Protonation
Adding a proton to form a conjugate acid.
Protonation, also called hydronation, is a chemical process where a proton—a hydrogen cation, often written as H⁺—is added to an atom, molecule, or ion. This reaction produces a conjugate acid. The reverse process, in which a proton is removed from a Brønsted–Lowry acid, is known as deprotonation. Common examples include the reaction of sulfuric acid with water to form hydronium and bisulfate ions, the addition of a proton to isobutene to create a carbocation using tetrafluoroboric acid, and the combination of ammonia gas with hydrogen chloride gas to yield solid ammonium chloride.
This type of reaction is a fundamental step in many stoichiometric and catalytic processes. Some substances, particularly many biological macromolecules, can accept more than one proton and are described as polybasic. Protonation and deprotonation are central to most acid–base reaction theories; a Brønsted–Lowry acid is specifically defined as a substance that protonates another species. When a substrate is protonated, both its mass and its charge increase by one unit, a property that is crucial in analytical methods like electrospray mass spectrometry. Adding or removing a proton can also alter other chemical properties, including solubility, hydrophilicity, reduction or oxidation potential, and optical characteristics.
Protonation reactions are often rapid, largely because protons move quickly through many solvents. The speed of protonation depends on the acidity of the proton donor: weak acids protonate a given base more slowly than strong acids do. The rates of both protonation and deprotonation can become especially slow when the process involves significant structural changes. Enantioselective protonations, which are controlled by reaction kinetics, are important in organic synthesis and also occur in various biological systems.
Protonation is usually reversible, and the structure and bonding of the conjugate base typically remain unchanged. However, in some cases, protonation can trigger isomerization—for instance, cis-alkenes can be converted to trans-alkenes using a catalytic amount of a protonating agent. Many enzymes, such as serine hydrolases, function through mechanisms that rely on reversible protonation of their substrates.
- field
- Chemistry
- known_for
- Adding a proton to form a conjugate acid; central to Brønsted–Lowry acid–base theory
- related_concepts
- Deprotonation, conjugate acid, polybasic species
Lore & Background
Protonation, also known as hydronation, is the chemical addition of a proton (a hydrogen cation, H⁺) to an atom, molecule, or ion, resulting in the formation of a conjugate acid. It is the inverse of deprotonation, where a proton is removed from a Brønsted–Lowry acid. This process is a fundamental step in many stoichiometric and catalytic reactions. Substances that can accept more than one proton are termed polybasic, a property common among many biological macromolecules. Upon protonation, both the mass and the positive charge of the species increase by one unit, a characteristic exploited in analytical techniques such as electrospray mass spectrometry. Beyond altering charge and mass, protonation can modify other chemical properties, including solubility, hydrophilicity, reduction or oxidation potential, and optical behavior. The rate of protonation is often rapid due to the high mobility of protons in many solvents, though it is slower when the protonating agent is a weak acid. Rates can also be notably slow if the process induces significant structural changes. Enantioselective protonations, which are under kinetic control, are important in organic synthesis and certain biological processes. While protonation is typically reversible without altering the conjugate base's structure, it can sometimes induce isomerization, such as converting cis-alkenes to trans-alkenes with a catalytic amount of a protonating agent. Many enzymes, including serine hydrolases, operate through mechanisms that involve reversible protonation of their substrates.
Reader's Guide
Protonation is a fundamental chemical reaction and a step in many stoichiometric and catalytic processes. It is the core of most acid–base reaction theories, with a Brønsted–Lowry acid defined as a substance that protonates another. Upon protonation, the mass and charge of the species each increase by one unit, making it essential in analytical procedures such as electrospray mass spectrometry. Protonation or deprotonation can change many chemical properties, including solubility, hydrophilicity, reduction or oxidation potential, and optical properties. Rates of protonation are often rapid due to the high mobility of protons in many solvents, but can be slow when significant structural changes occur. Enantioselective protonations are under kinetic control and are relevant to organic synthesis and biological processes. Protonation is usually reversible, though in some cases it induces isomerization, such as converting cis-alkenes to trans-alkenes with a catalytic amount of protonating agent. Many enzymes, like serine hydrolases, operate via mechanisms involving reversible protonation of substrates.
Did You Know?
- Protonation is also called hydronation.
- The rate of protonation by weak acids is slower than by strong acids.
- Protonation can change solubility, hydrophilicity, and optical properties of a molecule.
- Enantioselective protonations are under kinetic control and are relevant to biological processes.
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