chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, the mesomeric effect (or resonance effect) is a property of

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

s or

functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...

s in a

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

. It is defined as the polarity produced in the

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

by the interaction of two

pi bond In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbital ...

s or between a pi bond and

lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC ''Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. Lone ...

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s present on an adjacent atom. This change in electron arrangement results in the formation of

resonance structure In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

s that hybridize into the molecule's true structure. The pi electrons then move away from or toward a particular substituent group. The mesomeric effect is stronger in compounds with a lower

ionization potential In physics and chemistry, ionization energy (IE) is the minimum energy required to remove the most loosely bound electron of an isolated gaseous atom, positive ion, or molecule. The first ionization energy is quantitatively expressed as :X(g) ...

. This is because the

electron transfer Electron transfer (ET) occurs when an electron relocates from an atom, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions. Electrochemical processes are ET reactio ...

states will have lower energies.

Representations of the mesomeric effect

The effect is used in a qualitative way and describes the electron withdrawing or releasing properties of substituents based on relevant

resonance structures In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

and is symbolized by the letter M. The mesomeric effect is negative (−M) when the substituent is an

electron-withdrawing group An electron-withdrawing group (EWG) is a Functional group, group or atom that has the ability to draw electron density toward itself and away from other adjacent atoms. This electron density transfer is often achieved by resonance or inductive effe ...

, and the effect is positive (+M) when the substituent is an electron donating group. Below are two examples of the +M and −M effect. Additionally, the functional groups that contribute to each type of resonance are given below.

+M effect

The +M effect, also known as the positive mesomeric effect, occurs when the substituent is an electron donating group. The group must have one of two things: a lone pair of electrons, or a negative charge. In the +M effect, the pi electrons are transferred from the group towards the conjugate system, increasing the density of the system. Due to the increase in

electron density Electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial variables and is typical ...

, the conjugate system will develop a more negative charge. As a result, the system under the +M effect will be more reactive towards

electrophiles In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that carries ...

, which can take away the negative charge, than a

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

. +M effect order: :

−M effect

The −M effect, also known as the negative mesomeric effect, occurs when the substituent is an electron-withdrawing group. In order for a negative mesomeric (−M) effect to occur the group must have a positive charge or an empty orbital in order to draw the electrons towards it. In the −M effect, the pi electrons move away from the conjugate system and towards the electron drawing group. In the conjugate system, the density of electrons decreases and the overall charge becomes more positive. With the −M effect the groups and compounds become less reactive towards electrophiles, and more reactive toward nucleophiles, which can give up electrons and balance out the positive charge. −M effect order: :

Mesomeric effect vs. inductive effect

The net electron flow from or to the substituent is determined also by the

inductive effect In Organic chemistry, the inductive effect in a molecule is a local change in the electron density due to electron-withdrawing or electron-donating groups elsewhere in the molecule, resulting in a permanent dipole in a bond. It is present in a ...

. The mesomeric effect as a result of ''p''- orbital overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect has purely to do with the

electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...

of the atoms and their topology in the molecule (which atoms are connected to which). Specifically the inductive effect is the tendency for the substituents to repel or attract electrons purely based on electronegativity and not dealing with restructuring. The mesomeric effect however, deals with restructuring and occurs when the electron pair of the substituents shift around. The inductive effect only acts on alpha carbons, while the mesomeric utilizes pi bonds between atoms. While these two paths often lead to the similar molecules and resonance structures, the mechanism is different. As such, the mesomeric effect is stronger than the inductive effect. The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold in 1938 as an alternative to 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.

Mesomerism in conjugated systems

Mesomeric effect can be transmitted along any number of carbon atoms in a

conjugated system In physical organic chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in a molecule, which in general lowers the overall energy of the molecule and increases Chemical stability, stability. It is Reson ...

. This accounts for the resonance stabilization of the molecule due to delocalization of charge. It is important to note that the energy of the actual structure of the molecule, i.e. the resonance hybrid, may be lower than that of any of the contributing canonical structures. The difference in energy between the actual inductive structure and the (most stable contributing structures) worst kinetic structure is called the resonance energy or resonance stabilization energy. For the quantitative estimation of the mesomeric/resonance effect strength various substituent constants are used, i.e. Swain-Lupton resonance constant, Taft resonance constant or Oziminski and Dobrowolski

pEDA Peda () or Pera is an Indian sweet that originated in the city of Mathura, Uttar Pradesh, India. Traditionally prepared as thick, semi-soft round balls, its main ingredients are khoa, sugar and traditional flavourings including cardamom seeds. ...

parameter. Additionally, the resulting resonance structures can give the molecule properties that are not inherently evident from looking at one structure. Some of these properties include different reactivities, local

diamagnetic Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagn ...

shielding in aromatics, deshielding, and acid and base strengths.{{Cite journal , last1=Peter , first1=K. , last2=Vollhardt , first2=C. , date=January 1978 , title=A Review of: "The Place of Transition Metals in Organic Synthesis. Ed. D. W. Slocum. Annals of The New York Academy of Sciences, Volume 295, New York, N.Y., 1977, XXIV + 282 pp. $3 2.00" , url=http://dx.doi.org/10.1080/00945717808057443 , journal=Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry , volume=8 , issue=5–6 , pages=505–506 , doi=10.1080/00945717808057443 , issn=0094-5714, url-access=subscription

References

Chemical bonding