What Is The Process That Changes One Set Of Chemicals Into Another Set Of Chemicals?

Procedure that results in the interconversion of chemical species

A thermite reaction using iron(Three) oxide. The sparks flying outwards are globules of molten iron trailing fume in their wake.

A chemic reaction is a process that leads to the chemic transformation of i set of chemical substances to another.^[i] Classically, chemical reactions encompass changes that merely involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no modify to the nuclei (no change to the elements present), and can often be described by a chemical equation. Nuclear chemistry is a sub-discipline of chemistry that involves the chemical reactions of unstable and radioactive elements where both electronic and nuclear changes can occur.

The substance (or substances) initially involved in a chemical reaction are called reactants or reagents. Chemical reactions are usually characterized by a chemical alter, and they yield one or more products, which commonly have properties different from the reactants. Reactions often consist of a sequence of private sub-steps, the so-called elementary reactions, and the data on the precise course of activity is role of the reaction mechanism. Chemic reactions are described with chemical equations, which symbolically nowadays the starting materials, cease products, and sometimes intermediate products and reaction conditions.

Chemical reactions happen at a characteristic reaction rate at a given temperature and chemical concentration. Typically, reaction rates increase with increasing temperature because there is more thermal energy available to reach the activation energy necessary for breaking bonds between atoms.

Reactions may proceed in the forrard or contrary direction until they get to completion or reach equilibrium. Reactions that go on in the forward management to approach equilibrium are often described every bit spontaneous, requiring no input of energy to go frontwards. Not-spontaneous reactions require input of gratis energy to become forward (examples include charging a battery by applying an external electrical power source, or photosynthesis driven past absorption of electromagnetic radiations in the course of sunlight).

A reaction may be classified as redox in which oxidation and reduction occur or nonredox in which at that place is no oxidation and reduction occurring. Most simple redox reactions may be classified as combination, decomposition, or single displacement reactions.

Dissimilar chemic reactions are used during chemical synthesis in guild to obtain a desired product. In biochemistry, a sequent series of chemical reactions (where the product of i reaction is the reactant of the side by side reaction) form metabolic pathways. These reactions are often catalyzed by protein enzymes. Enzymes increase the rates of biochemical reactions, so that metabolic syntheses and decompositions impossible under ordinary conditions can occur at the temperatures and concentrations present within a cell.

The full general concept of a chemical reaction has been extended to reactions between entities smaller than atoms, including nuclear reactions, radioactive decays, and reactions between elementary particles, as described by quantum field theory.

History

Antoine Lavoisier developed the theory of combustion every bit a chemical reaction with oxygen.

Chemical reactions such as combustion in burn, fermentation and the reduction of ores to metals were known since artifact. Initial theories of transformation of materials were developed past Greek philosophers, such every bit the Four-Element Theory of Empedocles stating that whatsoever substance is composed of the 4 basic elements – fire, water, air and globe. In the Middle Ages, chemical transformations were studied by alchemists. They attempted, in particular, to convert atomic number 82 into gold, for which purpose they used reactions of lead and lead-copper alloys with sulfur.^[2]

The artificial production of chemical substances already was a key goal for medieval alchemists.^[3] Examples include the synthesis of ammonium chloride from organic substances equally described in the works (c. 850–950) attributed to Jābir ibn Ḥayyān,^[iv] or the production of mineral acids such as sulfuric and nitric acids past later alchemists, starting from c. 1300.^[5] The production of mineral acids involved the heating of sulfate and nitrate minerals such as copper sulfate, alum and saltpeter. In the 17th century, Johann Rudolph Glauber produced hydrochloric acid and sodium sulfate by reacting sulfuric acid and sodium chloride. With the evolution of the lead bedchamber process in 1746 and the Leblanc process, allowing large-scale production of sulfuric acid and sodium carbonate, respectively, chemical reactions became implemented into the industry. Further optimization of sulfuric acid technology resulted in the contact process in the 1880s,^[6] and the Haber process was adult in 1909–1910 for ammonia synthesis.^[vii]

From the 16th century, researchers including Jan Baptist van Helmont, Robert Boyle, and Isaac Newton tried to establish theories of the experimentally observed chemical transformations. The phlogiston theory was proposed in 1667 by Johann Joachim Becher. Information technology postulated the existence of a fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion. This proved to exist simulated in 1785 past Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air.^[viii]

Joseph Louis Gay-Lussac recognized in 1808 that gases always react in a sure relationship with each other. Based on this idea and the atomic theory of John Dalton, Joseph Proust had developed the police of definite proportions, which later resulted in the concepts of stoichiometry and chemical equations.^[9]

Regarding the organic chemistry, it was long believed that compounds obtained from living organisms were likewise complex to be obtained synthetically. According to the concept of vitalism, organic thing was endowed with a "vital force" and distinguished from inorganic materials. This separation was ended however by the synthesis of urea from inorganic precursors by Friedrich Wöhler in 1828. Other chemists who brought major contributions to organic chemical science include Alexander William Williamson with his synthesis of ethers and Christopher Kelk Ingold, who, among many discoveries, established the mechanisms of exchange reactions.

Characteristics

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The general characteristics of chemical reactions are:

Development of a gas
Germination of a precipitate
Change in temperature
Modify in state

Equations

As seen from the equation CH₄ + 2O₂ → CO₂ + 2 H₂O, a coefficient of 2 must exist placed before the oxygen gas on the reactants side and earlier the water on the products side in order for, as per the law of conservation of mass, the quantity of each element does non change during the reaction

Chemical equations are used to graphically illustrate chemical reactions. They consist of chemic or structural formulas of the reactants on the left and those of the products on the right. They are separated by an pointer (→) which indicates the management and type of the reaction; the arrow is read as the word "yields".^[10] The tip of the arrow points in the direction in which the reaction proceeds. A double pointer (⇌) pointing in opposite directions is used for equilibrium reactions. Equations should be counterbalanced co-ordinate to the stoichiometry, the number of atoms of each species should be the same on both sides of the equation. This is achieved by scaling the number of involved molecules (A, B, C and D in a schematic example below) by the appropriate integers a, b, c and d.^[eleven]

a A + b B → c C + d D

More elaborate reactions are represented by reaction schemes, which in add-on to starting materials and products show of import intermediates or transition states. Too, some relatively small additions to the reaction can be indicated in a higher place the reaction pointer; examples of such additions are water, heat, illumination, a catalyst, etc. Similarly, some minor products can exist placed below the arrow, often with a minus sign.

Retrosynthetic analysis can be applied to blueprint a circuitous synthesis reaction. Here the analysis starts from the products, for instance by splitting selected chemical bonds, to arrive at plausible initial reagents. A special arrow (⇒) is used in retro reactions.^[12]

Elementary reactions

The simple reaction is the smallest division into which a chemic reaction can exist decomposed, it has no intermediate products.^[13] Near experimentally observed reactions are built upward from many simple reactions that occur in parallel or sequentially. The bodily sequence of the private simple reactions is known as reaction mechanism. An unproblematic reaction involves a few molecules, usually one or two, considering of the depression probability for several molecules to meet at a certain time.^[14]

Isomerization of azobenzene, induced by light (hν) or heat (Δ)

The nigh important elementary reactions are unimolecular and bimolecular reactions. But one molecule is involved in a unimolecular reaction; information technology is transformed past an isomerization or a dissociation into one or more other molecules. Such reactions require the improver of energy in the grade of heat or light. A typical example of a unimolecular reaction is the cis–trans isomerization, in which the cis-class of a compound converts to the trans-form or vice versa.^[fifteen]

In a typical dissociation reaction, a bond in a molecule splits (ruptures) resulting in two molecular fragments. The splitting can be homolytic or heterolytic. In the starting time example, the bond is divided and then that each product retains an electron and becomes a neutral radical. In the second case, both electrons of the chemic bail remain with one of the products, resulting in charged ions. Dissociation plays an important part in triggering concatenation reactions, such every bit hydrogen–oxygen or polymerization reactions.

{\ce {AB -> A + B}}

Dissociation of a molecule AB into fragments A and B

For bimolecular reactions, ii molecules collide and react with each other. Their merger is called chemical synthesis or an addition reaction.

{\ce {A + B -> AB}}

Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid–base reactions. In redox reactions, the transferred particle is an electron, whereas in acid–base reactions it is a proton. This type of reaction is also called metathesis.

{\ce {HA + B -> A + HB}}

for example

{\ce {NaCl + AgNO3 -> NaNO3 + AgCl(v)}}

Chemical equilibrium

Most chemical reactions are reversible; that is, they tin can and do run in both directions. The forward and reverse reactions are competing with each other and differ in reaction rates. These rates depend on the concentration and therefore change with time of the reaction: the reverse rate gradually increases and becomes equal to the charge per unit of the forrard reaction, establishing the so-called chemic equilibrium. The time to accomplish equilibrium depends on parameters such as temperature, force per unit area, and the materials involved, and is determined past the minimum gratis energy. In equilibrium, the Gibbs free energy must be zero. The pressure dependence can be explained with the Le Chatelier's principle. For example, an increment in pressure level due to decreasing book causes the reaction to shift to the side with the fewer moles of gas.^[sixteen]

The reaction yield stabilizes at equilibrium, but can exist increased by removing the product from the reaction mixture or changed by increasing the temperature or force per unit area. A change in the concentrations of the reactants does non bear on the equilibrium constant, but does affect the equilibrium position.

Thermodynamics

Chemic reactions are determined past the laws of thermodynamics. Reactions can proceed by themselves if they are exergonic, that is if they release free energy. The associated free free energy of the reaction is composed of 2 different thermodynamic quantities, enthalpy and entropy:^[17]

$\Delta G=\Delta H-T\cdot \Delta Southward$ .: $M$ : free energy, $H$ : enthalpy, $T$ : temperature, $S$ : entropy, $Δ$ : difference(change between original and product)

Reactions can be exothermic, where ΔH is negative and energy is released. Typical examples of exothermic reactions are precipitation and crystallization, in which ordered solids are formed from disordered gaseous or liquid phases. In contrast, in endothermic reactions, heat is consumed from the environment. This can occur by increasing the entropy of the system, oft through the germination of gaseous reaction products, which have high entropy. Since the entropy increases with temperature, many endothermic reactions preferably have place at high temperatures. On the contrary, many exothermic reactions such as crystallization occur at low temperatures. Changes in temperature can sometimes opposite the sign of the enthalpy of a reaction, as for the carbon monoxide reduction of molybdenum dioxide:

{\ce {2CO(grand) + MoO2(south) -> 2CO2(g) + Mo(s)}}

;

\Delta H^{o}=+21.86\ {\text{kJ at 298 K}}

This reaction to form carbon dioxide and molybdenum is endothermic at low temperatures, becoming less so with increasing temperature.^[18] ΔH° is zero at 1855 K, and the reaction becomes exothermic higher up that temperature.

Changes in temperature can also opposite the management trend of a reaction. For example, the water gas shift reaction

{\ce {CO(g) + H2O({v}) <=> CO2(k) + H2(m)}}

is favored by low temperatures, but its opposite is favored by high temperature. The shift in reaction direction tendency occurs at 1100 K.^[18]

Reactions can besides exist characterized by the internal energy which takes into account changes in the entropy, volume and chemic potential. The latter depends, among other things, on the activities of the involved substances.^[nineteen]

${d}U=T\cdot {d}S-p\cdot {d}V+\mu \cdot {d}northward$: $U$ : internal energy, $S$ : entropy, $p$ : pressure, $μ$ : chemical potential, $n$ : number of molecules, $d$ : small change sign

Kinetics

The speed at which reactions takes place is studied by reaction kinetics. The rate depends on various parameters, such every bit:

Reactant concentrations, which unremarkably make the reaction happen at a faster rate if raised through increased collisions per unit time. Some reactions, however, have rates that are contained of reactant concentrations. These are called zero order reactions.
Area available for contact between the reactants, in particular solid ones in heterogeneous systems. Larger surface areas lead to higher reaction rates.
Pressure – increasing the force per unit area decreases the volume betwixt molecules and therefore increases the frequency of collisions betwixt the molecules.
Activation energy, which is defined as the amount of free energy required to brand the reaction first and carry on spontaneously. Higher activation energy implies that the reactants need more energy to get-go than a reaction with a lower activation energy.
Temperature, which hastens reactions if raised, since college temperature increases the energy of the molecules, creating more than collisions per unit of measurement time,
The presence or absenteeism of a catalyst. Catalysts are substances which alter the pathway (mechanism) of a reaction which in turn increases the speed of a reaction by lowering the activation energy needed for the reaction to take identify. A catalyst is not destroyed or changed during a reaction, and then it tin be used again.
For some reactions, the presence of electromagnetic radiations, most notably ultraviolet light, is needed to promote the breaking of bonds to start the reaction. This is particularly true for reactions involving radicals.

Several theories allow calculating the reaction rates at the molecular level. This field is referred to as reaction dynamics. The rate v of a get-go-order reaction, which could be disintegration of a substance A, is given by:

five=-{\frac {d[{\ce {A}}]}{dt}}=k\cdot [{\ce {A}}].

Its integration yields:

{\ce {[A]}}(t)={\ce {[A]}}_{0}\cdot e^{-g\cdot t}.

Hither yard is first-society rate constant having dimension 1/time, [A](t) is concentration at a fourth dimension t and [A]₀ is the initial concentration. The rate of a first-order reaction depends only on the concentration and the properties of the involved substance, and the reaction itself tin exist described with the feature one-half-life. More than 1 time constant is needed when describing reactions of higher order. The temperature dependence of the rate constant usually follows the Arrhenius equation:

thou=k_{0}e^{{-E_{a}}/{k_{B}T}}

where Eastward_a is the activation energy and k_B is the Boltzmann abiding. I of the simplest models of reaction rate is the collision theory. More realistic models are tailored to a specific trouble and include the transition state theory, the calculation of the potential free energy surface, the Marcus theory and the Rice–Ramsperger–Kassel–Marcus (RRKM) theory.^[20]

Reaction types

Four bones types

Representation of 4 basic chemical reactions types: synthesis, decomposition, single replacement and double replacement.

Synthesis

In a synthesis reaction, two or more elementary substances combine to grade a more than complex substance. These reactions are in the general class:

{\ce {A + B->AB}}

Two or more reactants yielding ane product is some other fashion to place a synthesis reaction. One case of a synthesis reaction is the combination of iron and sulfur to class atomic number 26(II) sulfide:

{\ce {8Fe + S8->8FeS}}

Another example is simple hydrogen gas combined with simple oxygen gas to produce a more circuitous substance, such as h2o.^[21]

Decomposition

A decomposition reaction is when a more than complex substance breaks downward into its more simple parts. It is thus the opposite of a synthesis reaction, and can be written as^[21] ^[22]

{\ce {AB->A + B}}

One example of a decomposition reaction is the electrolysis of water to brand oxygen and hydrogen gas:

{\ce {2H2O->2H2 + O2}}

Single displacement

In a Unmarried displacement reaction, a single uncombined element replaces some other in a compound; in other words, one element trades places with another element in a compound^[21] These reactions come in the general grade of:

{\ce {A + BC->Air conditioning + B}}

I example of a single deportation reaction is when magnesium replaces hydrogen in water to make magnesium hydroxide and hydrogen gas:

{\ce {Mg + 2H2O->Mg(OH)2 + H2 (^)}}

Double displacement

In a double displacement reaction, the anions and cations of two compounds switch places and form two entirely dissimilar compounds.^[21] These reactions are in the general form:^[22]

{\ce {AB + CD->Advertizement + CB}}

For example, when barium chloride (BaCl₂) and magnesium sulfate (MgSO₄) react, the So₄ ⁱⁱ⁻ anion switches places with the 2Cl⁻ anion, giving the compounds BaSO₄ and MgCl_two.

Another example of a double displacement reaction is the reaction of lead(Ii) nitrate with potassium iodide to form atomic number 82(II) iodide and potassium nitrate:

{\ce {Pb(NO3)2 + 2KI->PbI2(v) + 2KNO3}}

Combustion

In a combustion reaction, an element or chemical compound reacts with oxygen, often producing energy in the form of heat or light. Combustion reactions always involve oxygen, simply likewise frequently involve a hydrocarbon.

{\ce {2C8H18(l) + 25O2(g)->16CO2 + 18H2O(g)}}

A combustion reaction can also result from carbon, magnesium or sulfur reacting with oxygen.^[23]

{\ce {2Mg(s) + O2->2MgO(s)}}

{\ce {S(s) + O2(g)->SO2(g)}}

Oxidation and reduction

Analogy of a redox reaction

Sodium chloride is formed through the redox reaction of sodium metal and chlorine gas

Redox reactions can be understood in terms of transfer of electrons from i involved species (reducing amanuensis) to some other (oxidizing agent). In this process, the former species is oxidized and the latter is reduced. Though sufficient for many purposes, these descriptions are non precisely correct. Oxidation is amend defined every bit an increase in oxidation land, and reduction as a decrease in oxidation land. In do, the transfer of electrons will always alter the oxidation country, but there are many reactions that are classed as "redox" even though no electron transfer occurs (such as those involving covalent bonds).^[24] ^[25]

In the following redox reaction, hazardous sodium metallic reacts with toxic chlorine gas to form the ionic compound sodium chloride, or common table salt:

{\ce {2Na(s) + Cl2(thousand)->2NaCl(s)}}

In the reaction, sodium metallic goes from an oxidation state of 0 (equally information technology is a pure element) to +one: in other words, the sodium lost ane electron and is said to have been oxidized. On the other hand, the chlorine gas goes from an oxidation of 0 (it is too a pure element) to −i: the chlorine gains one electron and is said to have been reduced. Because the chlorine is the one reduced, information technology is considered the electron acceptor, or in other words, induces oxidation in the sodium – thus the chlorine gas is considered the oxidizing agent. Conversely, the sodium is oxidized or is the electron donor, and thus induces reduction in the other species and is considered the reducing agent.

Which of the involved reactants would be reducing or oxidizing amanuensis tin be predicted from the electronegativity of their elements. Elements with low electronegativity, such every bit about metals, easily donate electrons and oxidize – they are reducing agents. On the contrary, many ions with high oxidation numbers, such as H
₂ O
₂ , MnO ⁻
₄ , CrO
_iii , Cr
_ii O ²⁻
₇ , OsO
₄ tin gain one or ii extra electrons and are strong oxidizing agents.

For some chief-group elements the number of electrons donated or accepted in a redox reaction tin be predicted from the electron configuration of the reactant element. Elements try to reach the low-energy element of group 0 configuration, and therefore brine metals and halogens will donate and accept 1 electron respectively. Noble gases themselves are chemically inactive.^[26]

The overall redox reaction can exist balanced by combining the oxidation and reduction one-half-reactions multiplied by coefficients such that the number of electrons lost in the oxidation equals the number of electrons gained in the reduction.

An important class of redox reactions are the electrochemical reactions, where electrons from the power supply are used equally the reducing amanuensis. These reactions are particularly of import for the product of chemic elements, such as chlorine^[27] or aluminium. The reverse process in which electrons are released in redox reactions and can be used as electrical energy is possible and used in batteries.

Complexation

In complexation reactions, several ligands react with a metal atom to form a coordination circuitous. This is accomplished by providing lone pairs of the ligand into empty orbitals of the metallic cantlet and forming dipolar bonds. The ligands are Lewis bases, they tin 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 constitute using the 18-electron rule, saying that the valence shells of a transition metal volition collectively suit eighteen electrons, whereas the symmetry of the resulting complex tin can be predicted with the crystal field theory and ligand field theory. Complexation reactions also include ligand substitution, in which one or more than ligands are replaced by another, and redox processes which change the oxidation land of the central metallic cantlet.^[28]

Acid–base reactions

In the Brønsted–Lowry acid–base theory, an acid–base of operations reaction involves a transfer of protons (H⁺) from i species (the acid) to some other (the base). When a proton is removed from an acrid, the resulting species is termed that acrid'south conjugate base. When the proton is accepted by a base, the resulting species is termed that base of operations'south conjugate acid.^[29] In other words, acids act as proton donors and bases act as proton acceptors according to the following equation:

{\ce {{\underset {acid}{HA}}+{\underset {base of operations}{B}}<=>{\underset {conjugated\ base}{A^{-}}}+{\underset {conjugated\ acid}{HB+}}}}

The reverse reaction is possible, and thus the acid/base and conjugated base of operations/acid are ever in equilibrium. The equilibrium is adamant by the acid and base dissociation constants (Chiliad _a and K _b) of the involved substances. A special instance of the acid–base reaction is the neutralization where an acid and a base, taken at exactly same amounts, course a neutral salt.

Acid–base reactions tin take different definitions depending on the acrid–base concept employed. Some of the about common are:

Arrhenius definition: Acids dissociate in water releasing H_threeO⁺ 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.

Precipitation

Precipitation is the formation of a solid in a solution or inside another solid during a chemical reaction. It unremarkably takes identify when the concentration of dissolved ions exceeds the solubility limit^[30] and forms an insoluble salt. This process can be assisted by adding a precipitating agent or past removal of the solvent. Rapid precipitation results in an baggy or microcrystalline remainder and ho-hum process tin can yield single crystals. The latter can also be obtained by recrystallization from microcrystalline salts.^[31]

Solid-state reactions

Reactions can take place betwixt two solids. Yet, considering of the relatively small improvidence rates in solids, the respective chemic reactions are very slow in comparison to liquid and gas stage reactions. They are accelerated by increasing the reaction temperature and finely dividing the reactant to increase the contacting expanse.^[32]

Reactions at the solid|gas interface

Reaction can take place at the solid|gas interface, surfaces at very low pressure level such as ultra-high vacuum. Via scanning tunneling microscopy, it is possible to detect reactions at the solid|gas interface in real infinite, if the time calibration of the reaction is in the right range.^[33] ^[34] Reactions at the solid|gas interface are in some cases related to catalysis.

Photochemical reactions

In photochemical reactions, atoms and molecules absorb energy (photons) of the illumination light and convert into an excited land. They tin can and then release this energy past breaking chemical bonds, thereby producing radicals. Photochemical reactions include hydrogen–oxygen reactions, radical polymerization, chain reactions and rearrangement reactions.^[35]

Many important processes involve photochemistry. The premier example is photosynthesis, in which near plants use solar energy to convert carbon dioxide and water into glucose, disposing of oxygen as a side-production. Humans rely on photochemistry for the formation of vitamin D, and vision is initiated by a photochemical reaction of rhodopsin.^[xv] In fireflies, an enzyme in the abdomen catalyzes a reaction that results in bioluminescence.^[36] Many significant photochemical reactions, such equally ozone formation, occur in the Globe atmosphere and constitute atmospheric chemistry.

Catalysis

Schematic potential energy diagram showing the effect of a catalyst in an endothermic chemical reaction. The presence of a goad opens a different reaction pathway (in red) with a lower activation energy. The concluding 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 expanse. This exhaust converter is from a Peugeot 106 S2 1100

In catalysis, the reaction does not go on directly, but through reaction with a 3rd substance known as goad. Although the catalyst takes part in the reaction, information technology is returned to its original state by the end of the reaction and and so is not consumed. Even so, it can be inhibited, deactivated or destroyed past secondary processes. Catalysts can be used in a different phase (heterogeneous) or in the same phase (homogeneous) every bit the reactants. In heterogeneous catalysis, typical secondary processes include coking where the goad 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 tin can only speed up the reaction – chemicals that slow downwards the reaction are called inhibitors.^[37] ^[38] Substances that increase the activity of catalysts are called promoters, and substances that conciliate catalysts are called catalytic poisons. With a catalyst, a reaction which is kinetically inhibited by a high activation energy tin can take identify in circumvention of this activation energy.

Heterogeneous catalysts are ordinarily solids, powdered in society 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 instance of a homogeneous catalyst, they increase the nucleophilicity of carbonyls, allowing a reaction that would not otherwise proceed with electrophiles. The reward of homogeneous catalysts is the ease of mixing them with the reactants, just they may also be difficult to dissever from the products. Therefore, heterogeneous catalysts are preferred in many industrial processes.^[39]

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 equally oxygen, nitrogen, halogens, etc.). Many specific reactions in organic chemistry are name reactions designated after their discoverers.

Substitution

In a substitution reaction, a functional group in a item chemical compound is replaced past another group.^[forty] These reactions can be distinguished by the blazon of substituting species into a nucleophilic, electrophilic or radical substitution.

S_N1 mechanism

S_N2 mechanism

In the outset type, a nucleophile, an atom or molecule with an excess of electrons and thus a negative charge or fractional charge, replaces another atom or part of the "substrate" molecule. The electron pair from the nucleophile attacks the substrate forming a new bail, 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 blazon of reaction is found mainly in aliphatic hydrocarbons, and rarely in aromatic hydrocarbon. The latter have high electron density and enter nucleophilic aromatic substitution just with very strong electron withdrawing groups. Nucleophilic substitution tin take place past two different mechanisms, Due south_Northward1 and South_Nii. In their names, Southward stands for substitution, North for nucleophilic, and the number represents the kinetic order of the reaction, unimolecular or bimolecular.^[41]

The three steps of an S_Northward2 reaction. The nucleophile is green and the leaving group is ruby

S_North2 reaction causes stereo inversion (Walden inversion)

The Due south_North1 reaction gain in two steps. Commencement, the leaving grouping is eliminated creating a carbocation. This is followed by a rapid reaction with the nucleophile.^[42]

In the S_{Due north}2 mechanism, the nucleophile forms a transition land with the attacked molecule, and but then the leaving group is broken. These two mechanisms differ in the stereochemistry of the products. S_N1 leads to the non-stereospecific improver and does not outcome in a chiral center, but rather in a fix of geometric isomers (cis/trans). In contrast, a reversal (Walden inversion) of the previously existing stereochemistry is observed in the S_Ntwo mechanism.^[43]

Electrophilic commutation is the analogue of the nucleophilic exchange in that the attacking atom or molecule, an electrophile, has low electron density and thus a positive charge. Typical electrophiles are the carbon cantlet of carbonyl groups, carbocations or sulfur or nitronium cations. This reaction takes identify almost exclusively in aromatic hydrocarbons, where information technology is chosen electrophilic aromatic substitution. The electrophile set on results in the and then-chosen σ-complex, a transition land in which the aromatic system is abolished. And so, the leaving group, usually a proton, is split off and the aromaticity is restored. An culling to effluvious substitution is electrophilic aliphatic substitution. It is like to the nucleophilic aliphatic substitution and also has 2 major types, South_Eane and South_Eastwardtwo^[44]

Mechanism of electrophilic aromatic substitution

In the third blazon of substitution reaction, radical substitution, the attacking particle is a radical.^[40] This procedure usually takes the grade of a concatenation reaction, for instance in the reaction of alkanes with halogens. In the first step, lite or rut disintegrates the halogen-containing molecules producing the radicals. Then the reaction proceeds equally an avalanche until two radicals meet and recombine.^[45]

${\ce {X. + R-H -> 10-H + R.}}$
${\ce {R. + X2 -> R-X + X.}}$: Reactions during the chain reaction of radical substitution

Addition and elimination

The addition and its analogue, the elimination, are reactions which modify the number of substituents on the carbon atom, and course or carve multiple bonds. Double and triple bonds can be produced by eliminating a suitable leaving grouping. Similar to the nucleophilic substitution, at that place are several possible reaction mechanisms which are named after the respective reaction order. In the E1 mechanism, the leaving group is ejected showtime, forming a carbocation. The side by side footstep, formation of the double bond, takes identify with emptying of a proton (deprotonation). The leaving society is reversed in the E1cb machinery, that is the proton is split off beginning. This mechanism requires participation of a base.^[46] Because of the similar atmospheric condition, both reactions in the E1 or E1cb emptying ever compete with the S_Northward1 commutation.^[47]

E1 elimination

E1cb elimination

The E2 mechanism besides requires a base, just there the assail of the base and the emptying of the leaving group go on simultaneously and produce no ionic intermediate. In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 machinery, because the attack of the base of operations preferentially occurs in the anti-position with respect to the leaving grouping. Because of the similar atmospheric condition and reagents, the E2 elimination is ever in competition with the S_N2-commutation.^[48]

Electrophilic addition of hydrogen bromide

The analogue of emptying is the add-on where double or triple bonds are converted into single bonds. Similar to the exchange reactions, there are several types of additions distinguished past the blazon of the attacking particle. For instance, in the electrophilic addition of hydrogen bromide, an electrophile (proton) attacks the double bond forming a carbocation, which and then reacts with the nucleophile (bromine). The carbocation tin 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.^[49] This dominion states that "In the heterolytic add-on of a polar molecule to an alkene or alkyne, the more electronegative (nucleophilic) atom (or part) of the polar molecule becomes fastened to the carbon atom bearing the smaller number of hydrogen atoms."^[50]

If the addition of a functional group takes identify at the less substituted carbon atom of the double bond, then the electrophilic substitution with acids is non possible. In this case, 1 has to use the hydroboration–oxidation reaction, where in the commencement 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 cantlet.^[51]

While the add-on to the electron-rich alkenes and alkynes is mainly electrophilic, the nucleophilic addition plays an of import role for the carbon-heteroatom multiple bonds, and peculiarly its most of import representative, the carbonyl grouping. This process is ofttimes associated with an elimination, so that after the reaction the carbonyl group is nowadays again. It is therefore chosen improver-elimination reaction and may occur in carboxylic acid derivatives such as chlorides, esters or anhydrides. This reaction is often catalyzed past acids or bases, where the acids increase past the electrophilicity of the carbonyl group by binding to the oxygen atom, whereas the bases raise the nucleophilicity of the attacking nucleophile.^[52]

Acid-catalyzed addition-elimination machinery

Nucleophilic addition of a carbanion or some other nucleophile to the double bond of an alpha, beta unsaturated carbonyl compound tin can proceed via the Michael reaction, which belongs to the larger class of conjugate additions. This is ane of the almost useful methods for the mild formation of C–C bonds.^[53] ^[54] ^[55]

Some additions which can not be executed with nucleophiles and electrophiles, can be succeeded with complimentary radicals. As with the free-radical commutation, the radical addition gain as a chain reaction, and such reactions are the footing of the free-radical polymerization.^[56]

Other organic reaction mechanisms

The Cope rearrangement of 3-methyl-1,5-hexadiene

Machinery of a Diels-Alder reaction

Orbital overlap in a Diels-Alder reaction

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.^[57]

Cyclic rearrangements include cycloadditions and, more than generally, pericyclic reactions, wherein two or more than double bail-containing molecules form a cyclic molecule. An important instance of cycloaddition reaction is the Diels–Alder reaction (the so-called [4+two] cycloaddition) between a conjugated diene and a substituted alkene to form a substituted cyclohexene system.^[58]

Whether a sure cycloaddition would proceed depends on the electronic orbitals of the participating species, equally just orbitals with the same sign of wave office will overlap and interact constructively to form new bonds. Cycloaddition is normally assisted past light or rut. These perturbations result in different system of electrons in the excited state of the involved molecules and therefore in dissimilar effects. For example, the [iv+ii] Diels-Alder reactions can be assisted by heat whereas the [ii+2] cycloaddition is selectively induced by light.^[59] Because of the orbital grapheme, the potential for developing stereoisomeric products upon cycloaddition is limited, equally described by the Woodward–Hoffmann rules.^[60]

Biochemical reactions

Illustration of the induced fit model of enzyme action

Biochemical reactions are mainly controlled by enzymes. These proteins tin specifically catalyze a unmarried reaction, and then 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 scissure or pocket lined by amino acrid residues, and the residue of the enzyme is used mainly for stabilization. The catalytic action of enzymes relies on several mechanisms including the molecular shape ("induced fit"), bail strain, proximity and orientation of molecules relative to the enzyme, proton donation or withdrawal (acid/base catalysis), electrostatic interactions and many others.^[61]

The biochemical reactions that occur in living organisms are collectively known as metabolism. Among the most important of its mechanisms is the anabolism, in which different Deoxyribonucleic acid and enzyme-controlled processes result in the product of large molecules such equally proteins and carbohydrates from smaller units.^[62] Bioenergetics studies the sources of energy for such reactions. An important energy source is glucose, which tin can exist produced by plants via photosynthesis or assimilated from food. All organisms use this free energy to produce adenosine triphosphate (ATP), which can then exist used to energize other reactions.

Applications

Thermite reaction proceeding in railway welding. Shortly after this, the liquid fe flows into the mould around the rails gap

Chemical reactions are central to chemical technology where they are used for the synthesis of new compounds from natural raw materials such as petroleum and mineral ores. It is essential to make the reaction every bit efficient as possible, maximizing the yield and minimizing the corporeality of reagents, energy inputs and waste material. Catalysts are especially helpful for reducing the free energy required for the reaction and increasing its reaction charge per unit.^[63] ^[64]

Some specific reactions have their niche applications. For example, the thermite reaction is used to generate low-cal and rut in pyrotechnics and welding. Although it is less controllable than the more conventional oxy-fuel welding, arc welding and flash welding, information technology requires much less equipment and is nonetheless used to mend rail, specially in remote areas.^[65]

Monitoring

Mechanisms of monitoring chemical reactions depend strongly on the reaction rate. Relatively slow processes tin can be analyzed in situ for the concentrations and identities of the individual ingredients. Important tools of real fourth dimension analysis are the measurement of pH and analysis of optical absorption (color) and emission spectra. A less accessible but rather efficient method is introduction of a radioactive isotope into the reaction and monitoring how information technology changes over fourth dimension and where it moves to; this method is ofttimes used to clarify redistribution of substances in the human body. Faster reactions are ordinarily studied with ultrafast laser spectroscopy where utilization of femtosecond lasers allows short-lived transition states to be monitored at time scaled down to a few femtoseconds.^[66]

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Source: https://en.wikipedia.org/wiki/Chemical_reaction

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