a unique set of physicochemical properties that make it indispensable as a reaction medium and support for life: its high polarity, its significant dipole moment (≈1.85 D), its exceptional ability to form a dense and dynamic hydrogen bond network, its melting and boiling points compatible with a wide range of terrestrial temperatures, as well as its high dielectric constant (≈78.5 at 25 °C) which facilitates ionic dissociation.

A rare balance between fluidity and structuration

These combined characteristics ensure a rare balance between fluidity and structuration, between effective solvation and molecular mobility, which allows water to act simultaneously as a universal solvent and active catalyst. Moreover, its low molar mass and low viscosity promote rapid transport and effective molecular interactions.

Limitations of alternative solvents

Other molecules, such as liquid ammonia (NH₃), methanol (CH₃OH), or formic acid (HCOOH), have been considered as alternative solvents under extreme conditions, but none combines all of water's critical properties. For example, liquid ammonia is less polar, has a lower dielectric constant, and a less developed hydrogen network. Moreover, these solvents have temperature or pressure ranges compatible with life that are much more restricted.

Water is therefore an irreplaceable molecule

In conclusion, although other liquids could in theory fulfill some of water's roles, no known molecule possesses the full set of physicochemical properties that make water such an exceptional medium for complex chemistry and biology. This is why, to date, water remains the irreplaceable molecule under terrestrial conditions.