I have been in the classes of two different teachers of organic chemistry and they both agree on the fact that there is a difference between the mesomeric effect and resonance effect (they didn't agree mutually, I just compared their answers) but both of them tell the differences which are unique or simply don't match. So I decided to find the difference via research. I searched online and in the books but answers there also vary. Here are some findings of my search

  1. Answer by @NotEvans (here) mentions that in the gold book of IUPAC, they have been taken as synonym terms.
  2. Answer by @Raju (here) mentions the difference as "Resonance refers to delocalization of electrons in a given system. The mesomeric effect is the electron-donating or withdrawing nature of a substituent due to resonance." The answer matches @KshitizSharma answered on the link where @NotEvans answered.
  3. A page on the website Pediaa mentions the difference to be "Resonance is the effect that describes the polarity of a molecule that is induced by the interaction between lone electron pairs and bond electron pairs. The mesomeric effect is the effect of substituents or functional groups on chemical compounds."
  4. There are several other places but as expected, all their answers differ

My one teacher let's say A (don't want to disclose the name) says that the Resonance effect is a broader umbrella term that includes the mesomeric effect. In resonance, we have delocalization of electrons and the delocalization can be of several types such as pi-electron delocalization, sigma electron delocalization, dancing resonance, etc (these phenomena are shown in images below) but in mesomeric effect, we can only have pi-electron delocalization.

Dancing Resonance Dancing Resonance in methyl cyclopropyl carbocation

Sigma Bond Resonance sigma bond resonance in HCl

Pi Electron Resonance Pi electron resonance in benzene

Now my other teacher, let's say teacher B sort of agrees with the definition and difference of Resonance and Mesomeric Effect given on the website Pediaa which is mentioned above in point 3.

My opinion (not conclusion) on all of this is that technically my teacher A seems to be correct to me. But I can't decide and the definition on the IUPAC book confuses me even more so I came here to ask for help. Thanks.

Notice: The website called Pediaa might have its maintenance going on because of which the server seems down.So please believe my mentioned words, they have been exactly copied.


1 Answer 1


Have you read the poem "Five Blind Men and the Elephant", where each person tries to explore an elephant and comes with their own version. Someone touches elephant's trunk and calls it a snake, someone touches the legs and calls it a tree trunk and so on. The world of molecules and sub-atomic world is indeed that elephant, and chemists are unfortunately unable to see them- to see what is happening. Hence each one comes up with an intelligent story only to be improved or discarded later. So the lesson is that don't take everything literally (incuding the statements made by high school or college teachers). Moreover, do not rely on the web-answers including this one ultimate facts.

In the same way the concept of resonance was developed really a long time ago. Keep in mind that none of the structure you draw with resonance reflects a reality. It is a subtle and an esoteric way of human expression that we do not know how it looks like in reality. X-rays allow you see the arrangement of solids in space but it does not let you see electrons, there is no physical arrow pushing going on and there no electron dance going on. It is all human imagination.

Think of the classic benzene example, (image from Wikipedia). None of the structures reflect a reality, hence the term resonance hybrid, because benzene does not chemically behave as if its double bonds were "static". The C-C bond distances in the ring have to be different, otherwise.

enter image description here

There is a very nice article by Robert C. Kerber titled If It’s Resonance, What Is Resonating? in the Journal of Chemical Education (It is freely available on Google Scholar). Have you wondered why the name "Resonance"?

History The use of multiple structures to represent compounds with (what we now call) delocalized bonding (4) was pioneered in Germany by Arndt (5) and in Britain by Ingold (6). Their ideas were based essentially on chemical intuition. In the United States, Pauling $(7,8)$ in his pioneering applications of the principles of quantum mechanics to chemistry, came to a similar description from a different starting point. Whereas the concepts of describing a single substance in terms of multiple structures may not have differed significantly, the terminology did. Arndt coined the term $Zwischenstufe$ for the hybrid structure, while Ingold opted for the equivalent term (derived from Greek) mesomer. He proposed for the concept the terms mesomeric effect or mesomerism. The cognate term mesomerie came into use in French and German. Pauling preferred the term resonance, derived from valence bond theory (see below). Pauling and Ingold were well aware of each other's terminology, and each offered reasons for rejecting the alternative proposal: Because the resonating system does not have a structure intermediate between those involved in the resonance, but instead a structure which is further changed by the resonance stabilization, I prefer not to use the word 'mesomerism,' suggested by Ingold, for the resonance phenomenon.

  • L. Pauling (9) Pauling describes the phenomenon under the name 'resonance,' which, as is well known, is based on the mathematical analogy between mechanical resonance and the behavior of wave functions in quantum mechanical exchange phenomena. There appears, however, to be some possibility that this method of description may suggest an analogy which has never been intended.
  • C. K. Ingold (6) Pauling's argument conflates the concepts of structure and energy. Moreover, his chosen term resonance is derived from an analogy to coupled oscillators, which also lacks energetic implications: "There is no close classical analogue of resonance energy" (10). So neither term includes explicit reference to energy considerations, weakening Pauling's argument. For use in discussing the structure (i.e., geometry) of the hybrid, Ingold's argument seems the more persuasive. His recognition of the gratuitous oscillatory aspect of the analogy is prophetic of the subsequent experience of generations of students.*

Your next query on mesomerism and IUPAC. In terms of terminology, most chemists follow the IUPAC recommendation's on terminologies. You can call mesomerism as a type of resonance. See this reference "Introductory Organic Chemistry and Hydrocarbons: A Physical Chemistry Approach" by Caio Lima Firme on pg 111-112.

As Ingold himself defined: "Mesomerism is an extension of valency theory (that is the classical valence bond theory) and, like all valency theory, is founded in the quantum theory (...). The fundamental wave-property in the theory of valency is resonance $-$ the resonance of connected standing waves; their mutual perturbation replaces these waves by new standing waves" (Ingold 1938). Ingold exemplified the resonance theory by using the two resonance structures: $\mathrm{R}_{2} \mathrm{~N}-\mathrm{CH}=\mathrm{N}^{+} > \mathrm{R}_{3}$ and $\mathrm{R}_{3}^{+} > \mathrm{N}=\mathrm{C}(\mathrm{H})-\mathrm{NR}_{2}$ with their own standing waves which resonate to form the real molecule. Ingold stated that the term mesomerism was given to account for the special importance of resonance in organic chemistry. He also said: "When mesomerism was first recognized as a general phenomenon in organic chemistry, it was appreciated as an electron displacement than as an energy disappearance" (Ingold 1938). From this point on, we can say that mesomerim is equivalent to resonance type 3 (resonance involving only charged and/or neutral covalent structures), although IUPAC recognizes mesomerism as essentially synonymous with resonance (which could include all types of resonance). Nonetheless, IUPAC adds that mesomerism is "particularly associated with the picture of $\pi$-electrons as less localized in an actual molecule than in a Lewis formula” (IUPAC Gold Book). Then, mesomeric state is the same as resonance hybrid and energy of mesomerism is synonymous to resonance energy (Ingold 1938).

  • $\begingroup$ Thank you, seems like it's all the result of conflict between Ingold and Pauling. $\endgroup$
    – Ritanshu
    Sep 4, 2021 at 20:55
  • 1
    $\begingroup$ Again, just recall the blind men and the elephant (molecule)! Both were highly respected chemists. $\endgroup$
    – AChem
    Sep 4, 2021 at 21:04
  • $\begingroup$ Yes, I understand $\endgroup$
    – Ritanshu
    Sep 4, 2021 at 21:55

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