Dehydrohalogenation
Encyclopedia
Dehydrohalogenation is an organic reaction
from which an alkene
is obtained from an alkyl halide . It is also called a β-Elimination reaction and is a type of elimination reaction
.
In this reaction, the halogen
on the carbon in the alkyl halide reacts with hydrogen present on the β-carbon releasing hydrogen halide
. Hence an alkene is formed due to formation of a double bond
between the α- and β-carbon. Zaitsev's rule
helps to explain regioselectivity
for this reaction type. The opposite transformation is called hydrohalogenation
.
, giving ethene. Likewise, 1-chloropropane and 2-chloropropane give propene.
Chlorobenzene
does not react with potassium hydroxide due to the presence of the benzene
ring, which, due to stabilization as a result of aromaticity
, does not give conventional elimination, as it would lead to a very unstable benzyne intermediate.
with potassium hydroxide would compete with an Sn2 nucleophilic substitution
reaction (minor product) because OH- is a strong, sterically unhindered nucleophile
. Perhaps a better reagent would be a deprotonated alcohol such as potassium tert-butoxide
([CH3]3CO- K+) or, because it is an oxide
anion and thus a strong base
and nucleophile
, potassium ethoxide (CH3CH2O- K+), and because of steric hindrance, thereby promoting an elimination
mechanism instead of a substitution
mechanism.
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,...
from which 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...
is obtained from an alkyl halide . It is also called a β-Elimination reaction and is a type of elimination reaction
Elimination reaction
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism...
.
In this reaction, 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...
on the carbon in the alkyl halide reacts with hydrogen present on the β-carbon releasing hydrogen halide
Hydrogen halide
Hydrogen halides are acids resulting from the chemical reaction of hydrogen with one of the halogen elements , which are found in Group 17 of the periodic table. Astatine is not included in the list because it is very rare, unstable and not found as the acid in substantial quantities...
. Hence an alkene is formed due to formation of a double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
between the α- and β-carbon. Zaitsev's rule
Zaitsev's rule
In chemistry, Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich Zaitsev is a rule that states that if more than one alkene can be formed during dehalogenation by an elimination reaction, the more stable alkene is the major product...
helps to explain regioselectivity
Regioselectivity
In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions. It can often apply to which of many possible positions a reagent will affect, such as which proton a strong base will abstract from an organic molecule, or where...
for this reaction type. The opposite transformation is called hydrohalogenation
Hydrohalogenation
A hydrohalogenation reaction is the electrophilic addition of hydrohalic acids like hydrogen chloride or hydrogen bromide to alkenes to yield the corresponding haloalkanes....
.
Reactions
Here ethyl chloride reacts with potassium hydroxide dissolved in ethanolEthanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
, giving ethene. Likewise, 1-chloropropane and 2-chloropropane give propene.
Chlorobenzene
Chlorobenzene
Chlorobenzene is an aromatic organic compound with the chemical formula C6H5Cl. This colorless, flammable liquid is a common solvent and a widely used intermediate in the manufacture of other chemicals.-Uses:...
does not react with potassium hydroxide due to the presence of the benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
ring, which, due to stabilization as a result of aromaticity
Aromaticity
In organic chemistry, Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone. The earliest use of the term was in an article by August...
, does not give conventional elimination, as it would lead to a very unstable benzyne intermediate.
Promoting elimination
In general, the above reaction of haloalkaneHaloalkane
The haloalkanes are a group of chemical compounds derived from alkanes containing one or more halogens. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely used commercially and, consequently, are known under many chemical and...
with potassium hydroxide would compete with an Sn2 nucleophilic substitution
Nucleophilic substitution
In organic and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive...
reaction (minor product) because OH- is a strong, sterically unhindered nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
. Perhaps a better reagent would be a deprotonated alcohol such as potassium tert-butoxide
Potassium tert-butoxide
Potassium tert-butoxide is the chemical compound with the formula 3COK. This colourless solid is a strong base useful in organic synthesis. It exists as a tetrameric cubane-like cluster...
([CH3]3CO- K+) or, because it is an oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
anion and thus a strong base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
and nucleophile
Nucleophile
A nucleophile is a species that donates an electron-pair to an electrophile to form a chemical bond in a reaction. All molecules or ions with a free pair of electrons can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases.Nucleophilic describes the...
, potassium ethoxide (CH3CH2O- K+), and because of steric hindrance, thereby promoting an elimination
Elimination
- Science and medicine :*Elimination reaction, an organic reaction in which two functional groups split to form an organic product*Elimination, clearance of a drug or other foreign agent from the body...
mechanism instead of a substitution
Substitution
Substitution may refer to:- Sciences :* Substitution , a syntactic transformation on strings of symbols of a formal language* Substitution of variables* Substitution cipher, a method of encryption...
mechanism.