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A chemical reaction is a process that leads to the transformation of one set ofchemical
substancesto another.[1]
Chemical reactions can be eitherspontaneous, requiring no input of
energy, or non-spontaneous, typically following the input of some type of energy, viz. heat,
light or electricity. Classically, chemical reactions encompass changes that strictly involve
the motion ofelectronsin the forming and breaking ofchemical bonds, although the general
concept of a chemical reaction, in particular the notion of achemical equation, is applicabletotransformations of elementary particles, as well asnuclear reactions.
The substance (or substances) initially involved in a chemical reaction are calledreactants or
reagents. Chemical reactions are usually characterized by achemical change, and they yield
one or moreproducts, which usually have properties different from the reactants. Reactions
often consist of a sequence of individual sub-steps, the so-calledelementary reactions, and
the information on the precise course of action is part of thereaction mechanism. Chemical
reactions are described withchemical equations, which graphically present the starting
materials, end products, and sometimes intermediate products and reaction conditions.
Different chemical reactions are used in combination inchemical synthesisin order to obtaina desired product. Inbiochemistry, series of chemical reactionscatalyzedbyenzymesform
metabolic pathways, by which syntheses and decompositions impossible under ordinary
conditions are performed within acell.
Equations
Chemical equationsare used to graphically illustrate chemical reactions. They consist of
chemicalorstructural formulasof the reactants on the left and those of the products on the
right. They are separated by an arrow () which indicates the direction and type of the
reaction. The tip of the arrow points in the direction in which the reaction proceeds. A doublearrow ( ) pointing in opposite directions is used forequilibrium reactions. Equations should
be balanced according to thestoichiometry, 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, CandD in a schematic example below) by the appropriate integers a, b, c
and d.[7]
More complex reactions are represented by reaction schemes, which in addition to startingmaterials and products show important intermediates ortransition states. Also, some
relatively minor additions to the reaction can be indicated above the reaction arrow; examplesof such additions are water, heat, illumination, a catalyst, etc. Similarly, some minor products
can be placed below the arrow, often with a minus sign.
An example of organic reaction:oxidationofketonestoesterswithperoxycarboxylic acid
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Retrosynthetic analysiscan be applied to design a complex synthesis reaction. Here the
analysis starts from the products, for example by splitting selected chemical bonds, to arrive
at plausible initial reagents. A special arrow () is used in retro reactions.[8]
[edit] Elementary reactions
Theelementary reactionis the smallest division into which a chemical reaction can be
decomposed to, it has no intermediate products.[9]
Most experimentally observed reactions
are built up from many elementary reactions that occur in parallel or sequentially. The actual
sequence of the individual elementary reactions is known asreaction mechanism. An
elementary reaction involves a few molecules, usually one or two, because of the low
probability for several molecules to meet at a certain time.[10]
Isomerization ofazobenzene, induced by light (h) or heat ()
The most important elementary reactions are unimolecular and bimolecular reactions. Onlyone molecule is involved in a unimolecular reaction; it is transformed by an isomerization or
adissociationin one or more other molecules. Such reaction requires addition of energy in
the form of heat or light. A typical example of a unimolecular reaction is thecistrans
isomerization, in which the cis-form of a compound converts to the trans-form or vice
versa.[11]
In a typicaldissociationreaction, a bond in a molecule splits resulting in two molecular
fragments. The splitting can behomolyticorheterolytic. In the first case, the bond is divided
so that each product retains an electron and becomes a neutralradical. In the second case,
both electrons of the chemical bond remain with one of the products, resulting in charged
ions. Dissociation plays an important role in triggeringchain reactions, such ashydrogen
oxygenorpolymerizationreactions.
Dissociation of a molecule AB into fragments A and B
For bimolecular reactions, two molecules collide and react with each other. Their merger is
calledchemical synthesisor anaddition reaction.
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 calledmetathesis.
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for example
NaCl(aq) +AgNO3(aq)NaNO3(aq) +AgCl(s)
[edit] Chemical equilibrium
Main article:Chemical equilibrium
Most chemical reactions are reversible, that is they can and do run in both directions. The
forward and reverse reactions are competing with each other and differ inreaction 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 rate of the forward reaction,
establishing the so-called chemical equilibrium. The time to reach equilibrium depends on
such parameters as temperature, pressure and the materials involved, and is determined by theminimum free energy. In equilibrium, theGibbs free energymust be zero. The pressure
dependence can be explained with theLe Chatelier's principle. For example, an increase in
pressure due to decreasing volume causes the reaction to shift to the side with the fewer
moles of gas.[12]
The reaction yield stabilized at equilibrium, but can be increased by removing the product
from the reaction mixture or increasing temperature or pressure. Change in the initial
concentrations of the substances does not affect the equilibrium.
[edit] Thermodynamics
Chemical reactions are largely determined by the laws ofthermodynamics. Reactions can
proceed by themselves if they areexergonic, that is if they release energy. The associated free
energy of the reaction is composed of two different thermodynamic quantities,enthalpyand
entropy:[13]
G: free energy, H: enthalpy, T: temperature, S: entropy, : difference
Reactions can beexothermic, where H is negative and energy is released. Typical examples
of exothermic reactions areprecipitationandcrystallization, in which ordered solids areformed from disordered gaseous or liquid phases. In contrast, inendothermicreactions, heat
is consumed from the environment. This can occur by increasing the entropy of the system,
often through the formation of gaseous reaction products, which have high entropy. Since the
entropy increases with temperature, many endothermic reactions preferably take place at high
temperatures. On the contrary, many exothermic reactions such as crystallization occur at low
temperatures. Changes in temperature can sometimes reverse the direction of a reaction, as in
theBoudouard reaction:
This reaction betweencarbon dioxideandcarbonto formcarbon monoxideis endothermic attemperatures above approximately 800 C and is exothermic below this temperature.
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Reactions can also be characterized with theinternal energywhich takes into account
changes in the entropy, volume andchemical potential. The latter depends, among other
things, on theactivitiesof the involved substances.[15]
U: internal energy, S: entropy, p: pressure, : chemical potential, n: number of molecules, d:smallchange sign
[edit] Kinetics
The speed at which a reactions takes place is studied byreaction kinetics. The rate depends
on various parameters, such as:
Reactantconcentrations, which usually make the reaction happen at a faster rate ifraised through increased collisions per unit time. Some reactions, however, have rates
that are independentof reactant concentrations. These are calledzero order reactions.
Surface areaavailable for contact between the reactants, in particular solid ones inheterogeneous systems. Larger surface areas lead to higher reaction rates.
Pressureincreasing the pressure decreases the volume between molecules andtherefore increases the frequency of collisions between the molecules.
Activation energy, which is defined as the amount of energy required to make thereaction start and carry on spontaneously. Higher activation energy implies that the
reactants need more energy to start than a reaction with a lower activation energy.
Temperature, which hastens reactions if raised, since higher temperature increases theenergy of the molecules, creating more collisions per unit time,
The presence or absence of acatalyst. Catalysts are substances which change thepathway (mechanism) of a reaction which in turn increases the speed of a reaction bylowering theactivation energyneeded for the reaction to take place. A catalyst is not
destroyed or changed during a reaction, so it can be used again.
For some reactions, the presence ofelectromagnetic radiation, most notablyultraviolet light, is needed to promote the breaking of bonds to start the reaction. This
is particularly true for reactions involvingradicals.
Several theories allow calculating the reaction rates at the molecular level. This field is
referred to as reaction dynamics. The rate v of afirst-order reaction, which could be
disintegration of a substance A, is given by:
Its integration yields:
Here k is first-order rate constant having dimension 1/time, [A](t) is concentration at a time 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 can be
described with the characteristichalf-life. More than one time constant is needed when
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describing reactions of higher order. The temperature dependence of the rate constant usually
follows theArrhenius equation:
where Ea is the activation energy and kB is theBoltzmann constant. One of the simplestmodels of reaction rate is thecollision theory. More realistic models are tailored to a specific
problem and include thetransition state theory, the calculation of thepotential energy
surface, theMarcus theoryand theRiceRamspergerKasselMarcus (RRKM) theory.[16]
[edit] Reaction types
Four basic types
Synthesis
In a synthesis reaction, two or more simple substances combine to form a more complex
substance. Two or more reactants yielding one product is another way to identify a synthesis
reaction. For example, simple hydrogen gas combined with simple oxygen gas can produce a
more complex substance, such as water.[17]
Decomposition
A decomposition reaction is the opposite of a synthesis reaction, where a more complex
substance breaks down into its more simple parts.[17][18]
Single replacement
In a single replacement reaction, a single uncombined element replaces another in a
compound.[17]
Double replacement
In a double replacement reaction, parts of two compounds switch places to form two new
compounds.[17]
This is when the anions and cations of two different molecules switch places,
forming two entirely different compounds.[18]
These reactions are in the general form:
AB + CD ---> AD + CB
One example of a double displacement reaction is the reaction of lead (II) nitrate with
potassium iodide to form lead (II) iodide and potassium nitrate:
Pb(NO3)2 + 2 KI ---> PbI2 + 2 KNO3
[edit] Oxidation and reduction
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Illustration of a redox reaction
The two parts of a redox reaction
Redoxreactions 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 inoxidation
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 involvingcovalent
bonds).[19][20]
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
(thiosulfateanion) is oxidized to S4O62
.
Which of the involved reactants would be reducing or oxidizing agent can be predicted from
theelectronegativityof their elements. Elements with low electronegativity, such as most
metals, easily donate electrons and oxidizethey are reducing agents. On the contrary, many
ions with high oxidation numbers, such asH2O2,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 fromelectron configurationof the reactant element. Elements are trying to reach the low-energy
noble gasconfiguration, and therefore alkali metals and halogens will donate and accept one
electron, respectively, and the noble gases themselves are chemically inactive.[21]
An important class of redox reactions are theelectrochemicalreactions, 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 aschlorine[22]
oraluminium. 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 batteries.
[edit] Complexation
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Ferrocenean iron atom sandwiched between two C5H5ligands
In complexation reactions, severalligandsreact with a metal atom to form acoordination
complex. This is achieved by providinglone pairsof the ligand into emptyorbitalsof the
metal atom and formingdipolar bonds. The ligands areLewis bases, they can be both ions
and neutral molecules, such as carbon monoxide, ammonia or water. The number of ligandsthat react with a central metal atom can be found using the18-electron rule, saying that the
valence shellsof atransition metalwill collectively accommodate 18electrons, whereas the
symmetry of the resulting complex can be predicted with thecrystal field theoryandligand
field theory. Complexation reactions also includeligand exchange, in which one or more
ligands are replaced by another, and redox processes which change the oxidation state of the
central metal atom.[23]
[edit] Acid-base reactions
Acid-base reactionsinvolve transfer ofprotonsfrom one molecule (acid) to another (base).
Here,acidsact as proton donors andbasesas acceptors.
Acid-base reaction, HA: acid, B: Base, A: conjugated base, HB+: conjugated acid
The associated proton transfer results in the so-calledconjugate acidand conjugate base.[24]
The reverse reaction is possible, and thus the acid/base and conjugated base/acid are always
in equilibrium. The equilibrium is determined by theacid and base dissociation constants(Ka
and Kb) of the involved substances. A special case of the acid-base reaction is the
neutralizationwhere an acid and a base, taken at exactly same amounts, form a neutralsalt.
Acid-base reactions can have different definitions depending on the acid-base conceptemployed. Some of the most common are:
Arrheniusdefinition: Acids dissociate in water releasing H3O+ ions; bases dissociatein water releasing OH ions.
Brnsted-Lowrydefinition: Acids are proton (H+) donors, bases are proton acceptors;this includes the Arrhenius definition.
Lewisdefinition: Acids are electron-pair acceptors, bases are electron-pair donors;this includes the Brnsted-Lowry definition.
[edit] Precipitation
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Precipitation
Precipitationis 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 thesolubilitylimit[25]
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 anamorphous
or microcrystalline residue and slow process can yield singlecrystals. The latter can also be
obtained byrecrystallizationfrom microcrystalline salts.[26]
[edit] Solid-state reactions
Reactions can take place between two solids. However, because of the relatively small
diffusionrates 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.[27]
[edit] Photochemical reactions
In thisPaternoBchi reaction, a photoexcited carbonyl group is added to an unexcited
olefin, yielding anoxetane.
Inphotochemical reactions, atoms and molecules absorb energy (photons) of the illumination
light and convert into anexcited state. They can then release this energy by breaking
chemical bonds, thereby producing radicals. Photochemical reactions include hydrogen
oxygen reactions,radical polymerization,chain reactionsandrearrangement reactions.[28]
Many important processes involve photochemistry. The premier example isphotosynthesis,
in which most plants use solar energy to convertcarbon dioxideand water intoglucose,
disposing ofoxygenas a side-product. Humans rely on photochemistry for the formation of
vitamin D, andvisionis initiated by a photochemical reaction ofrhodopsin.[11]
Infireflies, an
enzymein the abdomen catalyzes a reaction that results inbioluminescence.[29]
Many
significant photochemical reactions, such as ozone formation, occur in the Earth atmosphereand constituteatmospheric chemistry.
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iagram.svghttp://en.wikipedia.org/wiki/File:Chemical_precipitation_diagram.svghttp://en.wikipedia.org/wiki/File:Paterno-Buchi_reaction.svghttp://en.wikipedia.org/wiki/File:Paterno-Buchi_reaction.svghttp://en.wikipedia.org/wiki/File:Chemical_precipitation_diagram.svghttp://en.wikipedia.org/wiki/File:Chemical_precipitation_diagram.svghttp://en.wikipedia.org/wiki/File:Paterno-Buchi_reaction.svghttp://en.wikipedia.org/wiki/File:Paterno-Buchi_reaction.svghttp://en.wikipedia.org/wiki/File:Chemical_precipitation_diagram.svghttp://en.wikipedia.org/wiki/File:Chemical_precipitation_diagram.svghttp://en.wikipedia.org/wiki/Atmospheric_chemistryhttp://en.wikipedia.org/wiki/Chemical_reaction#cite_note-28http://en.wikipedia.org/wiki/Bioluminescencehttp://en.wikipedia.org/wiki/Enzymehttp://en.wikipedia.org/wiki/Fireflieshttp://en.wikipedia.org/wiki/Chemical_reaction#cite_note-rh-10http://en.wikipedia.org/wiki/Rhodopsinhttp://en.wikipedia.org/wiki/Visual_perceptionhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Carbon_dioxidehttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Chemical_reaction#cite_note-27http://en.wikipedia.org/wiki/Rearrangement_reactionhttp://en.wikipedia.org/wiki/Chain_reactionhttp://en.wikipedia.org/wiki/Radical_polymerizationhttp://en.wikipedia.org/wiki/Excited_statehttp://en.wikipedia.org/wiki/Photonhttp://en.wikipedia.org/wiki/Photochemistryhttp://en.wikipedia.org/wiki/Oxetanehttp://en.wikipedia.org/wiki/Olefinhttp://en.wikipedia.org/wiki/Paterno%E2%80%93B%C3%BCchi_reactionhttp://en.wikipedia.org/w/index.php?title=Chemical_reaction&action=edit§ion=18http://en.wikipedia.org/wiki/Chemical_reaction#cite_note-26http://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/w/index.php?title=Chemical_reaction&action=edit§ion=17http://en.wikipedia.org/wiki/Chemical_reaction#cite_note-25http://en.wikipedia.org/wiki/Recrystallization_%28chemistry%29http://en.wikipedia.org/wiki/Crystalhttp://en.wikipedia.org/wiki/Amorphoushttp://en.wikipedia.org/wiki/Chemical_reaction#cite_note-24http://en.wikipedia.org/wiki/Solubilityhttp://en.wikipedia.org/wiki/Precipitation_%28chemistry%29 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[edit] Catalysis
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 ceramiccatalytic convertersin order to
maximize their surface area. This exhaust converter is from aPeugeot 106S2 1100
Incatalysis, the reaction does not proceed directly, but through a third substance known ascatalyst. 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 includecokingwhere the catalyst becomes covered bypolymericside products.
Additionally, heterogeneous catalysts can dissolve into the solution in a solidliquid system
or evaporate in a solidgas system. Catalysts can only speed up the reactionchemicals that
slow down the reaction are called inhibitors.[30][31]
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 energycan 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 theplatinum groupmetals and other
transition metals, which are used inhydrogenations,catalytic reformingand in the synthesis
of commodity chemicals such asnitric acidandammonia. Acids are an example of a
homogeneous catalyst, they increase the nucleophilicity ofcarbonyls, 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.[32]
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[edit] 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 involvecovalent bondsbetween carbon atoms or carbon
andheteroatoms(such as oxygen, nitrogen,halogens, etc.). Many specific reactions in
organic chemistry arename reactionsdesignated after their discoverers.
[edit] Substitution
In asubstitution reaction, afunctional groupin a particularchemical compoundis replaced
by another group.[33]
These reactions can be distinguished by the type of substituting species
into anucleophilic,electrophilicorradical substitution.
SN1 mechanism
SN2 mechanism
In the first type, anucleophile, an atom or molecule with an excess of electrons and thus anegative charge orpartial 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 groupdeparts 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 arehydroxideion,alkoxides,aminesandhalides. This type of reaction is
found mainly inaliphatic hydrocarbons, and rarely inaromatic hydrocarbon. The latter have
high electron density and enternucleophilic aromatic substitutiononly with very strong
electron withdrawing groups. Nucleophilic substitution can take place by two different
mechanisms,SN1andSN2. In their names, S stands for substitution, N for nucleophilic, and
the number represents thekinetic orderof the reaction, unimolecular or bimolecular.[34]
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The three steps of anSN2 reaction. The nucleophile is green and the leaving group is red
SN2 reaction causes stereo inversion (Walden inversion)
The SN1 reaction proceeds in two steps. First, theleaving groupis eliminated creating a
carbocation. This is followed by a rapid reaction with the nucleophile.[35]
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
stereochemistryof the products. SN1 leads to the non-stereospecific addition and does not
result in a chiral center, but rather in a set ofgeometric isomers(cis/trans). In contrast, a
reversal (Walden inversion) of the previously existing stereochemistry is observed in the SN2
mechanism.[36]
Electrophilic substitutionis the counterpart of the nucleophilic substitution in that the
attacking atom or molecule, anelectrophile, has low electron density and thus a positive
charge. Typical electrophiles are the carbon atom ofcarbonyl groups, carbocations orsulfur
ornitroniumcations. This reaction takes place almost exclusively in aromatic hydrocarbons,
where it is calledelectrophilic aromatic substitution. The electrophile attack results in the so-
called -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[37]
Mechanism of electrophilic aromatic substitution
In the third type of substitution reaction, radical substitution, the attacking particle is a
radical.[33]
This process usually takes the form of achain reaction, for example in the reaction
of alkanes with halogens. In the first step, light or heat disintegrates the halogen-containing
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molecules producing the radicals. Then the reaction proceeds as an avalanche until two
radicals meet and recombine.[38]
Reactions during the chain reaction of radical substitution
[edit] Addition and elimination
Theadditionand its counterpart, theelimination, are reactions which change the number of
substituents on the carbon atom, and form or cleavemultiple bonds.Doubleandtriple 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.[39]Because of thesimilar conditions, both reactions in the E1 or E1cb elimination always compete with the SN1
substitution.[40]
E1 elimination E1cb elimination
E2 elimination
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.[41]
Electrophilic addition of hydrogen bromide
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