Field Effect in Organic Chemistry

Inductive Effect

It is an electron delocalisation effect via σ bonds that arises due to the difference in electronegativities. For example, in a σ bonded organic compound like C-C-C-Cl, the carbon attached to the chlorine atom can be referred to as the α-carbon, and the one adjacent to that carbon as the ß-carbon and so on.

Now, since chlorine is more electronegative than carbon, it withdraws the electrons that are present via the σ bond toward itself, thereby making Cα fractionally positive. Since it is devoid of electrons, Cα, now being slightly electropositive than Cß, pulls the sigma-bonded electrons of Cα-Cß bond toward itself, and in this process, it makes Cß slightly electropositive.

The electron-withdrawing effect of the chlorine atom is transmitted through the carbon chain via the σ bonds. This transmission of charges decreases rapidly with the number of intervening σ bonds. We can practically ignore this effect beyond Cß.

The arrow is pointed more towards the electronegative atom. If a group withdraws the electron from carbon, it makes carbon slightly electropositive. Such groups are called -I groups, and the effect is termed -I effect. For example, -Cl, -Br, -CN and -NO₂ are -I groups.

Groups that release electrons towards carbon are termed +I groups, and the effect is termed as +I inductive effect in organic chemistry. For example, alkyl groups like -CH3 are +I groups.

Electromeric Effect

It is the temporary delocalisation of π-electrons in a compound containing multiple covalent bonds. It is important to note that it is only a temporary effect, that is, it occurs only when a reagent is added. The Electromagnetic Effect in organic chemistry can be classified into two types:

Positive Electromeric Effect
Negative Electromeric Effect

Positive Electromeric Effect

When the π-electrons are given to the attacking reagent, for example, the reactions alkenes and alkynes mostly occur via +E, this reaction is also called electrophilic addition.

Positive Electromeric Effect

The proton adds at C-1 as the π-electrons were given to the attacking reagent (H⁺). This results in the formation of a carbocation.

Negative Electromeric Effect

When the π-electrons are shifted to a more electronegative atom (O, N, S) joined via multiple bonds, it is called the negative electromeric effect. For example, the reactions of aldehydes and ketones occur predominantly by the -E effect. It is also called nucleophilic addition.

Negative Electromeric Effect

The CN- ion adds to the C atom of the carboxy group opposite to the movement of the π-electron cloud.

Mesomeric Effect

Molecules possessing sigma bonds and pi-bonds alternatively exhibit the mesomeric effect. The effect is exhibited due to the permanent delocalisation of π-bonds. This increases the number of resonating structures which makes the molecules of organic chemistry more stable. Such kind of a system, where there are alternative sigma and pie bonds, is called conjugated.

Types of Mesomeric Effect

Positive mesomeric effect
Negative mesomeric effect:

Positive Mesomeric Effect

This effect is exhibited when the direction of the delocalisation of electrons is away from the position where the group is attached. Normally, groups having a lone pair of electrons attached to a conjugated system push electrons into the conjugated system, that is, away from them.

Groups in organic chemistry showing positive mesomeric effect (+M) effect are -OH, -OR, -NH₂, -SH, -X, etc.

Negative Mesomeric Effect

This effect is exhibited when the direction of the delocalisation of electrons is towards the position where the group is attached. These are generally electron-withdrawing groups of organic chemistry.

Resonance Effect

For certain molecules like carbonate ion (CO₃^2-), one single Lewis structure would not be enough to explain all of the properties. In that case, the molecule is said to have more than one structure.

Each of those structures can explain some of the properties but not all of the properties. The actual structure of the molecule is a hybrid of all the possible structures (canonical forms). This phenomenon is called resonance in organic chemistry. If resonance occurs, each bond would be both a single bond and a double bond at the same time, i.e. the bond order would lie between one and two.

Resonating structures should fulfil the following criteria:

All atoms should have the same positions in all the structures.
There should have the same number of paired and unpaired electrons.
The structures should have almost the same energies.

Note: Canonical forms do not have any existence in reality.

Resonance Energy

The energy difference between the most stable canonical form and the resonance hybrid is known as Resonance Energy. The more the resonance energy, the more the stability.

Rules for finding out the most stable canonical form:

The canonical form with no charges is the most stable
The canonical form with more number of covalent bonds is more stable
The canonical form where unlike charges are in close proximity are more stable
If they are to be charged, the negative charge should be on an electronegative atom. Then, this canonical form is said to be more stable.