mesomeric effect and inductive effect

mesomeric effect and inductive effect

The mesomeric effect or resonance effect in chemistry is a property of substituents or functional groups in a chemical compound. 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 based on resonance and the substituent is an electron releasing group.

Examples of -M substituents: acetyl (IUPAC ethanoyl) - nitrile - nitro

Examples of +M substituents: alcohol - amine-benzene

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 is purely to do with the electronegativity of the atoms and their topology in the molecule (which atoms are connected to which).

The concepts of mesomeric effect, mesomerism and mesomer were introduced by Ingold in 1938 as an alternative to the Pauling's synonymous concept of resonance.[1] "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.

The electron withdrawing or releasing effect attributed to a substituent through delocalization of p or π electrons, which can be visualized by drawing various canonical forms, is known as mesomeric effect or resonance effect. It is symbolized by M or R.

Negative resonance or mesomeric effect (-M or -R): It is shown by substituents or groups that withdraw electrons by delocalization mechanism from rest of the molecule and are denoted by -M or -R. The electron density on rest of the molecular entity is decreased due to this effect.

E.g. -NO₂, Carbony group (C=O), -C≡N, -COOH, -SO₃H etc.

Positive resonance or mesomeric effect (+M or +R):  The groups show  positive mesomeric effect when they release electrons to the rest of the molecule by delocalization. These groups are denoted by +M or +R. Due to this effect, the electron density on rest of the molecular entity is increased.

 E.g. -OH, -OR, -SH, -SR, -NH₂, -NR₂ etc.

ILLUSTRATIONS & APPLICATIONS OF RESONANCE EFFECT (OR) MESOMERIC EFFECT

1) The negative resonance effect (-R or -M) of carbonyl group is shown below. It withdraws electrons by delocalization of π electrons and reduces the electron density particularly on 3rd carbon.

2) The negative mesomeric effect (-R or -M) shown by cyanide group in acrylonitrile is illustrated below. The electron density on third carbon decreases due to delocalization of π electrons towards cyanide group.

Because of negative resonance effect, the above compounds act as good micheal acceptors.

3) The nitro group, -NO₂, in nitrobenzene shows -M effect due to delocalization of conjugated π electrons as shown below. Note that the electron density on benzene ring is decreased particularly on ortho and para positions.

 This is the reason for why nitro group deactivates the benzene ring towards electrophilic substitution reaction.

4) In phenol, the -OH group shows +M effect due to delocalization of lone pair on oxygen atom towards the ring. Thus the electron density on benzene ring is increased particularly on ortho and para positions.

Hence phenol is more reactive towards electrophilic substitution reactions. The substitution is favored more at ortho and para positions. 

5) The -NH₂ group in aniline also exhibits +R effect. It releases electrons towards benzene ring through delocalization. As a result, the electron density on benzene ring increases particularly at ortho and para positions. Thus aniline activates the ring towards electrophilic substitution.

It is also worth mentioning that the electron density on nitrogen in aniline decreases due to delocalization which is the reason for its less basic strength when compared to ammonia and alkyl amines.

Inductive effect

In chemistry and physics, the 'Inductive Effect' is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule. The permanent dipole induced in one bond by another is called inductive effect. The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge (δ–) and the other atom has a slight positive charge (δ+).

If the electronegative atom is then joined to a chain of atoms, usually carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the -I effect.

Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. This is electron releasing character and is indicated by the +I effect. In short, alkyl groups tend to give electrons, leading to induction effect.

As the induced change in polarity is less than the original polarity, the inductive effect rapidly dies out. Therefore, the effect is significant only over a short distance. The inductive effect is permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other stronger factors may overshadow this effect.

Relative inductive effects have been experimentally measured with reference to hydrogen:

(Decreasing order of - I effect or increasing order of + I effect)

—NR₃ > —NO₂ > —SO₂R > —CN > —COOH > —F > —Cl > —Br > —I > —OR > —COR > —OH > —C₆H₅ > —CH=CH₂ > —H

Also the inductive effect is dependent on the distance between the substituent group and the main group that react. That is, as the distance of the substituent group increases the Inductive effect weakens or decreases.

Inductive effects can be measured through the Hammett equation.

The inductive effect can also be used to determine the stability of a molecule depending on the charge present on the atom and the groups bonded to it. For example, if an atom has a positive charge and is attached to a −I group its charge becomes 'amplified' and the molecule becomes more unstable. Similarly, if an atom has a negative charge and is attached to a +I group its charge becomes 'amplified' and the molecule becomes more unstable. But, contrary to the above two cases, if an atom has a negative charge and is attached to a −I group its charge becomes 'de-amplified' and the molecule becomes more stable than if I-effect was not taken into consideration. Similarly, if an atom has a positive charge and is attached to a +I group its charge becomes 'de-amplified' and the molecule becomes more stable than if I-effect was not taken into consideration. The explanation for the above is given by the fact that more charge on an atom decreases stability and less charge on an atom increases stability.

The inductive effect also plays a vital role in deciding the acidity and basicity of a molecule. Groups having +I effect attached to a molecule increases the overall electron density on the molecule and the molecule is able to donate electrons, making it basic. Similarly groups having -I effect attached to a molecule decreases the overall electron density on the molecule making it electron deficient which results in its acidity. As the number of -I groups attached to a molecule increases, its acidity increases; as the number of +I groups on a molecule increases, its basicity increases.

Aliphatic carboxylic acids. The strength of a carboxylic acid depends on the extent of its ionization: the more ionized it is, the stronger it is. As an acid becomes stronger, the numerical value of its pKa drops. In aliphatic acids, the electron-releasing inductive effect of the methyl group increases the electron density on oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization. Greater ionization in formic acid when compared to acetic acid makes formic acid (pKa=3.75) stronger than acetic acid (pKa=4.76). Monochloroacetic acid (pKa=2.82), though, is stronger than formic acid, since the electron-withdrawing effect of chlorine promotes ionization.

Aromatic carboxylic acids. In benzoic acid, the carbon atoms which are present in the ring are sp2 hybridised.As a result, benzoic acid(pKa=4.20) is a stronger acid than cyclohexane carboxylic acid(pKa=4.87). Also, electron-withdrawing groups substituted at the ortho and para positions, enhance the acid strength.

Dioic acids. Since the carboxyl group is itself an electron-withdrawing group, the dioic acids are, in general, stronger than their monocarboxyl analogues.

In the so-called Baker–Nathan effect the observed order in electron-releasing alkyl substituents is apparently reversed.

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