What is protonation?

Protonation is an acid-base chemical reaction in which a Brønsted-Lowry acid donates a proton or H ⁺ ion to a Brønsted-Lowry base. In other words, it is the process by which an atom, ion or molecule captures an H ⁺ ion through a pair of free electrons that it has in its structure .

The proton corresponds to the only cation that can form the hydrogen atom. Since the hydrogen atom is made up of only a proton surrounded by an electron, when it loses that electron, all that remains is a naked proton, hence its name.

The protonation reaction is of great importance in chemistry. In addition to representing a type of acid-base reaction itself, there are many chemical reactions that occur in multiple stages in which one or more of these stages correspond to protonation processes of different species.

How does protonation occur?

This concept of protonation is compatible with Lewis acid-base theory, since the proton represents an electron-deficient species (thus acting as a Lewis acid), whereas the species to be protonated is a rich Lewis base. into electrons and that it has a lone pair of electrons that it can share.

A typical example of a protonation reaction is the reaction between sulfuric acid and water, presented below:

The figure below clearly shows how this reaction occurs. The process involves the attack of oxygen from water on one of the two hydrogens in sulfuric acid to become the hydronium ion.

The blue arrows show the movements of electrons from the Lewis base (water) to the Lewis acid. However, the protonation process as such, that is, the movement of the hydrogen ion, occurs in the opposite direction. While the water is the one that attacks the acid with its pair of electrons, acting as a base, we could say that the sulfuric acid protonates the water molecule, since the acid is the one that gives up the proton. In this case, the hydronium ion is called the protonated species.

Characteristics of protonation

Protonation is usually a reversible and remarkably fast reaction (ie it reaches equilibrium in a very short time). In addition, these reactions tend to convert the substrate into an electron-deficient species, which in turn makes them good electrophiles, capable of carrying out different types of reactions.

In this sense, many acid catalytic processes begin with the protonation of one of the reactants, after which the protonated species reacts more easily to become products. A typical example is the acid-catalyzed dehydration of alcohols to form alkenes; Its reaction mechanism is presented below:

Protonation vs. hydrogenation

Protonation is often confused with the concept of hydrogenation, however, despite the fact that in both cases an atom, molecule or ion forms new bonds with hydrogen, they are not the same. The main difference has to do with electrical charge.

During protonation, as the base receives a proton which has a positive charge of +1, the reaction implies that the base undergoes a change in its total electrical charge. In fact, its charge is increased by +1.

On the other hand, the hydrogenation reaction consists of a type of addition reaction in which a molecule of neutral hydrogen (H ₂ ) is added to another molecule. In general, the second molecule corresponds to an unsaturated organic compound that can use its pi bonds to bond with hydrogen. However, since a neutral molecule is being added to another species, then the hydrogenation reaction does not involve a change in overall charge.

To further complicate matters, the reaction mechanisms of some hydrogenation reactions involve at least one protonation step.

Examples of protonation reactions

Autoprotolysis of water

In this reaction, water is both the acidic species and the species that is protonated at the same time.

Protonation of nitric acid by sulfuric acid.

Although both substances are strong acids, sulfuric acid is stronger than nitric acid and is actually capable of protonating it, forcing it to act as a base.

Protonation of an alcohol

These protonation reactions are frequent in the chemical reactions of alcohols. In general, a fairly strong acid is required to protonate alcohols, since these are very weak bases.

Protonation of a hydroxide ion

We can view the neutralization reaction between a hydroxide ion and a proton as a protonation reaction of the hydroxide ion to form water. You can see the electric charge change from -1 in the case of hydroxide, to neutral in the case of water.

Protonation of a carbonyl compound:

Carbonyl compounds can be protonated with some ease thanks to the formation of resonance structures that stabilize the protonated species.

References

Ashenhurst, J. (2020, October 16). How Protonation and Deprotonation Affect Reactivity . Master Organic Chemistry. https://www.masterorganicchemistry.com/2012/05/30/acid-base-reactions-whats-the-point/

Carey, F. (2021). Organic Chemistry (9th ^ed .). MCGRAW HILL EDUCATION.

Chang, R. (2021). Chemistry (11th ^ed .). MCGRAW HILL EDUCATION.

What does PROTONATION mean? (nd). Definitions. https://www.definitions.net/definition/PROTONATION