Proton affinity
Encyclopedia
The proton affinity, Epa, of an anion or of a neutral atom
or molecule
is a measure of its gas-phase basicity. It is the energy released in the following reactions:
These reactions are always exergonic
in the gas phase, i.e. energy is released when the reaction advances in the direction shown. However, proton affinities are conventionally quoted with the opposite sign convention
from most other thermodynamic properties, a positive Epa indicating an intake of energy by the system. This is the same sign convention as is used for electron affinity
.
ion (Epa = 1675 kJ/mol), making methane
the weakest proton acid in the gas phase, followed by dihydrogen. The weakest known base is the helium
atom (Epa = 177.8 kJ/mol), making the hydrohelium(1+) ion
the strongest known proton acid.
in aqueous-phase Brønsted acidity. Hydrofluoric acid
is a weak acid in aqueous solution (pKa = 3.15) but a very weak acid in the gas phase (Epa (F−) = 1554 kJ/mol): the fluoride
ion is as strong a base as SiH3− in the gas phase, but its basicity is reduced in aqueous solution because it is strongly hydrated, and therefore stabilized. The contrast is even more marked for the hydroxide
ion (Epa = 1635 kJ/mol), one of the strongest known proton acceptors in the gas phase. Suspensions of potassium hydroxide
in dimethyl sulfoxide
(which does not solvate the hydroxide ion as strongly as water) are markedly more basic than aqueous solutions, and are capable of deprotonating such weak acids as triphenylmethane
(pKa = ca. 30).
To a first approximation, the proton affinity of a base in the gas phase can be seen as offsetting (usually only partially) the extremely favorable hydration energy of the gaseous proton (ΔE = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity:
These estimates suffer from the fact the free energy change of dissociation is in effect the small difference of two large numbers. However, hydrofluoric acid is correctly predicted to be a weak acid in aqueous solution and the estimated value for the pKa of dihydrogen is in agreement with the behaviour of saline hydrides (e.g., sodium hydride
) when used in organic synthesis
.
A second difference arises in noting that pKa reflects a thermal free energy for the proton transfer process, in which both enthalpic and entropic terms are considered together. Therefore, pKa is influenced both by the stability of the molecular anion, as well as the entropy associated of forming and mixing new species. Proton affinity, on the other hand, is not a measure of free energy.
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
or molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
is a measure of its gas-phase basicity. It is the energy released in the following reactions:
-
- A− + H+ → HA
- B + H+ → BH+
These reactions are always exergonic
Exergonic
Exergonic means "releasing energy in the form of work". By thermodynamic standards, work, a form of energy, is defined as moving from the system to the surroundings...
in the gas phase, i.e. energy is released when the reaction advances in the direction shown. However, proton affinities are conventionally quoted with the opposite sign convention
Sign convention
In physics, a sign convention is a choice of the physical significance of signs for a set of quantities, in a case where the choice of sign is arbitrary. "Arbitrary" here means that the same physical system can be correctly described using different choices for the signs, as long as one set of...
from most other thermodynamic properties, a positive Epa indicating an intake of energy by the system. This is the same sign convention as is used for electron affinity
Electron affinity
The Electron affinity of an atom or molecule is defined as the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion....
.
Acid/base chemistry
The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase. The strongest known base is the methanide anion (Epa = 1743 kJ/mol), slightly stronger than the hydrideHydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
ion (Epa = 1675 kJ/mol), making methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
the weakest proton acid in the gas phase, followed by dihydrogen. The weakest known base is the helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
atom (Epa = 177.8 kJ/mol), making the hydrohelium(1+) ion
Hydrohelium(1+) ion
The hydrohelium cation, HeH+, is a positively charged ion formed by the reaction of a proton with a helium atom in the gas phase, first observed in 1925. It is the strongest known acid, with a proton affinity of 177.8 kJ/mol. This ion is also called helium-hydride molecular ion. It has been...
the strongest known proton acid.
Hydration
Proton affinities illustrate the role of hydrationHydration reaction
In organic chemistry, a hydration reaction is a chemical reaction in which a hydroxyl group and a hydrogen cation are added to the two carbon atoms bonded together in the carbon-carbon double bond which makes up an alkene functional group. The reaction usually runs in a strong acidic, aqueous...
in aqueous-phase Brønsted acidity. Hydrofluoric acid
Hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
is a weak acid in aqueous solution (pKa = 3.15) but a very weak acid in the gas phase (Epa (F−) = 1554 kJ/mol): the fluoride
Fluoride
Fluoride is the anion F−, the reduced form of fluorine when as an ion and when bonded to another element. Both organofluorine compounds and inorganic fluorine containing compounds are called fluorides. Fluoride, like other halides, is a monovalent ion . Its compounds often have properties that are...
ion is as strong a base as SiH3− in the gas phase, but its basicity is reduced in aqueous solution because it is strongly hydrated, and therefore stabilized. The contrast is even more marked for the hydroxide
Hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
ion (Epa = 1635 kJ/mol), one of the strongest known proton acceptors in the gas phase. Suspensions of potassium hydroxide
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...
in dimethyl sulfoxide
Dimethyl sulfoxide
Dimethyl sulfoxide is an organosulfur compound with the formula 2SO. This colorless liquid is an important polar aprotic solvent that dissolves both polar and nonpolar compounds and is miscible in a wide range of organic solvents as well as water...
(which does not solvate the hydroxide ion as strongly as water) are markedly more basic than aqueous solutions, and are capable of deprotonating such weak acids as triphenylmethane
Triphenylmethane
Triphenylmethane, or triphenyl methane, is the hydrocarbon with the formula 3CH. This colorless solid is soluble in nonpolar organic solvents and not in water. Triphenylmethane has the basic skeleton of many synthetic dyes called triarylmethane dyes, many of them are pH indicators, and some display...
(pKa = ca. 30).
To a first approximation, the proton affinity of a base in the gas phase can be seen as offsetting (usually only partially) the extremely favorable hydration energy of the gaseous proton (ΔE = −1530 kJ/mol), as can be seen in the following estimates of aqueous acidity:
| Proton affinity | HHe+(g) | → | H+(g) | + He(g) | +178 kJ/mol | HF(g) | → | H+(g) | + F−(g) | +1554 kJ/mol | H2(g) | → | H+(g) | + H−(g) | +1675 kJ/mol | |||||
| Hydration of acid | HHe+(aq) | → | HHe+(g) | +973 kJ/mol | HF(aq) | → | HF(g) | +23 kJ/mol | H2(aq) | → | H2(g) | −18 kJ/mol | ||||||||
| Hydration of proton | H+(g) | → | H+(aq) | −1530 kJ/mol | H+(g) | → | H+(aq) | −1530 kJ/mol | H+(g) | → | H+(aq) | −1530 kJ/mol | ||||||||
| Hydration of base | He(g) | → | He(aq) | +19 kJ/mol | F−(g) | → | F−(aq) | −13 kJ/mol | H−(g) | → | H−(aq) | +79 kJ/mol | ||||||||
| Dissociation equilibrium | HHe+(aq) | → | H+(aq) | + He(aq) | −360 kJ/mol | HF(aq) | → | H+(aq) | + F−(aq) | +34 kJ/mol | H2(aq) | → | H+(aq) | + H−(aq) | +206 kJ/mol | |||||
| Estimated pKa | −63 | +6 | +36 | |||||||||||||||||
These estimates suffer from the fact the free energy change of dissociation is in effect the small difference of two large numbers. However, hydrofluoric acid is correctly predicted to be a weak acid in aqueous solution and the estimated value for the pKa of dihydrogen is in agreement with the behaviour of saline hydrides (e.g., sodium hydride
Sodium hydride
Sodium hydride is the chemical compound with the empirical formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as...
) when used in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
.
Difference from pKa
Both proton affinity and pKa are measures of the acidity of a molecule, and so both reflect the thermodynamic gradient between a molecule and the anionic form of that molecule upon removal of a proton from it. Implicit in the definition of pKa however is that the acceptor of this proton is water, and an equilibrium is being established between the molecule and bulk solution. More broadly, pKa can be defined with reference to any solvent, and many weak organic acids have measured pKa values in DMSO. Large discrepancies between pKa values in water versus DMSO (i.e., the pKa of water in water is 15.7, but water in DMSO is 32) demonstrate that the solvent is an active partner in the proton equilibrium process, and so pKa does not represent an intrinsic property of the molecule in isolation. In contrast, proton affinity is an intrinsic property of the molecule, without explicit reference to the solvent.A second difference arises in noting that pKa reflects a thermal free energy for the proton transfer process, in which both enthalpic and entropic terms are considered together. Therefore, pKa is influenced both by the stability of the molecular anion, as well as the entropy associated of forming and mixing new species. Proton affinity, on the other hand, is not a measure of free energy.