Reaction typesOxidation and reduction
Redox reactions can be understood in terms of transfer of electrons from one involved species (reducing agent) to another (oxidizing agent). In this process, the former species is oxidized and the latter is reduced, thus the term redox.
Though sufficient for many purposes, these descriptions are not
precisely correct. Oxidation is better defined as an increase in oxidation number,
and reduction as a decrease in oxidation number. In practice, the
transfer of electrons will always change the oxidation number, but
there are many reactions that are classed as "redox" even though no
electron transfer occurs (such as those involving covalent bonds).
An example of a redox reaction is:
- 2 S2O32−(aq) + I2(aq) → S4O62–(aq) + 2 I−(aq)
Here I2 is reduced to I– and S2O32– (thiosulfate anion) is oxidized to S4O62–.
Which of the involved reactants would be reducing or oxidizing agent can be predicted from the electronegativity of their elements. Elements with low electronegativity, such as most metals,
easily donate electrons and oxidize – they are reducing agents. On the
contrary, many ions with high oxidation numbers, such as H2O2, MnO−
4, CrO3, Cr2O2−
7, OsO4) can gain one or two extra electrons and are strong oxidizing agents.
The number of electrons donated or accepted in a redox reaction can be predicted from electron configuration of the reactant element. Elements are trying to reach the low-energy noble gas
configuration, and therefore alkali metals and halogens will donate and
accept one electron, respectively, and the noble gases themselves are
An important class of redox reactions are the electrochemical
reactions, where the electrons from the power supply are used as a
reducing agent. These reactions are particularly important for the
production of chemical elements, such as chlorine or aluminium.
The reverse process in which electrons are released in redox reactions
and can be used as electrical energy is possible and is used in the
ComplexationIn complexation reactions, several ligands react with a metal atom to form a coordination complex. This is achieved by providing lone pairs of the ligand into empty orbitals of the metal atom and forming dipolar bonds. The ligands are Lewis bases,
they can be both ions and neutral molecules, such as carbon monoxide,
ammonia or water. The number of ligands that react with a central metal
atom can be found using the 18-electron rule, saying that the valence shells of a transition metal will collectively accommodate 18 electrons, whereas the symmetry of the resulting complex can be predicted with the crystal field theory and ligand field theory. Complexation reactions also include ligand exchange,
in which one or more ligands are replaced by another, and redox
processes which change the oxidation state of the central metal atom.
Acid-base reactions involve transfer of protons from one molecule (acid) to another (base). Here, acids act as proton donors and bases as acceptors.
- Acid-base reaction, HA: acid, B: Base, A–: conjugated base, HB+: conjugated acid
The associated proton transfer results in the so-called conjugate acid and conjugate base.
The reverse reaction is possible, and thus the acid/base and conjugated
base/acid are always in equilibrium. The equilibrium is determined by
the acid and base dissociation constants (Ka and Kb) of the involved substances. A special case of the acid-base reaction is the neutralization where an acid and a base, taken at exactly same amounts, form a neutral salt.
Acid-base reactions can have different definitions depending on the acid-base concept employed. Some of the most common are:
- Arrhenius definition: Acids dissociate in water releasing H3O+ ions; bases dissociate in water releasing OH– ions.
- Brønsted-Lowry definition: Acids are proton (H+) donors, bases are proton acceptors; this includes the Arrhenius definition.
- Lewis definition: Acids are electron-pair acceptors, bases are electron-pair donors; this includes the Brønsted-Lowry definition.
is the formation of a solid in a solution or inside another solid
during a chemical reaction. It usually takes place when the
concentration of dissolved ions exceeds the solubility limit
and forms an insoluble salt. This process can be assisted by adding a
precipitating agent or by removal of the solvent. Rapid precipitation
results in an amorphous or microcrystalline residue and slow process can yield single crystals. The latter can also be obtained by recrystallization from microcrystalline salts.
Reactions can take place between two solids. However, because of the relatively small diffusion
rates in solids, the corresponding chemical reactions are very slow.
They are accelerated by increasing the reaction temperature and finely
dividing the reactant to increase the contacting surface area.
Photochemical reactionsIn photochemical reactions, atoms and molecules absorb energy (photons) of the illumination light and convert into an excited state.
They can then release this energy by breaking chemical bonds, thereby
producing radicals. Photochemical reactions include hydrogen-oxygen
reactions, radical polymerization, chain reactions and rearrangement reactions.
Many important processes involve photochemistry. The premier example is photosynthesis, in which most plants use solar energy to convert carbon dioxide and water into glucose, disposing of oxygen as a side-product. Humans rely on photochemistry for the formation of vitamin D, and vision is initiated by a photochemical reaction of rhodopsin. In fireflies, an enzyme in the abdomen catalyzes a reaction that results in bioluminescence. Many significant photochemical reactions, such as ozone formation, occur in the Earth atmosphere and constitute atmospheric chemistry.
Schematic potential energy diagram showing the effect of a catalyst in
an endothermic chemical reaction. The presence of a catalyst opens a
different reaction pathway (in red) with a lower activation energy. The
final result and the overall thermodynamics are the same.
Solid heterogeneous catalysts are plated on meshes in ceramic catalytic converters in order to maximize their surface area. This exhaust converter is from a Peugeot 106 S2 1100
In catalysis, the reaction does not proceed directly, but through a third substance known as catalyst.
Unlike other reagents that participate in the chemical reaction, a
catalyst is not consumed by the reaction itself; however, it can be
inhibited, deactivated or destroyed by secondary processes. Catalysts
can be used in a different phase (heterogeneous) or in the same phase (homogenous) as the reactants. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by polymeric
side products. Additionally, heterogeneous catalysts can dissolve into
the solution in a solid-liquid system or evaporate in a solid-gas
system. Catalysts can only speed up the reaction – chemicals that slow
down the reaction are called inhibitors.
Substances that increase the activity of catalysts are called
promoters, and substances that deactivate catalysts are called
catalytic poisons. With a catalyst, a reaction which is kinetically
inhibited by a high activation energy can take place in circumvention
of this activation energy.
Heterogeneous catalysts are usually solids, powdered in order to
maximize their surface area. Of particular importance in heterogeneous
catalysis are the platinum group metals and other transition metals, which are used in hydrogenations, catalytic reforming and in the synthesis of commodity chemicals such as nitric acid and ammonia. Acids are an example of a homogeneous catalyst, they increase the nucleophilicity of carbonyls,
allowing a reaction that would not otherwise proceed with
electrophiles. The advantage of homogeneous catalysts is the ease of
mixing them with the reactants, but they may also be difficult to
separate from the products. Therefore, heterogeneous catalysts are
preferred in many industrial processes.
Reactions in organic chemistry
In organic chemistry, in addition to oxidation, reduction or
acid-base reactions, a number of other reactions can take place which
involve covalent bonds between carbon atoms or carbon and heteroatoms (such as oxygen, nitrogen, halogens, etc.). Many specific reactions in organic chemistry are name reactions designated after their discoverers.
In a substitution reaction, a functional group in a particular chemical compound is replaced by another group. These reactions can be distinguished by the type of substituting species into a nucleophilic, electrophilic or radical substitution.
In the first type, a nucleophile, an atom or molecule with an excess of electrons and thus a negative charge or partial charge,
replaces another atom or part of the "substrate" molecule. The electron
pair from the nucleophile attacks the substrate forming a new bond,
while the leaving group
departs with an electron pair. The nucleophile may be electrically
neutral or negatively charged, whereas the substrate is typically
neutral or positively charged. Examples of nucleophiles are hydroxide ion, alkoxides, amines and halides. This type of reaction is found mainly in aliphatic hydrocarbons, and rarely in aromatic hydrocarbon. The latter have high electron density and enter nucleophilic aromatic substitution only with very strong electron withdrawing groups. Nucleophilic substitution can take place by two different mechanisms, SN1 and SN2. In their names, S stands for substitution, N for nucleophilic, and the number represents the kinetic order of the reaction, unimolecular or bimolecular.
steps of an
is green and
group is red
The SN1 reaction proceeds in two steps. First, the leaving group is eliminated creating a carbocation. This is followed by a rapid reaction with the nucleophile.
In the SN2 mechanism, the nucleophile forms a transition
state with the attacked molecule, and only then the leaving group is
cleaved. These two mechanisms differ in the stereochemistry of the products. SN1 leads to the non-stereospecific addition and does not result in a chiral center, but rather in a set of geometric isomers (cis/trans). In contrast, a reversal (Walden inversion) of the previously existing stereochemistry is observed in the SN2 mechanism.
Electrophilic substitution is the counterpart of the nucleophilic substitution in that the attacking atom or molecule, an electrophile, has low electron density and thus a positive charge. Typical electrophiles are the carbon atom of carbonyl groups, carbocations or sulfur or nitronium cations. This reaction takes place almost exclusively in aromatic hydrocarbons, where it is called electrophilic aromatic substitution.
The electrophile attack results in the so-called σ-complex, a
transition state in which the aromatic system is abolished. Then, the
leaving group, usually a proton, is split off and the aromaticity is
restored. An alternative to aromatic substitution is electrophilic
aliphatic substitution. It is similar to the nucleophilic aliphatic
substitution and also has two major types, SE1 and SE2
Mechanism of electrophilic aromatic substitution
In the third type of substitution reaction, radical substitution, the attacking particle is a radical. This process usually takes the form of a chain reaction,
for example in the reaction of alkanes with halogens. In the first
step, light or heat disintegrates the halogen-containing molecules
producing the radicals. Then the reaction proceeds as an avalanche
until two radicals meet and recombine.
- Reactions during the chain reaction of radical substitution
Addition and elimination
The addition and its counterpart, the elimination, are reactions which change the number of substituents on the carbon atom, and form or cleave multiple bonds. Doubletriple bonds
can be produced by eliminating a suitable leaving group. Similar to the
nucleophilic substitution, there are several possible reaction
mechanisms which are named after the respective reaction order. In the
E1 mechanism, the leaving group is ejected first, forming a
carbocation. The next step, formation of the double bond, takes place
with elimination of a proton (deprotonation).
The leaving order is reversed in the E1cb mechanism, that is the proton
is split off first. This mechanism requires participation of a base. Because of the similar conditions, both reactions in the E1 or E1cb elimination always compete with the SN1 substitution. and
The E2 mechanism also requires a base, but there the attack of the
base and the elimination of the leaving group proceed simultaneously
and produce no ionic intermediate. In contrast to the E1 eliminations,
different stereochemical configurations are possible for the reaction
product in the E2 mechanism, because the attack of the base
preferentially occurs in the anti-position with respect to the leaving
group. Because of the similar conditions and reagents, the E2
elimination is always in competition with the SN2-substitution.
Electrophilic addition of hydrogen bromide
The counterpart of elimination is the addition where double or
triple bonds are converted into single bonds. Similar to the
substitution reactions, there are several types of additions
distinguished by the type of the attacking particle. For example, in
the electrophilic addition of hydrogen bromide, an electrophile (proton) attacks the double bond forming a carbocation,
which then reacts with the nucleophile (bromine). The carbocation can
be formed on either side of the double bond depending on the groups
attached to its ends, and the preferred configuration can be predicted
with the Markovnikov's rule.
This rule states that "In the heterolytic addition of a polar molecule
to an alkene or alkyne, the more electronegative (nucleophilic) atom
(or part) of the polar molecule becomes attached to the carbon atom
bearing the smaller number of hydrogen atoms."
If the addition of a functional group takes place at the less
substituted carbon atom of the double bond, then the electrophilic
substitution with acids is not possible. In this case, one has to use
the hydroboration–oxidation reaction, where in the first step, the boron atom acts as electrophile and adds to the less substituted carbon atom. At the second step, the nucleophilic hydroperoxide or halogen anion attacks the boron atom.
While the addition to the electron-rich alkenes and alkynes is mainly electrophilic, the nucleophilic addition
plays an important role for the carbon-heteroatom multiple bonds, and
especially its most important representative, the carbonyl group. This
process is often associated with an elimination, so that after the
reaction the carbonyl group is present again. It is therefore called
addition-elimination reaction and may occur in carboxylic acid
derivatives such as chlorides, esters or anhydrides. This reaction is
often catalyzed by acids or bases, where the acids increase by the
electrophilicity of the carbonyl group by binding to the oxygen atom,
whereas the bases enhance the nucleophilicity of the attacking
Acid-catalyzed addition-elimination mechanism
Nucleophilic addition of a carbanion or another nucleophile to the double bond of an alpha, beta unsaturated carbonyl compound can proceed via the Michael reaction, which belongs to the larger class of conjugate additions. This is one of the most useful methods for the mild formation of C-C bonds.
Some additions which can not be executed with nucleophiles and
electrophiles, can be succeeded with free radicals. As with the
free-radical substitution, the radical additionfree-radical polymerization proceeds as a chain reaction, and such reactions are the basis of the
Other organic reaction mechanisms
The Cope rearrangement of 3-methyl-1,5-hexadiene
Mechanism of a
Orbital overlap in a
In a rearrangement reaction, the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. These include hydride shift reactions such as the Wagner-Meerwein rearrangement, where a hydrogen, alkyl or aryl
group migrates from one carbon to a neighboring carbon. Most
rearrangements are associated with the breaking and formation of new
carbon-carbon bonds. Other examples are sigmatropic reaction such as the Cope rearrangement.
Cyclic rearrangements include cycloadditions and, more generally, pericyclic reactions,
wherein two or more double bond-containing molecules form a cyclic
molecule. An important example of cycloaddition reaction is the Diels–Alder reactiondiene and a substituted alkene to form a substituted cyclohexene system (the so-called [4+2] cycloaddition) between a conjugated
Whether or not a certain cycloaddition would proceed depends on the
electronic orbitals of the participating species, as only orbitals with
the same sign of wave function
will overlap and interact constructively to form new bonds.
Cycloaddition is usually assisted by light or heat. These perturbations
result in different arrangement of electrons in the excited state of
the involved molecules and therefore in different effects. For example,
the [4+2] Diels-Alder reactions can be assisted by heat whereas the
[2+2] cycloaddition is selectively induced by light. Because of the orbital character, the potential for developing stereoisomeric products upon cycloaddition is limited, as described by the Woodward-Hoffmann rules.
Illustration of the induced fit model of enzyme activity
Biochemical reactions are mainly controlled by enzymes. These proteins can specifically catalyze a single reaction, so that reactions can be controlled very precisely. The reaction takes place in the active site, a small part of the enzyme which is usually found in a cleft or pocket lined by amino acid
residues, and the rest of the enzyme is used mainly for stabilization.
The catalytic action of enzymes relies on several mechanisms including
the molecular shape ("induced fit"), bond strain, proximity and
orientation of molecules relatively to the enzyme, proton donation or
withdrawal (acid/base catalysis), electrostatic interactions and many
The biochemical reactions that occur in living organisms are collectively known as metabolism. Among the most important its mechanisms is the anabolism, in which different DNA and enzyme-controlled processes result in the production of large molecules such as proteins and carbohydrates from smaller units. Bioenergeticsglucose, which can be produced by plants via photosynthesis or assimilated from food. All organisms use this energy to produce adenosine triphosphate (ATP), which can then be used to energise other reactions studies the sources of energy for such reactions. An important energy sources is