The resonance is a chemical phenomenon which is observed in certain organic compounds possessing double bonds at a suitable position. Certain organic compounds can be represented by more than one structure and they differ only in the position of
bonding and lone pair of electrons. Such structures are called resonance structures (canonical structures) and this phenomenon is called resonance. This phenomenon is also called mesomerism or mesomeric effect.
For example, the structure of aromatic compounds such as benzene and conjugated systems tike 1,3 - butadiene cannot be represented by a single structure, and their observed properties can be explained on the basis of a resonance hybrid.
In 1, 3 buta diene, it is expected that the bond between C₁ - C₂ and C₃ - C₄ should be shorter than that of C₂ - C₃, but the observed bond lengths are of same. This property cannot be explained by a simple structure in which two \(\pi\) bonds localised between C₁ - C₂ and C₃ - C₄ . Actually the \(\pi\) electrons are delocalised

These resonating structures are called canonical forms and the actual structure lies between these three resonating strucfures, and is called a resonance hybrid. The resonance hybrid is represented
Similar to the other electron displacement effect, mesomeric effect is also classified into positive mesomeric effect (+M or +R) and negative mesomeric effect (-M of -R) based on the nature of the functional group present adjacent to the multiple bond.
Positive Mesomeric Effect :
Positive resonance effect occurs, when the electrons move away from substituent attached to the conjugated system. It occurs, if the electron releasing substituents are attached to the conjugated system. In such cases, the attached group has a tendency to release electrons through resonance. These electron releasing groups are usually denoted as +R or +M groups.
Examples : -OH, -SH, -OR, -SR, -NH₂, -O- etc...
Negative Mesomeric Effect :
Negative resonance effect occurs, when the electrons move towards the substituent attached to the conjugated system. It occurs if the electron withdrawing substituents are attached to the conjugated system.

In such cases, the attached group has a tendency to withdraw electrons through resonance. These electron withdrawing groups are usually denoted as -R or -M groups.
Examples : NO₂, > C = O, -COOH, -C = N etc......
Resonance is useful in explaining certain properties such as acidity oi phenol. The phenoxide ion is more stabilised than phenol by resonance effect (+M effect) and hencp resonance tavours ionisation of phenol to form H⁺ and shows acidity.
The structures shows that there is a charge separation in the resonance structure of phenol which needs energy, where as there is no such hybrid structures in the case of phenoxide ion. This increased stability accounts for the acidic character of phenol.

Electromeric is a temporary effect which operates in unsaturated compounds (containing >C = C <, > C = O, etc...) in the presence of an attacking reagent.
Let us consider two different compounds
(i) Compounds containing carbonyl group (> C = O) and
(ii) Unsaturated compounds such as alkenes (> C = C <).
When a nucleophile approaches the carbonyl compound, the \(\pi\) electrons between C and O is instantaneously shifted to the more electronegative oxygen. This makes the carbon electron deficient and thus facilitating the formation of a new bond between the incoming nucleophile and the carbonyl carbon atom.

On the other hand When an electrophile such as H⁺ approaches an alkene molecule, the π electrons are instantaneously shified to the electrophile and a new bond is formed between carbon and hydrogen. is makes the other carbon electron decient and hence it acquires a positive charge.

The electromeric effect, is denoted as E effect. Like the inductive effect, the electromeric effect is also classified as +E and -E based on the direction in which the pair of electron is transfered to form a new bond with the attacking agent.
When the π electron is transfer red towards the attacking reagent, it is called + E (positive electromeric) effect.
The addition of H+ to alkene as shown above is an example of +E effect.
When the π electron is transfered away from the attacking reagent, it is called, -E (negativc electromeric) effect.
The attack of CN- on a carbonyl carbon, as shown above, is an example of -E effect.

a) Electrophiles:
Electrophiles are reagents that are attracted towards negative charge or electron rich center. They are either positively charged ions or electron deficient neutral molecules. All Lewis acids act as electrophiles.
Neutral molecules like SnCl₄ can also act as an electrophile, as it has vacant d-orbitals which can accommodate the electrons from others.

b) Nucleophiles:
Nucleophiles are reagents that has high affinity for electro positive centers. They possess an atom has an unshared pair of electrons, and hence it is in search for an electro positive centre where it can have an opportunity to share its elections to form a covalent bond, and gets stabilised.
They are usually negatively charged ions or electron rich neutral molecules (contains one or more lone pair of electrons). AII Lewis bases act as nucleophiles.

Inductive effect (I):
(i) Inductive effect is defined as the change in the polarisation of a covalent bond due to the presence of adjacent bonds, atoms or groups in the molecule. This is a permanent phenomenon.
(ii) Let us explain the inductive effect by considering ethylchloride as example. The C-C bond in ethyl chloride is polar.
We know that chlorine is more electronegative than carbon, and hence it attracts the shared pair of electron between C-Cl in ethyl chloride towards itself. is develops a slight negative charge on Chlorine and a slight positive charge on carbon to which chlorine is attached.
To compensate it, the C₁ draws the shared pair of electron between itself and. C₂ . This polarisation effect is called inductive effect.
(iii) The magnitude of the charge separation decreases rapidly, as we move away from C₁ and is observed maximum for 2 carbons and almost insignicant after 4 bonds from the active group.
\(\overset { \delta }{ C } \overset { \delta + }{ { H }_{ 3 } } \longrightarrow \underset { 1 }{ C\overset { \delta + }{ { H }_{ 2 } } } \twoheadrightarrow \overset { \delta - }{ C } { l }_{ 2 }\)
It is important to note that the inductive effect does not transfer electrons from one atom to another but the displacement effect is permanent. The inductive effect represents the ability of a particular atom or a group to either withdraw or donate electron density to the attached carbon. Based on this ability the substituents are classified as +I groups and -I groups. Their ability to release or withdraw the electron through sigma covalent bond is called +I effect and -I effect respectively.
Highly electronegative atoms and atoms of groups which are carry a positive charge are electron withdrawing or -I group
Example : -F, -CI, -COOH, -NO₂, NH₂,
Higher the electronegativity of the substitutent, greater is the -I effect.
The order of the -I effect of some groups are given below :
NH³⁺ > NO₂ > CN > SO₃H > CHO > CO > COOH > COCI > CONH₂ > F > Cl > Br > I > OH > OR, NH₂ > C₆H₅ > H
Highly electropositive atoms and atoms are groups which carry a negative charge are electron donating or +I groups.
Example. Alkali metals, alkyl groups such as methyl, ethyl, negatively charged groups such as CH₃O^-, C₂H₅O^-, COO^- etc.
Lesser the electronegativity of the elements, greater is the +I effect. The relative order of +I effect of some alkyl groups is given below
\(-\mathrm{C}\left(\mathrm{CH}_3\right)_3>-\mathrm{CH}\left(\mathrm{CH}_3\right)_2>-\mathrm{CH}_2 \mathrm{CH}_3>-\mathrm{CH}_3\)
Let us understand the influence of inductive effect on some properties of organic compounds.
Reactivity :
When a highly electronegative atom such as halogen is attached to a carbon then it makes the C-X bond polar. In such cases the -I effect of halogen facilitates the attack of an incoming nucleophile at the polarised carbon, and hence increases the reactivity.

If a - I group is attached nearer to a carbonyl carbon, it decreases the availability of electron density-on the carbonyl carbon, and hence increases the rate of the nucelophilic addition reaction.
Aciditv of carborvlic acids :
When a halogen atom is attached to the carbon which is nearer to the carboxylic acid group, its -I effect withdraws the bonded electrons towards itself and makes the ionisation of H⁺ easy. The acidity of various chloro acetic acid is in the following order. The strength of the acid increases with increase in the -I effect of the group attached to the carboxyl group.
Tiichloro acetic acid > Dichloro acetic acid > Chloro acetic acid > acetic acid