Mesomeric effect

See also: Resonance (chemistry)

The mesomeric effect is the polarity created between atoms of a conjugated system via electron transfer or pi–bond electron transfer. In simple terms, the mesomeric effect happens when electrons in a conjugated orbital system move away from or towards a substituent group. The effect, which is symbolised by the letter M, is used in a qualitative way to explain the electron withdrawing or releasing properties of substituents based on relevant resonance structures. Here, we will discuss the mesomeric effect, its type, and their order, as well as the resonance effect. We will also look into the differences between inductive and mesomeric effects. Mesomeric Effect OrderThere are two types of mesomeric effects namely-(+M) i.e. positive mesomeric effect, which is when an electron-donating  group is attached to the chain/ring.(-M) i.e. negative mesomeric effect, which is when an electron-withdrawing group is attached to the chain/ring.+M Effect Order :−O−>−NH2>−OR>−NHCOR>−OCOR>−Ph>−CH3>−F>−Cl>−Br>−I−O−>−NH2>−OR>−NHCOR>−OCOR>−Ph>−CH3>−F>−Cl>−Br>−I-\mathrm{O}^{-}>-\mathrm{NH}_{2}>-\mathrm{OR}>-\mathrm{NHCOR}>-\mathrm{OCOR}>-\mathrm{Ph}>-\mathrm{CH}_{3}>-\mathrm{F}>-\mathrm{Cl}>-\mathrm{Br}>-\mathrm{I} -M Effect Order :−NO2>−CN>−SO3H>−CHO>−COR>−COOCOR>−COO>−COOH>−CONH2>−COO−−NO2>−CN>−SO3H>−CHO>−COR>−COOCOR>−COO>−COOH>−CONH2>−COO−\begin{align} &-\mathrm{NO}_{2}>-\mathrm{CN}>-\mathrm{SO}_{3} \mathrm{H}>-\mathrm{CHO}>-\mathrm{COR}>-\mathrm{COOCOR}>-\mathrm{COO }>-\mathrm{COOH}>-\mathrm{CONH}_{2}> \\ &-\mathrm{COO}^{-} \end{align}Resonance EffectWhen a single structure cannot represent all the properties of a molecule, then more than one different structure is used to explain all these properties, known as resonating structures or canonical form.Resonating structures are hypothetical.The real structure is somewhere between all these resonance structures and is called a resonance hybrid. 

 Resonance Structure of Vinyl ChloridePositive Mesomeric EffectThe (+M) effect or positive mesomeric effect occurs when electrons or pi electrons are moved from a specific group to a conjugate system, hence boosting the electron density of the conjugated system.The electrons are here because of the lone pair or (–ve) charge in the first atom's p orbital are given to the conjugated system at a distance. Further linked atoms or groups will increase the +M effect if they are pushed by the +I or +M effect, but will lower the +M effect if they are pushed by the -I or -M effect.The order of the +M impact between the following groups, for example, is−NH−CH3>−NH2>−NH−COCH3>−N(CH3)2−NH−CH3>−NH2>−NH−COCH3>−N(CH3)2-\mathrm{NH}-\mathrm{CH}_{3}>-\mathrm{NH}_{2}>-\mathrm{NH}-\mathrm{COCH}{ }_{3}>-\mathrm{N}\left(\mathrm{CH}_{3}\right)_{2}
   
   
   
   
 Example of Positive Mesomeric EffectWhat is Resonance Mesomeric Effect?DefinitionResonance Effect- The interaction between lone electron pairs and bond electron pairs causes resonance, which characterises the polarity of a molecule. Mesomeric Effect- The influence of substituted or functional groups on chemical compounds is known as the mesomeric effect.The primary distinction between resonance and mesomeric effect is that resonance is caused by the interaction of lone electron pairs and bond electron pairs, whereas mesomeric effect is caused by the presence of substituents or functional groups.Difference Between Inductive effect and Mesomeric effectSr-NoInductive EffectMesomeric Effect1.Distance dependent.Distance independent.2.Vanishes after four carbon chains.Operates up to the end of the conjugate system.3.Involves displacement of sigma electrons.Involves delocalization of pi and lone pair electrons.4.Operates in both saturated and unsaturated compounds.Operates only in unsaturated and conjugated compounds.5.Permanent effect.Temporary effect.Mesomeric Effect Examples
   
   
   
   
 Example of +M effect
   
   
   
   
 Example of -M effect
   
   
   
   
 Example of -M Effect
   
   
   
   
 Example of +M EffectResonance Effect and Its ExamplesLet’s explain the resonance effect with an example. A group of two or more Lewis structures that collectively represent the electronic bonding of a single polyatomic species, including fractional bonds and fractional charges, is known as a resonance structure. Resonance structures can be used to describe delocalized electrons that can't be described by a single Lewis formula with an integer number of covalent bonds.Resonance structure of carbonate  ion (CO3-2) The carbonate ion's electronic structure, like that of ozone, is not represented by a single Lewis electron structure. Unlike O3, however, CO32-  real structure is a composite of three resonance structures.We put carbon in the middle since it is the least electronegative element.There are 4 valence electrons in carbon, 6 valence electrons in each oxygen, and 2 extra for the 2 charges  4 + 3×63×63\times6 + 2 = 24 valence electrons are obtained.Between the oxygen atoms and the carbon atoms, six electrons are employed to form three bonding pairs.By placing three lone pairs on each oxygen atom and marking the 2 charges, we split the remaining 18 electrons evenly among the three oxygen atoms.The core atom has no electrons left.Since the carbon atom only has 6 valence electrons at this point, we must use one lone pair from oxygen to build a carbon–oxygen double bond. However, in this scenario, three options are available.
   
   
   
   
 Example of Resonance Effect of Carbonate Ion.ConclusionOne of the features of functional groups or substituents within a chemical molecule is mesomeric effects. It is defined as the polarity of a molecule formed by the interaction of two pi bonds or two pi bonds and lone electron pairs on a nearby atom. The effect is defined in terms of electron-withdrawing behaviour and is discussed qualitatively. The symbol “M” is used to denote the mesomeric effect.Mesomeric effects are negative (-M) for substituents belonging to electron-drawing groups and positive (+M) for substituents belonging to electron-donating groups

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 and is symbolized by the letter M. 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.

+M effect from a methoxy (−OCH₃) substituent

+M effect order:

−O⁻ > −NH₂ > −NHR > −NR₂ > −OH > −OR > −NHCOR > −OCOR > −Ph > −F > −Cl > −Br > −I > −NO

-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 from a formyl (−CHO) substituent

-M effect order:

−NO₂ > −CN > −SO₃H > −CHO > −COR > −COOCOR > −COOR > −COOH > −CONH₂ > −COO⁻

Mesomeric effect vs. inductive effect

The net electron flow from or to the substituent is determined also by the inductive effect. 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 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. 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 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 shielding in aromatics, deshielding, and acid and base strengths.