Halogen addition reaction
Encyclopedia
A halogen addition reaction is a simple organic reaction
where a halogen
molecule is added to the carbon–carbon double bond of an alkene
functional group
.
The general chemical formula
of the halogen addition reaction is:
(X represents the halogen
s bromine
or chlorine
, and in this case, a solvent could be CH2Cl2
or CCl4
). The product is a vicinal
dihalide.
This type of reaction is a halogenation
and an electrophilic addition
.
for an alkene bromination can be described as follows. In the first step of the reaction, a bromine molecule approaches the electron-rich alkene
carbon–carbon double bond. The bromine atom closer to the bond takes on a partial positive charge as its electron
s are repelled by the electrons of the double bond.
The atom is electrophilic
at this time and is attacked by the pi electrons of the alkene [carbon–carbon double bond]. It forms for an instant a single sigma bond
to both of the carbon atoms involved. The bonding of bromine
is special in this intermediate, due to its relatively large size compared to carbon
, the bromide ion
is capable to interacting with both carbons which once shared the π-bond
, making a three-membered ring. The bromide ion acquires a positive formal charge. At this moment the halogen ion
is called a "bromonium ion" or "chloronium ion", respectively.
When the first bromine atom attacks the carbon–carbon pi-bond, it leaves behind one of its electrons with the other bromine that it was bonded to in Br2. That other atom is now a negative bromide
anion and is attracted to the slight positive charge on the carbon atoms. It is blocked from nucleophilic attack on one side of the carbon chain by the first bromine atom and can only attack on the other side. As it attacks and forms a bond with one of the carbons, the bond between the first bromine atom and the other carbon atoms breaks, leaving each carbon atom with a halogen substituent.
In this way the two halogens add in an anti addition fashion, and when the alkene is part of a cycle the dibromide adopts the trans configuration. For maximum overlap of the C-Br σ* antibonding
molecular orbital (the LUMO
, shown to the right in red) and the nucleophile (X−) lone pair
(the HOMO
, shown to the right below in green), X− must attack the bromonium ion from behind, at carbon.
This reaction mechanism was proposed by Roberts and Kimball in 1937. With it they explained the observed stereospecific trans-additions in brominations of maleic acid
and fumaric acid
. Maleic acid with a cis-double bond forms the dibromide as a mixture of enantiomer
s:
while the trans-isomer fumaric acid forms a single meso compound
:
The reaction is even stereospecific in alkenes with two bulky tert-butyl groups in a cis-position as in the compound cis-di-tert-butylethylene. Despite the steric repulsion present in the chloronium ion, the only product formed is the anti-adduct.
.
For reactions taking place through this mechanism no stereospecificity
is expected and indeed not found.
Roberts and Kimball in 1937 already accounted for the fact that brominations with the maleate ion resulted in cis-addition driven by repulsion between the negatively charged carboxylic acid anions being stronger than halonium ion formation. In alkenes such as anethole
s and stilbene
s the substituents are able to stabilize the carbocation by donating electrons at the expense of the halonium ion.
Halonium ions can be identified by means of NMR spectroscopy
. In 1967 the group of George A. Olah obtained NMR spectra of tetramethylethylenebromonium ions by dissolving 2,3-dibromo-2,3-dimethylbutane in magic acid
at −60°C. The spectrum for the corresponding fluorine
compound on the other hand was consistent with a rapidly equilibrating
pair of β-fluorocarbocations.
Organic reaction
Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, photochemical reactions and redox reactions. In organic synthesis,...
where a halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
molecule is added to the carbon–carbon double bond of an alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
.
The general chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
of the halogen addition reaction is:
- C=C + X2 → X−C−C−X
(X represents the halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
s bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
or chlorine
Chlorine
Chlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
, and in this case, a solvent could be CH2Cl2
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
or CCl4
Carbon tetrachloride
Carbon tetrachloride, also known by many other names is the organic compound with the formula CCl4. It was formerly widely used in fire extinguishers, as a precursor to refrigerants, and as a cleaning agent...
). The product is a vicinal
Vicinal (chemistry)
In chemistry vicinal stands for any two functional groups bonded to two adjacent carbon atoms. For example the molecule 2,3-dibromobutane carries two vicinal bromine atoms and 1,3-dibromobutane does not....
dihalide.
This type of reaction is a halogenation
Halogenation
Halogenation is a chemical reaction that incorporates a halogen atom into a molecule in substitution of hydrogen atom. Halogenation takes place in the gas phase. There are four types of halogenation: fluorination, chlorination, bromination, and iodination...
and an electrophilic addition
Electrophilic addition
In organic chemistry, an electrophilic addition reaction is an addition reaction where, in a chemical compound, a π bond is broken and two new σ bonds are formed...
.
Reaction mechanism
The reaction mechanismReaction mechanism
In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs.Although only the net chemical change is directly observable for most chemical reactions, experiments can often be designed that suggest the possible sequence of steps in...
for an alkene bromination can be described as follows. In the first step of the reaction, a bromine molecule approaches the electron-rich alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
carbon–carbon double bond. The bromine atom closer to the bond takes on a partial positive charge as its electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s are repelled by the electrons of the double bond.
| Bromine addition to alkene reaction mechanism | A bromide ion attacks the C-Br σ* antibonding molecular orbital of a bromonium ion |
The atom is electrophilic
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...
at this time and is attacked by the pi electrons of the alkene [carbon–carbon double bond]. It forms for an instant a single sigma bond
Sigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
to both of the carbon atoms involved. The bonding of bromine
Bromine
Bromine ") is a chemical element with the symbol Br, an atomic number of 35, and an atomic mass of 79.904. It is in the halogen element group. The element was isolated independently by two chemists, Carl Jacob Löwig and Antoine Jerome Balard, in 1825–1826...
is special in this intermediate, due to its relatively large size compared to carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, the bromide ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
is capable to interacting with both carbons which once shared the π-bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
, making a three-membered ring. The bromide ion acquires a positive formal charge. At this moment the halogen ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
is called a "bromonium ion" or "chloronium ion", respectively.
When the first bromine atom attacks the carbon–carbon pi-bond, it leaves behind one of its electrons with the other bromine that it was bonded to in Br2. That other atom is now a negative bromide
Bromide
A bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
anion and is attracted to the slight positive charge on the carbon atoms. It is blocked from nucleophilic attack on one side of the carbon chain by the first bromine atom and can only attack on the other side. As it attacks and forms a bond with one of the carbons, the bond between the first bromine atom and the other carbon atoms breaks, leaving each carbon atom with a halogen substituent.
In this way the two halogens add in an anti addition fashion, and when the alkene is part of a cycle the dibromide adopts the trans configuration. For maximum overlap of the C-Br σ* antibonding
Antibonding
Antibonding is a type of chemical bonding. An antibonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei...
molecular orbital (the LUMO
Lumo
Lumo is a 2007 documentary film about twenty-year-old Lumo Sinai, a woman who fell victim to "Africa's First World War." While returning home one day, Lumo and another woman were gang-raped by a group of soldiers fighting for control of the Democratic Republic of the Congo during the 1994 Rwandan...
, shown to the right in red) and the nucleophile (X−) lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
(the HOMO
Homo
Homo may refer to:*the Greek prefix ὅμο-, meaning "the same"*the Latin for man, human being*Homo, the taxonomical genus including modern humans...
, shown to the right below in green), X− must attack the bromonium ion from behind, at carbon.
This reaction mechanism was proposed by Roberts and Kimball in 1937. With it they explained the observed stereospecific trans-additions in brominations of maleic acid
Maleic acid
Maleic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Maleic acid is the cis-isomer of butenedioic acid, whereas fumaric acid is the trans-isomer...
and fumaric acid
Fumaric acid
Fumaric acid or trans-butenedioic acid is the chemical compound with the formula HO2CCH=CHCO2H. This white crystalline compound is one of two isomeric unsaturated dicarboxylic acids, the other being maleic acid. In fumaric acid the carboxylic acid groups are trans and in maleic acid they are cis...
. Maleic acid with a cis-double bond forms the dibromide as a mixture of enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...
s:
while the trans-isomer fumaric acid forms a single meso compound
Meso compound
A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereocenters it is not chiral. A meso compound is superimposable on its mirror image, and it does not produce a ""...
:
The reaction is even stereospecific in alkenes with two bulky tert-butyl groups in a cis-position as in the compound cis-di-tert-butylethylene. Despite the steric repulsion present in the chloronium ion, the only product formed is the anti-adduct.
β-Halocarbocations
In an alternative reaction scheme depicted below the reactive intermediate is a β-bromocarbocation or β-bromocarbonium ion with one of the carbon atoms a genuine carbocationCarbocation
A carbocation is an ion with a positively-charged carbon atom. The charged carbon atom in a carbocation is a "sextet", i.e. it has only six electrons in its outer valence shell instead of the eight valence electrons that ensures maximum stability . Therefore carbocations are often reactive,...
.
For reactions taking place through this mechanism no stereospecificity
Stereospecificity
In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one of the stereoisomers."Overlap Control of Carbanionoid Reactions. I. Stereoselectivity in Alkaline...
is expected and indeed not found.
Roberts and Kimball in 1937 already accounted for the fact that brominations with the maleate ion resulted in cis-addition driven by repulsion between the negatively charged carboxylic acid anions being stronger than halonium ion formation. In alkenes such as anethole
Anethole
Anethole is a phenylpropene, a type of aromatic compound that occurs widely in nature, in essential oils...
s and stilbene
Stilbene
-Stilbene, is a diarylethene, i.e., a hydrocarbon consisting of a trans ethene double bond substituted with a phenyl group on both carbon atoms of the double bond. The name stilbene is derived from the Greek word stilbos, which means shining....
s the substituents are able to stabilize the carbocation by donating electrons at the expense of the halonium ion.
Halonium ions can be identified by means of NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. In 1967 the group of George A. Olah obtained NMR spectra of tetramethylethylenebromonium ions by dissolving 2,3-dibromo-2,3-dimethylbutane in magic acid
Magic acid
Magic acid , is a superacid consisting of a mixture, most commonly in a 1:1 molar ratio, of fluorosulfonic acid and antimony pentafluoride...
at −60°C. The spectrum for the corresponding fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
compound on the other hand was consistent with a rapidly equilibrating
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...
pair of β-fluorocarbocations.