Editing Mesomeric effect

{{short description|Chemical polarity due to interactions between pi bonds or lone pairs}}
{{See also|Resonance (chemistry)}}

In [[chemistry]], the '''mesomeric effect''' (or '''resonance effect''') is a property of [[substituent]]s or [[functional group]]s in a [[chemical compound]]. It is defined as the [[Chemical polarity|polarity]] produced in the [[molecule]] by the interaction of two [[pi bond]]s or between a pi bond and [[lone pair]] of [[electron]]s present on an adjacent atom.<ref name=":0"/> This change in electron arrangement results in the formation of [[resonance structure]]s that [[Orbital hybridisation|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]]. This is because the [[electron transfer]] 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 (chemistry)|resonance structures]] and is symbolized by the letter '''M'''.<ref name=":1">{{Cite journal |last1=Grover |first1=Nitika |last2=Emandi |first2=Ganapathi |last3=Twamley |first3=Brendan |last4=Khurana |first4=Bhavya |last5=Sol |first5=Vincent |last6=Senge |first6=Mathias O. |date=2020-11-08 |title=Synthesis and Structure of meso-Substituted Dibenzihomoporphyrins |journal=European Journal of Organic Chemistry |volume=2020 |issue=41 |pages=6489–6496 |doi=10.1002/ejoc.202001165 |issn=1434-193X |pmc=7702178 |pmid=33328793}}</ref> The mesomeric effect is negative ('''−M''') when the substituent is an [[electron-withdrawing group]], 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]], the conjugate system will develop a more negative charge. As a result, the system under the +M effect will be more reactive towards [[electrophiles]], which can take away the negative charge, than a [[nucleophile]].{{Citation needed|date=May 2024}}

[[File:Mesomeric effect (+M) V.1.png|thumb|500px|+M effect from a methoxy ({{chem2|\sOCH3}}) substituent|center]]'''+M effect order:'''<ref name=":0">{{Cite journal |last=Murrell |first=J N |date=1955-11-01 |title=The Electronic Spectrum of Aromatic Molecules VI: The Mesomeric Effect |url=https://iopscience.iop.org/article/10.1088/0370-1298/68/11/303 |journal=Proceedings of the Physical Society. Section A |volume=68 |issue=11 |pages=969–975 |doi=10.1088/0370-1298/68/11/303 |bibcode=1955PPSA...68..969M |issn=0370-1298}}</ref>

:{{chem2|[[Oxide ion|\sO-]] > [[Amine|\sNH2]] > \sNHR > \sNR2 > [[Alcohols|\sOH]] > [[ether|\sOR]] > [[amide|\sNHCOR]] > [[ester|\sOCOR]] > [[Phenyl group|\sPh]] > [[fluorine|\sF]] > [[Chlorine|\sCl]] > [[bromine|\sBr]] > [[iodine|\sI]] > [[Nitroxyl|\sNO]]}}

=== −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.<ref name=":2">{{Cite journal |last=Chemistry (IUPAC) |first=The International Union of Pure and Applied |title=IUPAC - mesomeric effect (M03844) |url=https://goldbook.iupac.org/terms/view/M03844 |access-date=2022-10-25 |website=goldbook.iupac.org|doi=10.1351/goldbook.M03844 |doi-access=free }}</ref>

[[File:Mesomeric effect (–M) V.1.png|500x500px|thumb|−M effect from a formyl ({{chem2|\sCHO}}) substituent|center]]
'''−M effect order:'''

:{{chem2|[[Nitro compound|\sNO2]] > [[cyanide |\sCN]] > [[sulfonic acid|\sSO3H]] > [[aldehyde|\sCHO]] > [[ketone|\sCOR]] > [[Organic acid anhydride|\sCOOCOR]] > [[Ester|\sCOOR]] > [[carboxylic acid|\sCOOH]] > [[amide|\sCONH2]] > \sCOO-}}

== Mesomeric effect vs. inductive effect ==
The net electron flow from or to the substituent is determined also by the [[inductive effect]].<ref name=":2" /> The mesomeric effect as a result of ''p''-[[Atomic orbital|orbital]] overlap (resonance) has absolutely no effect on this inductive effect, as the inductive effect has purely to do with the [[electronegativity]] 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.<ref>{{Cite journal |last1=Clark |first1=D. T. |last2=Murrell |first2=J. N. |last3=Tedder |first3=J. M. |date=1963 |title=234. The magnitudes and signs of the inductive and mesomeric effects of the halogens |url=http://xlink.rsc.org/?DOI=jr9630001250 |journal=Journal of the Chemical Society (Resumed) |language=en |pages=1250–1253 |doi=10.1039/jr9630001250 |issn=0368-1769}}</ref> 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.<ref name=":3">{{Cite journal |last1=Streets |first1=D.G. |last2=Ceasar |first2=Gerald P. |date=October 1973 |title=Inductive and mesomeric effects on the π orbitals of halobenzenes |url=http://www.tandfonline.com/doi/abs/10.1080/00268977300102271 |journal=Molecular Physics |language=en |volume=26 |issue=4 |pages=1037–1052 |doi=10.1080/00268977300102271 |bibcode=1973MolPh..26.1037S |issn=0026-8976}}</ref>

The concepts of mesomeric effect, '''mesomerism''' and '''mesomer''' were introduced by [[Christopher Kelk Ingold|Ingold]] in 1938 as an alternative to [[Linus Pauling|Pauling's]] synonymous concept of resonance.<ref>{{cite journal|title=If It's Resonance, What Is Resonating? |last=Kerber |first=Robert C. |journal=[[J. Chem. Educ.]] |date=2006-02-01 |volume=83 |number=2 |page=223 |url=http://www.jce.divched.org/Journal/Issues/2006/Feb/abs223.html |doi=10.1021/ed083p223 |url-status=dead |archive-url=https://web.archive.org/web/20061004085840/http://www.jce.divched.org/Journal/Issues/2006/Feb/abs223.html |archive-date=2006-10-04 |bibcode=2006JChEd..83..223K }}</ref> "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]]. This accounts for the resonance stabilization of the molecule due to delocalization of charge.<ref name=":4">{{Citation |last=Balci |first=Metin |title=12 - Chemical Shift |date=2005-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780444518118500127 |work=Basic 1H- and 13C-NMR Spectroscopy |pages=283–292 |editor-last=Balci |editor-first=Metin |place=Amsterdam |publisher=Elsevier Science |language=en |doi=10.1016/b978-044451811-8.50012-7 |isbn=978-0-444-51811-8 |access-date=2022-10-25}}</ref>  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.<ref name=":5">{{Cite book |title=Elsevier Enhanced Reader | chapter=Chapter 2-2 - Theory of the Chemical Shift |url=https://reader.elsevier.com/reader/sd/pii/B9780080229539500119?token=5A916B90719B3D354C0B813BC4EFBA7F642C6C025D8A2D5F023C5074CCA975BAE0F8DBB064700B375C959401D6EFC297&originRegion=us-east-1&originCreation=20221025163518 |access-date=2022-10-25 |website=reader.elsevier.com | series=International Series in Organic Chemistry | date=January 1969 | pages=61–113 | publisher=Pergamon | doi=10.1016/B978-0-08-022953-9.50011-9 | isbn=9780080229539 |language=en}}</ref> 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 [[Pi electron donor-acceptor|pEDA]] 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 [[Diamagnetism|diamagnetic]] shielding in aromatics, deshielding, and acid and base strengths.<ref name=":6">{{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}}</ref>

== References ==
<references />

[[Category:Chemical bonding]]