Editing Mesomeric effect (section)

== 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-}}