Halohydrin formation reaction
Encyclopedia
The halohydrin formation reaction is a chemical reaction
in which a halogen
is added to an alkene
in aqueous solution
to form a halohydrin
. The reaction is a form of electrophilic addition
; it is similar to the halogen addition reaction
.
The basic chemical equation for this reaction is as follows:
The reaction occurs with anti addition, leaving the newly added X and OH groups in a trans configuration.
When bromination is desired, N-bromosuccinimide
is preferable to bromine
because fewer side-products are produced.
of the alkene attacks the halogen. For the case where bromine is the halogen, a three-membered C, C, Br ring, known as a bromonium ion, is formed.
The addition of water, which provides the hydroxide anion, gives the desired halohydrin with high anti stereospecificity
as a consequence of the SN2 opening of the bromonium ion by hydroxide or water. Markovnikov's rule
applies: the positive charge resides on the more-substituted carbon, hence the hydroxide anion adds here.
However, in the case of bromine water reacting with cyclohexene
, the solubility of bromine in water is about 0.21 moles per litre, and the concentration of HOBr formed in the equilibrating reaction between bromine and water is about 1.15 x 10−3 moles per litre. So it is thousands of times more likely that Br2 will be the species that attacks the C=C bond.
A couple of other factors have some bearing too:
So while there can be no doubt that some halohydrin is formed when a halogen-water mixture is mixed with an alkene, the main product will be the dihalo compound. For example, when bromine water reacts with cyclohexene
, the main product is 1,2-dibromocyclohexane.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
in which 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...
is added to 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...
in aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
to form a halohydrin
Halohydrin
A halohydrin or a haloalcohol is a type of organic compound or functional group in which one carbon atom has a halogen substituent, and an adjacent carbon atom has a hydroxyl substituent. They are derived from alcohols are therefore characterized by the presence of both the hydroxyl functional...
. The reaction is a form of 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...
; it is similar to the halogen addition reaction
Halogen addition reaction
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:...
.
The basic chemical equation for this reaction is as follows:
- C=C + X2 + H2O → X-C-C-OH (X represents a halogen, either Cl or Br).
The reaction occurs with anti addition, leaving the newly added X and OH groups in a trans configuration.
When bromination is desired, N-bromosuccinimide
N-Bromosuccinimide
N-Bromosuccinimide or NBS is a chemical reagent which is used in radical substitution and electrophilic addition reactions in organic chemistry. NBS can be considered a convenient source of cationic bromine.-Preparation:...
is preferable to 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...
because fewer side-products are produced.
Reaction mechanism
In the first step, the pi bondPi 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...
of the alkene attacks the halogen. For the case where bromine is the halogen, a three-membered C, C, Br ring, known as a bromonium ion, is formed.
The addition of water, which provides the hydroxide anion, gives the desired halohydrin with high anti 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...
as a consequence of the SN2 opening of the bromonium ion by hydroxide or water. Markovnikov's rule
Markovnikov's rule
In organic chemistry, Markovnikov's rule or Markownikoff's rule is an observation based on Zaitsev's rule. It was formulated by the Russian chemist Vladimir Vasilevich Markovnikov in 1870....
applies: the positive charge resides on the more-substituted carbon, hence the hydroxide anion adds here.
However, in the case of bromine water reacting with cyclohexene
Cyclohexene
Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes...
, the solubility of bromine in water is about 0.21 moles per litre, and the concentration of HOBr formed in the equilibrating reaction between bromine and water is about 1.15 x 10−3 moles per litre. So it is thousands of times more likely that Br2 will be the species that attacks the C=C bond.
A couple of other factors have some bearing too:
- Br2 is much more soluble in the non-polar alkene than either water or HOBr.
- When the Br+ ion is attacked by the C=C bond electrons, the Br- is right on the spot to add to the other side of the bond. In this manner, steric issues are minimized.
So while there can be no doubt that some halohydrin is formed when a halogen-water mixture is mixed with an alkene, the main product will be the dihalo compound. For example, when bromine water reacts with cyclohexene
Cyclohexene
Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes...
, the main product is 1,2-dibromocyclohexane.