Acid catalysis
Encyclopedia
In acid catalysis and base catalysis a chemical reaction
is catalyzed
by an acid
or a base
. The acid is often the proton
and the base is often a hydroxyl
ion. Typical reactions catalyzed by proton transfer are esterfications and aldol reaction
s. In these reactions the conjugate acid
of the carbonyl
group is a better electrophile
than the neutral carbonyl group itself. Catalysis by either acid or base can occur in two different ways: specific catalysis and general catalysis.
chemical reactions. There are many possible chemical compounds that can act as sources for the protons to be transferred in an acid catalysis system. A compound such as sulfuric acid
H2SO4 can be used. Usually this is done to create a more likely leaving group
, such as converting an OH group to a H2O+ group, which can then be eliminated as water (H2O).
With carbonyl compounds such as ester
s, synthesis and hydrolysis go through a tetrahedral transition state, where the central carbon has an oxygen, an alcohol group, and the original alkyl group. Strong acids protonate the carbonyl, which makes the oxygen positively charged, so that it can easily receive the double bond electrons when the alcohol attacks the carbonyl carbon. This enables ester synthesis and hydrolysis. The reaction is an equilibrium between the ester and its cleavage to carboxylic acid and alcohol. On the contrary, strong bases deprotonate the attacking alcohol or amine, which also promotes the reaction. However, bases also deprotonate the acid, which is irreversible. Therefore, in a strongly basic, aqueous environment, esters only hydrolyze.
is proportional to the concentration
of the protonated solvent molecules SH+. The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the chemical equilibrium
between solvent S and AH in favor of the SH+ species.
S + AH → SH+ + A-
For example in an aqueous buffer solution
the reaction rate for reactants R depends on the pH
of the system but not on the concentration
s of different acids.
This type of chemical kinetics
is observed when reactant R1 in a fast equilibrium with its conjugate acid R1H+ which proceeds to react slowly with R2 to the reaction product for example in the acid catalysed aldol reaction
.
acceleration. The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example diazonium coupling reactions.
When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
is catalyzed
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
by 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...
or 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...
. The acid is often the proton
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
and the base is often a hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
ion. Typical reactions catalyzed by proton transfer are esterfications and aldol reaction
Aldol reaction
The aldol reaction is a powerful means of forming carbon–carbon bonds in organic chemistry.Discovered independently by Charles-Adolphe Wurtz and Alexander Porfyrevich Borodin in 1872, the reaction combines two carbonyl compounds to form a new β-hydroxy carbonyl compound...
s. In these reactions the conjugate acid
Conjugate acid
Within the Brønsted–Lowry acid-base theory , a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. A conjugate acid can also be seen as the chemical substance that releases, or donates, a proton in the forward chemical...
of the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
group is a better electrophile
Electrophile
In general electrophiles are positively charged species that are attracted to an electron rich centre. In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile...
than the neutral carbonyl group itself. Catalysis by either acid or base can occur in two different ways: specific catalysis and general catalysis.
Use in synthesis
Acid catalysis is mainly used for organicOrganic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...
chemical reactions. There are many possible chemical compounds that can act as sources for the protons to be transferred in an acid catalysis system. A compound such as sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
H2SO4 can be used. Usually this is done to create a more likely leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
, such as converting an OH group to a H2O+ group, which can then be eliminated as water (H2O).
With carbonyl compounds such as ester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
s, synthesis and hydrolysis go through a tetrahedral transition state, where the central carbon has an oxygen, an alcohol group, and the original alkyl group. Strong acids protonate the carbonyl, which makes the oxygen positively charged, so that it can easily receive the double bond electrons when the alcohol attacks the carbonyl carbon. This enables ester synthesis and hydrolysis. The reaction is an equilibrium between the ester and its cleavage to carboxylic acid and alcohol. On the contrary, strong bases deprotonate the attacking alcohol or amine, which also promotes the reaction. However, bases also deprotonate the acid, which is irreversible. Therefore, in a strongly basic, aqueous environment, esters only hydrolyze.
Specific catalysis
In specific acid catalysis taking place in solvent S , the reaction rateReaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
is proportional to the concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of the protonated solvent molecules SH+. The acid catalyst itself (AH) only contributes to the rate acceleration by shifting the chemical equilibrium
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
between solvent S and AH in favor of the SH+ species.
S + AH → SH+ + A-
For example in an aqueous buffer solution
Buffer solution
A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. It has the property that the pH of the solution changes very little when a small amount of strong acid or base is added to it. Buffer solutions are used as a...
the reaction rate for reactants R depends on the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of the system but not on the concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
s of different acids.
This type of chemical kinetics
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
is observed when reactant R1 in a fast equilibrium with its conjugate acid R1H+ which proceeds to react slowly with R2 to the reaction product for example in the acid catalysed aldol reaction
Aldol reaction
The aldol reaction is a powerful means of forming carbon–carbon bonds in organic chemistry.Discovered independently by Charles-Adolphe Wurtz and Alexander Porfyrevich Borodin in 1872, the reaction combines two carbonyl compounds to form a new β-hydroxy carbonyl compound...
.
General catalysis
In general acid catalysis all species capable of donating protons contribute to reaction rateReaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
acceleration. The strongest acids are most effective. Reactions in which proton transfer is rate-determining exhibit general acid catalysis, for example diazonium coupling reactions.
When keeping the pH at a constant level but changing the buffer concentration a change in rate signals a general acid catalysis. A constant rate is evidence for a specific acid catalyst.