Mesomeric effect
Encyclopedia
The mesomeric effect or resonance effect in chemistry
is a property of substituent
s or functional group
s in a chemical compound
. 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 and the substituent is an electron releasing group.
The net electron flow from or to the substituent is determined also by the inductive effect
. The mesomeric effect as a result of p-orbital
overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect is purely to do with the electronegativity
of the atoms and their topology in the molecule (which atoms are connected to which).
The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold
in 1938 as an alternative to the Pauling's
synonymous concept of resonance. "Mesomerism" in this context is often encountered in German and French literature but in English literature the term "resonance" dominates.
. This accounts for the resonance stabilization of the molecule due to delocalization of charge.
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....
is a property of substituent
Substituent
In 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 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...
s in a chemical compound
Chemical compound
A 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 and the substituent is an electron releasing group.
- Examples of -M substituents: acetylAcetylIn organic chemistry, acetyl is a functional group, the acyl with chemical formula COCH3. It is sometimes represented by the symbol Ac . The acetyl group contains a methyl group single-bonded to a carbonyl...
(IUPAC ethanoyl) - nitrileNitrileA nitrile is any organic compound that has a -C≡N functional group. The prefix cyano- is used interchangeably with the term nitrile in industrial literature. Nitriles are found in many useful compounds, one example being super glue .Inorganic compounds containing the -C≡N group are not called...
- nitroNitro compoundNitro 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 and is impure. The nitro group is one of the most common explosophores used globally... - Examples of +M substituents: alcoholAlcoholIn chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
- amineAmineAmines 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,...
-benzeneBenzeneBenzene 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....
The net electron flow from or to the substituent is determined also by the inductive effect
Inductive effect
In chemistry and physics, the inductive effect 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-orbital
Atomic orbital
An 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...
overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect is purely to do with the 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...
of the atoms and their topology in the molecule (which atoms are connected to which).
The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold
Christopher Kelk Ingold
Sir 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's
Linus Pauling
Linus Carl Pauling was an American chemist, biochemist, peace activist, author, and educator. He was one of the most influential chemists in history and ranks among the most important scientists of the 20th century...
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 systemConjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...
. This accounts for the resonance stabilization of the molecule due to delocalization of charge.
See also
- Important publications in organic chemistry