9
$\begingroup$

While studying electron displacement effects in organic chemistry, I read that the Resonance Effect and Mesomeric Effect are the same. Every source I checked used the heading or opening "The Resonance (or Mesomeric) Effect is..."

However, I was able to find the difference between Resonance Structures and Mesomeric Structures is and found out that the two differ in how they depict conjugated pi bonding. The Mesomeric Structures are all the compounds with all possible arrangements or the pi bonds drawn separately. The Resonance Structure is one diagram that uses dashes and lines to denote the partial double bond characters in the compound.

So my understanding from this difference between the structures is that the effects must only differ in how they are depicted. But I was unable to find a source that could help confirm this. Can someone please confirm or explain it for me?

$\endgroup$
11
$\begingroup$

The IUPAC gold book (IUPAC are the organisation which 'define' standards for things such as chemical nomenclature) describes them as being essentially synonymous. The following definitions are given (link):

Resonance In the context of chemistry, the term refers to the representation of the electronic structure of a molecular entity in terms of contributing structures. Resonance among contributing structures means that the wavefunction is represented by 'mixing' the wavefunctions of the contributing structures. The concept is the basis of the quantum mechanical valence bond methods. The resulting stabilization is linked to the quantum mechanical concept of 'resonance energy'. The term resonance is also used to refer to the delocalization phenomenon itself.

Mesomerism Essentially synonymous with resonance. The term is particularly associated with the picture of π-electrons as less localized in an actual molecule than in a Lewis formula. The term is intended to imply that the correct representation of a structure is intermediate between two or more Lewis formulae.

There does seem to have been some historic difference that has largely been reconciled into a single definition however one use of Mesomerism that still gets commonly used is the definition of groups as having a positive (+M) or negative (-M) effect to describe the effect of a group on a conjugated system.

The Mesomeric Structures are all the compounds with all possible arrangements or the pi bonds drawn separately. The Resonance Structure is one diagram that uses dashes and lines to denote the partial double bond characters in the compound.

This statement is not entirely correct. The 'Mesomeric' structures you talk about are usually refered to as resonance structures/forms. The depiction with the dashed lines and partial bonds is a resonance hybrid.

enter image description here

$\endgroup$
1
$\begingroup$

Mesomerism and resonance are identical, completely fungible, used interchangeably. Hückel-Lewis-Projection Method: A “Weights Watcher” for Mesomeric Structures: Y. Carissan, D. Hagebaum-Reignier, N. Goudard, S. Humbel J. Phys. Chem. A. 2008, 112, 13256–13262

$\endgroup$
-1
$\begingroup$

Yes, they're almost synonymous. The only minute difference between them is this:

Mesomerism = donation/withdrawal of electrons

Resonance = delocalization of electrons

$\endgroup$

protected by andselisk Oct 5 at 3:31

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.