Zaitsev's rule
Encyclopedia
In chemistry
, Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich Zaitsev
(number of different spellings due to the name being transliterated from Russian, За́йцев) 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. In general, the compound that has a more highly substituted C=C
double bond
is more stable.
Alexander M. Zaitsev was a professor at the University of Kazan (Russia) in the late 19th century. In 1875 he put forth a generalization about the regioselectivity of β elimination reaction
s from alcohols.
He said, "The alkene
formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents."
Another way to state it is as follows: in elimination reactions, the major reaction product is the alkene
with the more highly substituted (more stable) double bond
. This most-substituted alkene is also the most stable. Under thermodynamic conditions β elimination occurs to form the most stable alkene. This principle is similar to Markovnikov's rule
which applies to the reverse process, addition reaction
s.
The textbooks offer different explanations for the physical origin of the rule. The more-substituted double bond is more stable due to the electron donating properties of the alkyl group. The effect can also be attributed to hyperconjugation
with a stabilizing interaction between the HOMO
of the alkyl group with the LUMO
of the double bond. In terms of orbital hybridization it is noted that C(sp2)-C(sp3) bonds are stronger than C(sp3)-C(sp3) bonds. The higher stability of substituted double bonds can also be inferred from heats of formation
. A dominant stabilizing hyperconjugation effect of 6 kcal/mole per alkyl group can be found by computation
The Zaitsev rule is correct only when there are no other substituents beside carbon and hydrogen. Once other atoms are added; electronegativity
, resonance
, and other factors complicate the situation and invalidate the rule.
forms.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, Zaitsev's rule, Saytzeff's rule or Saytsev's rule named after Alexander Mikhailovich Zaitsev
Alexander Mikhailovich Zaitsev
Alexander Mikhaylovich Zaitsev , also spelled as Zaitsev, Saytzeff, and Saytzev , was a Russian chemist from Kazan...
(number of different spellings due to the name being transliterated from Russian, За́йцев) is a rule that states that if more than one alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
can be formed during dehalogenation by an 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...
, the more stable alkene is the major product. In general, the compound that has a more highly substituted C=C
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...
double bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
is more stable.
Alexander M. Zaitsev was a professor at the University of Kazan (Russia) in the late 19th century. In 1875 he put forth a generalization about the regioselectivity of β 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...
s from alcohols.
He said, "The alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
formed in greatest amount is the one that corresponds to removal of the hydrogen from the β-carbon having the fewest hydrogen substituents."
Another way to state it is as follows: in elimination reactions, the major reaction product is the alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
with the more highly substituted (more stable) 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...
. This most-substituted alkene is also the most stable. Under thermodynamic conditions β elimination occurs to form the most stable alkene. This principle is similar to 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....
which applies to the reverse process, addition reaction
Addition reaction
An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one....
s.
The textbooks offer different explanations for the physical origin of the rule. The more-substituted double bond is more stable due to the electron donating properties of the alkyl group. The effect can also be attributed to hyperconjugation
Hyperconjugation
In organic chemistry, hyperconjugation is the interaction of the electrons in a sigma bond with an adjacent empty non-bonding p-orbital or antibonding π orbital or filled π orbital, to give an extended molecular orbital that increases the stability of the system...
with a stabilizing interaction between 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...
of the alkyl group with 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...
of the double bond. In terms of orbital hybridization it is noted that C(sp2)-C(sp3) bonds are stronger than C(sp3)-C(sp3) bonds. The higher stability of substituted double bonds can also be inferred from heats of formation
Heat of formation group additivity
Heat of formation group additivity methods in thermochemistry enable the calculation and prediction of heat of formation of organic compounds based on additivity. This method was pioneered by S. W...
. A dominant stabilizing hyperconjugation effect of 6 kcal/mole per alkyl group can be found by computation
The Zaitsev rule is correct only when there are no other substituents beside carbon and hydrogen. Once other atoms are added; electronegativity
Electronegativity
Electronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
, resonance
Resonance (chemistry)
In chemistry, resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula...
, and other factors complicate the situation and invalidate the rule.
Steric Hindrance
During the elimination reaction, the base that causes the double bond to form has to be sterically unhindered for the reaction to follow Zaitsev. If the base, for example, is (CH3)3CONa, the bulkiness prohibits the base from pulling the proton off the most substituted carbon. A less-hindered carbon atom is chosen and the Hofmann ProductHofmann elimination
Hofmann elimination is a process where an amine is reacted to create a tertiary amine and an alkene by treatment with excess methyl iodide followed by treatment with silver oxide, water, and heat.After the first step, a quaternary ammonium iodide salt is created...
forms.