The
mesomeric effect or
resonance effect in
chemistryChemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...
is a property of
substituentIn organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
s or
functional groupIn 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...
s in a
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter
M. The mesomeric effect is negative (
-M) when the substituent is an electron-withdrawing group and the effect is positive (
+M) when based on resonance the substituent is an electron releasing group.
- Examples of -M substituents: acetyl
In organic chemistry, acetyl is a functional group, the acyl with chemical formula COCH3. It is sometimes abbreviated as Ac . The acetyl group contains a methyl group single-bonded to a carbonyl...
(IUPAC ethanoyl) - nitrileA nitrile is any organic compound which has a -C≡N functional group. The -C≡N functional group is called a nitrile group. In the -CN group, the carbon atom and the nitrogen atom are triple bonded together. The prefix cyano is used in chemical nomenclature to indicate the presence of a nitrile...
- nitroNitro compounds are organic compounds that contain one or more nitro functional groups . They are often highly explosive, especially when the compound contains more than one nitro group. The presence of impurities or improper handling can trigger a violent exothermic decomposition...
- Examples of +M substituents: alcohol
In chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
- amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
The net electron flow from or to the substituent is determined also by the
inductive effectThe inductive effect in chemistry is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction...
.
The
mesomeric effect or
resonance effect in
chemistryChemistry is the science concerned with the composition, behavior, structure, and properties of matter, as well as the changes it undergoes during chemical reactions...
is a property of
substituentIn organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
s or
functional groupIn 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...
s in a
chemical compoundA chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
. The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant resonance structures and is symbolized by the letter
M. The mesomeric effect is negative (
-M) when the substituent is an electron-withdrawing group and the effect is positive (
+M) when based on resonance the substituent is an electron releasing group.
- Examples of -M substituents: acetyl
In organic chemistry, acetyl is a functional group, the acyl with chemical formula COCH3. It is sometimes abbreviated as Ac . The acetyl group contains a methyl group single-bonded to a carbonyl...
(IUPAC ethanoyl) - nitrileA nitrile is any organic compound which has a -C≡N functional group. The -C≡N functional group is called a nitrile group. In the -CN group, the carbon atom and the nitrogen atom are triple bonded together. The prefix cyano is used in chemical nomenclature to indicate the presence of a nitrile...
- nitroNitro compounds are organic compounds that contain one or more nitro functional groups . They are often highly explosive, especially when the compound contains more than one nitro group. The presence of impurities or improper handling can trigger a violent exothermic decomposition...
- Examples of +M substituents: alcohol
In chemistry, an alcohol is any organic compound in which a hydroxyl group is bound to a carbon atom of an alkyl or substituted alkyl group. An important group of acohols is formed by the simple acyclic alcohols, the general formula for which is C
nH
2n+1OH...
- amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
The net electron flow from or to the substituent is determined also by the
inductive effectThe inductive effect in chemistry is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction...
. The mesomeric effect as a result of
p-
orbitalAn atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons, in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus. These functions...
overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect is purely to do with the
electronegativityElectronegativity, symbol χ, is a chemical property that describes the ability of an atom to attract electrons towards itself in a covalent bond. An atom's electronegativity is affected by both its atomic weight and the distance that its valence electrons reside from the charged nucleus...
of the atoms and their structural chemistry (which atoms are connected to which).
The concepts of mesomeric effect,
mesomerism and
mesomer were introduced by
IngoldSir Christopher Kelk Ingold FRS was a British chemist based in Leeds and London. His groundbreaking work in the 1920s and 1930s on reaction mechanisms and the electronic structure of organic compounds was responsible for the introduction into mainstream chemistry of concepts such as nucleophile,...
in 1938 as an alternative to the
Pauling'sLinus Carl Pauling was an American chemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists in any field of the 20th century. Pauling was among the first scientists to work in the fields of quantum...
synonymous concept of resonance. "Mesomerism" in this context is often encountered in German and French literature but in English literature the term "resonance" dominates.
Mesomerism in conjugated systems
Mesomeric effect can be transmitted along any number of carbon atoms in a
conjugated systemA conjugated system occurs in an organic compound where atoms covalently bond with alternating single and multiple bonds and influence each other to produce a region called electron delocalization. In this region electrons do not belong to a single bond or atom, but rather a group...
. This accounts for the resonance stabilization of the molecule due to delocalization of charge.
See also
- Important publications in organic chemistry