What are Electronic Effects?

The effect of electrons that are located in the chemical bonds within the atoms of the molecule is termed an electronic effect. The electronic effect is also explained as the effect through which the reactivity of the compound in one portion is controlled by the electron repulsion or attraction producing in another portion of the molecule.

An electronic effect impacts the reactivity, properties, and structure of the compound. The relation between the electronic structure and geometry of the compound can also be emphasized by the term ‘stereo electronic effect’.

Types of electronic effects

The electronic effects are divided into four different types which include,

• Inductive effect.
• Mesomeric effect or resonance effect.
• Electromeric effect.
• Hyperconjugation.

Inductive effect

In inductive effect, a permanent dipole is produced in the compound by the nonequivalent distribution of the bonding electrons of the two atoms (different atoms) in a molecule. Inductive effect mainly arises in sigma bonds. The polarization created in this effect is permanent.

Subdivisions of inductive effect

The inductive effect could be categorized into two types which are,

• -I effect.
• +I effect.

-I effect

If an electronegative atom like halogen is introduced to the chain, it consists of carbon atoms that will create a +ve charge because of the unequal distribution of electrons, and this+ve charge is transmitted via the chain.

This will create a permanent dipole moment in the compound where the electron-rich atom contains a -ve charge. This is termed the electron-withdrawing inductive effect or –I effect.

+I effect

With the addition of an electron releasing substituent, for example, an alkyl substituent to the chain containing atoms of C, the charge will be relayed via the chain. This is the +I effect.

Prediction of basicity and acidity using inductive effect

The basicity and acidity of the molecule could be determined by using the inductive effect. The acidity of the compound is decreased by the electron-donating substituent and is increased due to the electron-withdrawing substituent.

Example

When we consider RCOO^-, the electron releasing substituent is considered R, then the conjugate base is destabilized due to inter-electronic repulsion. When an electron-withdrawing substituent is considered R, then the conjugate base is stabilized through the delocalization of the formed negative charge.

Mesomeric effect

Due to the transfer of π electrons, polarity is formed within the atoms of the resonance system. This effect is termed a mesomeric effect. This effect occurs if the π electrons move away or towards the substituent in the conjugated orbital arrangement. The figure below depicts the mesomeric effect.

The mesomeric effect could be categorized as follows:

• +M effect.
• -M effect.

+M effect

The density of electrons in a conjugate structure is increased when the π electrons from a certain group towards the conjugate system. This conjugate system shows less reactivity towards the nucleophile and more reactivity towards the electrophiles. This is named as +M effect. The figure below depicts the +M effect.

In order to undergo a mesomeric effect, the group must contain a –ve charge or lone pair of electrons.

The substituents which show the positive mesomeric effects are,

-NH, -NH₂, -NHR, -NR₂ -O, -OH, -OR, -F, -Cl, -O-COR, -NHCOR, -SH, -SR, etc.

-M effect

The density of electrons in a conjugate structure is decreased when the π electrons are shifted from a conjugate structure to a certain group. In –M effect, the compound shows less reactive in the presence of electrophile and highly reactive in the presence of nucleophile. This is named called as –M effect. The figure below depicts the –M effect.

In order to undergo a mesomeric effect, the substituent must contain either a +ve  charge or vacant orbital.

The substituents which show –M effect are,

-NO₂,-CN,-COX,-SO₃H,-CHO,-CONH₂,-COR,-COOH,-COOR, etc.

Electromeric effect

The electromeric effect is the effect where an absolute shift of a π electron pair occurs due to the impact of a nucleophile or an electrophile. The electronic effect is a reversible reaction. During the withdrawal of the attacking reagent, this effect will disappear. This effect is possible only if there is an electron striking reagent. The electromeric effect is happening in organic compounds that have at least one multiple bond. If an atom participating in these multiple bonds belongs to the controlling of the striking reagent, one π bonding pair of electrons is completely moved to one of the 2 atoms.

Subdivisions of electromeric effect

The electromeric effect is categorized into two classes, which depend upon the direction by which the electron pair is transferred. The two types are,

• +E effect.
• -E effect.

+E effect

If the electron pair located in the bond is transferred towards the striking reagent, then that is named as +E effect. +E effect is caused due to the addition of acid into alkenes. This effect usually occurs if the striking reagent is an electrophile and the π electrons are moved into the +vely charged atoms. For example, during the protonation of ethene, the +E effect will occur, which is shown as follows:

-E Effect

If the electron pair located in the π bond is transferred away from the striking reagent, then that is termed as the –E effect. In the case of the –E effect, the striking reagent connects itself to the+vely charged atom located in the compound i.e the atom that loses the pair of an electron in the transfer. If the striking reagent is a nucleophile, then the –E effect will occur. –E effect occurs in the introduction of nucleophiles into the carbonyl compounds. The figure below indicates the –E effect.

Hyperconjugation

Hyperconjugation is the effect where the localization of the electrons of the C-H group of the alkyl substituent is directly joined to the atom of the unsaturated compound. This hyperconjugation is mainly helpful in the carbocation stabilization and it helps in the +ve charge distribution. Hence, the higher the number of alkyl substituents connected to the atom of carbon-containing positive charge, the higher will be the hyperconjugation, and it will result in greater carbocation stabilization. Based on the hyperconjugation, the carbocation stability is indicated in the figure as follows:

Comparison of steric effect with electronic effect

The steric effect is a nonbonding reaction while the electronic effect is the bonding interaction. This is a major variation between the steric effect and the electronic effect. The electronic effect expresses the effect of electrons that are located in the chemical bonds within the atoms of the molecule whereas the steric effect expresses the impacts of the electrons that are not participating in the chemical bonding but occurs due to the lone pair of electrons or nonbonding electrons. The steric effect is contrasted by the electronic effect implying the impacts of the effects like conjunction, induction, electrostatic interaction, orbital symmetry, and spin state.  

Context and Applications

This topic is significant for all undergraduates and postgraduates especially for Bachelors and Masters in Chemistry.

Practice Problems

Question 1: Among the following, the effect which shows the permanent electron displacement is

a) Electromeric

b) Inductive

c) Inductomeric

d) All the above

Answer: Option (b) is correct.

Explanation: Inductive effect is defined as the effect in where a permanent dipole is produced in a molecule because of the nonequivalent sharing of the bonding electrons of the two unlike atoms in the compound. The electron displacement which is occurring in the inductive effect is permanent whereas the electron replacement happens in electromeric and inductomeric effects are temporary.

Question 2: Among the following, which is the resonance effect?

1) Electromeric effect

2) Mesomeric effect

3)Inductomeric effect

4) Inductive effect

Answer: Option 2 is correct.

Explanation: The other name of the mesomeric effect is the resonance effect. It is a permanent effect and it operates in the subtances having at least 1 double bond.

Question 3: Among the following, which is the temporary effect that is brought into play at the need of attacking reagent?

1. Mesomeric effect
2. Inductive effect
3. Inductometric effect
4. Electromeric effect

Answer: Option 4 is correct.

Explanation: Among the given effects, the Electromeric effect is the temporary effect that is brought into play at the need of attacking reagent. This effect is referred to as the molecular polarizability effect happening by the intramolecular electron displacement.

Question 4: Greater the number of hyperconjugation, the stability of free radicals will

1. Decrease
2. Remains same
3. Increase
4. None of the above

Answer: Option 3 is correct.

Explanation: The hyperconjugation and the stability of free radicals are directly proportional to each other. Hence the greater the number of hyperconjugation, the stability of the free radicals will increase.

Question 5: Choose the right statement from the following,

1) –I effect is weaker than the conjugate effect

2) –I effect is stronger than the conjugate effect

3) Both effects are the same

4) None of the above

Answer: Option 1 is correct.

Explanation: -I effect is weaker than the conjugate effect because the conjugate effect is the effect where MOs are conjugated to new MOs which are greatly delocalized and hence usually lower in energy.