Brønsted-Lowry
Encyclopedia
In chemistry
, the Brønsted–Lowry theory is an acid-base theory, proposed independently by Johannes Nicolaus Brønsted
and Thomas Martin Lowry in 1923. In this system, Brønsted acids and Brønsted bases are defined, by which an acid
is a molecule or ion that is able to lose, or "donate," a hydrogen
cation (proton
, H+), and a base
is a species with the ability to gain, or "accept," a hydrogen cation (proton).
The conjugate base is the ion or molecule remaining after the acid has lost a proton, and the conjugate acid is the species created when the base accepts the proton. The reaction can proceed in either forward or backward direction; in each case, the acid donates a proton to the base.
Water is amphoteric and can act as an acid or as a base. In the reaction between acetic acid
, CH3CO2H, and water, H2O, water acts as a base.
The acetate
ion, CH3CO2-, is the conjugate base of acetic acid and the hydronium
ion, H3O+, is the conjugate acid of the base, water.
Water can also act as an acid, for instance when it reacts with ammonia. The equation given for this reaction is:
in which H2O donates a proton to NH3. The hydroxide
ion is the conjugate base of water acting as an acid and the ammonium
ion is the conjugate acid of the base, ammonia.
A strong acid, such as hydrochloric acid
, dissociates completely. A weak acid, such as acetic acid, may be partially dissociated; the acid dissociation constant
, pKa, is a quantitative measure of the strength of the acid.
A wide range of compounds can be classified in the Brønsted–Lowry framework: mineral acid
s and derivatives such as sulfonate
s, phosphonate
s, etc., carboxylic acid
s, amine
s, carbon acids, 1,3-diketone
s such as acetylacetone
, ethyl acetoacetate
, and Meldrum's acid
, and many more.
A Lewis base, defined as an electron-pair donor, can act as a Brønsted–Lowry base as the pair of electrons can be donated to a proton. This means that the Brønsted–Lowry concept is not limited to aqueous solutions. Any donor solvent S can act as a proton acceptor.
Typical donor solvents used in acid-base chemistry, such as dimethyl sulfoxide
or liquid ammonia
have an oxygen or nitrogen atom with a lone pair of electrons that can be used to form a bond with a proton.
s, defined as electron-pair acceptors, also act as Brønsted–Lowry acids. For example, the aluminium ion, Al3+ can accept electron pairs from water molecules, as in the reaction
The aqua ion formed is a weak Brønsted–Lowry acid.
The overall reaction is described as acid hydrolysis of the aluminium ion.
However not all Lewis acid
s generate Brønsted–Lowry acidity. The magnesium ion similarly reacts as a Lewis acid with six water molecules
but here very few protons are exchanged since the Brønsted–Lowry acidity of the aqua ion is negligible (Ka = 3.0 × 10-12).
Boric acid
also exemplifies the usefulness of the Brønsted–Lowry concept for an acid that does not dissociate but does effectively donate a proton to the base, water. The reaction is
Here boric acid acts as a Lewis acid and accepts an electron pair from the oxygen of one water molecule, which in turn donates a proton to a second water molecule and, therefore, acts as a Brønsted acid.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, the Brønsted–Lowry theory is an acid-base theory, proposed independently by Johannes Nicolaus Brønsted
Johannes Nicolaus Brønsted
Johannes Nicolaus Brønsted born in Varde was a Danish physical chemist.He received a degree in chemical engineering in 1899 and his Ph. D. in 1908 from the University of Copenhagen. He was immediately appointed professor of inorganic and physical chemistry at Copenhagen.In 1906 he published his...
and Thomas Martin Lowry in 1923. In this system, Brønsted acids and Brønsted bases are defined, by which an acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
is a molecule or ion that is able to lose, or "donate," a hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
cation (proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
, H+), and a base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
is a species with the ability to gain, or "accept," a hydrogen cation (proton).
Properties of acids and bases
It follows that, if a compound is to behave as an acid, donating a proton, there must be a base to accept the proton. So the Brønsted–Lowry concept can be defined by the reaction:- acid + base conjugate base + conjugate acid.
The conjugate base is the ion or molecule remaining after the acid has lost a proton, and the conjugate acid is the species created when the base accepts the proton. The reaction can proceed in either forward or backward direction; in each case, the acid donates a proton to the base.
Water is amphoteric and can act as an acid or as a base. In the reaction between acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
, CH3CO2H, and water, H2O, water acts as a base.
- CH3COOH + H2O CH3COO− + H3O+
The acetate
Acetate
An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
ion, CH3CO2-, is the conjugate base of acetic acid and the hydronium
Hydronium
In chemistry, a hydronium ion is the cation , a type of oxonium ion produced by protonation of water. This cation is often used to represent the nature of the proton in aqueous solution, where the proton is highly solvated...
ion, H3O+, is the conjugate acid of the base, water.
Water can also act as an acid, for instance when it reacts with ammonia. The equation given for this reaction is:
- H2O + NH3 OH− + NH4+
in which H2O donates a proton to NH3. 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 is the conjugate base of water acting as an acid and the ammonium
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
ion is the conjugate acid of the base, ammonia.
A strong acid, such as hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
, dissociates completely. A weak acid, such as acetic acid, may be partially dissociated; the acid dissociation constant
Acid dissociation constant
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
, pKa, is a quantitative measure of the strength of the acid.
A wide range of compounds can be classified in the Brønsted–Lowry framework: mineral acid
Mineral acid
A mineral acid is an acid derived from one or more inorganic compounds. A mineral acid is not organic and all mineral acids release hydrogen ions when dissolved in water.-Characteristics:...
s and derivatives such as sulfonate
Sulfonate
A sulfonate is a salt or ester of a sulfonic acid. It contains the functional group R-SO2O-.- Sulfonate salts:Anions with the general formula RSO2O− are called sulfonates. They are the conjugate bases of sulfonic acids with formula RSO2OH. As sulfonic acids tend to be strong acids, the...
s, phosphonate
Phosphonate
Phosphonates or phosphonic acids are organic compounds containing C-PO2 or C-PO2 groups . Bisphosphonates were first synthesized in 1897 by Von Baeyer and Hofmann. An example of such a bisphosphonate is HEDP . Since the work of Schwarzenbach in 1949, phosphonic acids are known as effective...
s, etc., carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s, amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s, carbon acids, 1,3-diketone
Diketone
A diketone is a molecule containing two ketone groups. The simpliest diketone is diacetyl, also known as 2,3-butanedione. Diacetyl, acetylacetone, and hexane-2,5-dione are examples of 1,2-, 1,3-, and 1,4-diketones, respectively...
s such as acetylacetone
Acetylacetone
Acetylacetone is an organic compound that famously exists in two tautomeric forms that rapidly interconvert. The less stable tautomer is a diketone formally named pentane-2,4-dione. The more common tautomer is the enol form. The pair of tautomers rapidly interconvert and are treated as a single...
, ethyl acetoacetate
Ethyl acetoacetate
The organic compound ethyl acetoacetate is the ethyl ester of acetoacetic acid. It is mainly used as a chemical intermediate in the production of a wide variety of compounds, such as amino acids, analgesics, antibiotics, antimalarial agents, antipyrine and aminopyrine, and vitamin B1; as well as...
, and Meldrum's acid
Meldrum's acid
Meldrum's acid or 2,2-dimethyl-1,3-dioxane-4,6-dione is an organic compound. The compound was first made in 1908 by Andrew Norman Meldrum by a condensation reaction of malonic acid with acetone in acetic anhydride and sulfuric acid. Meldrum misidentified the structure as a β-lactone of...
, and many more.
A Lewis base, defined as an electron-pair donor, can act as a Brønsted–Lowry base as the pair of electrons can be donated to a proton. This means that the Brønsted–Lowry concept is not limited to aqueous solutions. Any donor solvent S can act as a proton acceptor.
- AH + S: A− + SH+
Typical donor solvents used in acid-base chemistry, such as 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...
or liquid ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
have an oxygen or nitrogen atom with a lone pair of electrons that can be used to form a bond with a proton.
Brønsted acidity of some Lewis acids
Some Lewis acidLewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
s, defined as electron-pair acceptors, also act as Brønsted–Lowry acids. For example, the aluminium ion, Al3+ can accept electron pairs from water molecules, as in the reaction
- Al3+ + 6H2O → Al(H2O)63+
The aqua ion formed is a weak Brønsted–Lowry acid.
- Al(H2O)63+ + H2O Al(H2O)5OH2+ + H3O+ .....KaAcid dissociation constantAn acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
= 1.2 × 10−5
The overall reaction is described as acid hydrolysis of the aluminium ion.
However not all Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
s generate Brønsted–Lowry acidity. The magnesium ion similarly reacts as a Lewis acid with six water molecules
- Mg2+ + 6H2O → Mg(H2O)62+
but here very few protons are exchanged since the Brønsted–Lowry acidity of the aqua ion is negligible (Ka = 3.0 × 10-12).
Boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
also exemplifies the usefulness of the Brønsted–Lowry concept for an acid that does not dissociate but does effectively donate a proton to the base, water. The reaction is
- B(OH)3 + 2H2O B(OH)4− + H3O+
Here boric acid acts as a Lewis acid and accepts an electron pair from the oxygen of one water molecule, which in turn donates a proton to a second water molecule and, therefore, acts as a Brønsted acid.