Electronic Effects: Saturated hydrocarbons are non-polar because the \({\rm{C – C}}\) bond is non-polar and has negligible electronegativity difference in \({\rm{C – H}}\) bonds. However, the presence of certain substituents in the saturated hydrocarbons imparts polarity to the carbon chain, leading to different behaviour than what would be expected in a non-polar version of the compound, where no sections were electron-rich or electron-deficient.

Elements with higher electronegativity, such as oxygen or halogens, can alter the electron density around an organic molecule and make the molecule less stable and more reactive. However, electronic effects tend to stabilise these molecules, make a molecule more likely to react in the desired fashion, or affect the acidity or basicity. Let’s learn some of these electronic effects and how they influence the mechanisms of a chemical reaction.

Inductive Effect

The inductive effect is a permanent effect in which the polarisation of a σ-bond is caused by the polarisation of an adjacent σ-bond. 

In a σ bond between two unlike atoms having electronegativity difference in an organic molecule, the electron density is more inclined towards the atom having higher electronegativity than the other. This uneven distribution of electrons between atoms having electronegativity differences causes bond polarisation and affects the adjacent bonds.

The inductive effect is a distance-dependent phenomenon and is illustrated as shown:

\({{\rm{C}}^{\partial {\rm{ + }}}}{\rm{ – }}{{\rm{X}}^{\partial – }}\)

The atom \({\rm{X}}\) is more electronegative than the carbon atom and acquires a slightly negative charge \(\left( {\partial – } \right),\) and the carbon atom a slightly positive charge \(\left( {\partial + } \right),\) which means the bond is polarised:

If the electronegative atom X is connected to a chain of carbon atoms, then the positive charge is relayed to the neighbouring carbon atoms. \({{\rm{C}}_{\rm{1}}},\) the carbon atom adjacent to the \({\rm{X}}\) atom, with its positive δ charge, exerts a pull on the electrons of \({{\rm{C}}_{\rm{2}}},\) but the pull is weaker than it is between \({\rm{X}}\) and \({{\rm{C}}_{\rm{1}}}.\) The effect rapidly dies out and is usually not significant after three bonds.

The inductive effect is permanent but relatively weak and can be easily overshadowed by other electronic effects. The inductive effect is related to substituent(s) ability to either withdraw or donate electron density to the attached carbon atom. Based on this ability, the substituents can be classified as electron-withdrawing and electron-donating or electron-releasing groups relative to hydrogen.

These relative inductive effects are measured with reference to hydrogen:

\({\rm{N}}{{\rm{O}}_2} > {\rm{COOH}} > {\rm{F}} > {\rm{Cl}} > {\rm{Br}} > {\rm{I}} > {\rm{OR}} > {{\rm{C}}_6}{{\rm{H}}_6} > {\rm{H}} > {\rm{M}}{{\rm{e}}_2}{\rm{CH}} > {\rm{MeCH}}_2^ – > {\rm{CH}}_3^ – \)

-I effect: Electron-withdrawing Effect

When an electronegative atom, such as a halogen, is introduced to a chain of carbon atoms, an unequal sharing of electrons occurs, generating a negative charge over the halide ion and a positive charge over the carbon atom attached to it. This positive charge is relayed and transmitted through the carbon chain and causes a permanent dipole to arise in the molecule. The corresponding effect is called the electron-withdrawing inductive effect or the -\({\rm{I}}\) effect.

It is electron-withdrawing because the electronegative atom withdraws electrons from the adjacent carbon atom, affecting the entire carbon chain.

Electron-withdrawing groups include halogen, nitro \(\left( {{\rm{ – N}}{{\rm{O}}_{\rm{2}}}} \right),\) cyano \(( – {\rm{CN}}),\) carboxy \(( – {\rm{COOH}}),\) ester \(( – {\rm{COOR}}),\) and aryloxy\(( – {\rm{OAr}}).\)

Decreasing order of \({\rm{ – I}}\) effect of these groups when attached to a molecule:

\({{\rm{R}}_{\rm{3}}}{{\rm{N}}^{\rm{ + }}}{\rm{ > N}}{{\rm{O}}_{\rm{2}}}{\rm{ > CN > F > Cl > OH > OC}}{{\rm{H}}_{\rm{3}}}{\rm{ > Br > I > – CH = C}}{{\rm{H}}_{\rm{2}}}\)

+I effect: Electron Releasing or Donating Effect

When a chemical species with the tendency to release or donate electrons, such as an alkyl group, is introduced to a carbon chain, the negative charge is relayed through the chain of carbon atoms. This effect is called the Positive Inductive Effect or the \({\rm{ + I}}\) Effect

Decreasing order of +I effect of these groups when attached to a molecule:

\(\left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{C}} – > \left( {{\rm{C}}{{\rm{H}}_3}} \right){\rm{CH}} – > {\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2} – > {\rm{C}}{{\rm{H}}_3} – \)

Inductive Effect on Stability of Molecules

The charge on a given or on a group bonded to an atom plays an important role in determining the stability of the resulting molecules. The electron-withdrawing and electron-donating groups can impart stability to a negatively and positively charged species, respectively. 

When a group displaying the \({\rm{ – I}}\) effect is bonded to a positively charged atom, the positive charge on the resulting molecule gets amplified, resulting in an unstable molecule. However, when a negatively charged species is attached to the same group displaying the \({\rm{ – I}}\) effect, the positive charge on the group is minimalised by the negatively charged atom, and the resulting molecule would be stable as per the inductive effect. The group displaying the \({\rm{ – I}}\) effect is more likely to accept electrons, thereby increasing the molecule’s acidity.

The order of acidities of the molecules are as follows:

\({\rm{C}}{{\rm{F}}_3}{\rm{COOH}} > {\rm{C}}{{\rm{H}}_2}\;{\rm{F}} – {\rm{COOH}} > {\rm{C}}{{\rm{H}}_2}{\rm{Cl}} – {\rm{COOH}} > {\rm{C}}{{\rm{H}}_3}{\rm{COOH}}\)

Similarly, when a group displaying the \({\rm{ + I}}\) effect is bonded to a negatively charged atom, the negative charge on the resulting molecule gets amplified, resulting in an unstable molecule. However, when a positively charged species is attached to the same group displaying the \({\rm{ + I}}\) effect, the negative charge on the group is minimalised by the positively charged atom. The resulting molecule would be stable as per the inductive effect. The group displaying the \({\rm{ + I}}\) effect (negatively charged) is more likely to donate electrons, thereby increasing the molecule’s basicity.

Again the order of basicities of the molecules are as follows:

\({\left( {{{\rm{C}}_2}{{\rm{H}}_5}} \right)_2} – {\rm{NH}} > {\left( {{\rm{C}}{{\rm{H}}_3}} \right)_2} – {\rm{NH}} > {\rm{C}}{{\rm{H}}_3} – {\rm{N}}{{\rm{H}}_2} > {\rm{N}}{{\rm{H}}_3}\)

Resonance or Mesomeric Effect

Resonance is a phenomenon used to represent a molecule in more than one form when a single Lewis structure cannot explain all of its properties. A common example of resonance is that of benzene whose cyclic structure contains alternating \({\rm{C – C}}\) single and \({\rm{C = C}}\) double bonds.

The above structure of benzene is inadequate in explaining all of its characteristic properties. Moreover, as determined experimentally, benzene has a uniform \({\rm{C – C}}\) bond distance of 139 pm, which is intermediate between the \({\rm{C – C}}\) single \(\left( {{\rm{154}}\,{\rm{pm}}} \right)\) and \({\rm{C = C}}\) double \(\left( {{\rm{134}}\,{\rm{pm}}} \right)\) bonds. Thus, the structure of benzene cannot be represented adequately by the above structure. Hence, benzene is represented equally well by the energetically identical structures \({\rm{I}}\) and \({\rm{II}},\) as shown below.

The above structures I and II are known as resonance structures or canonical structures, or contributing structures of the actual molecule. These structures are only hypothetical and have no real existence. They only contribute to the stability of the actual structure. The actual structure that has existence is called the resonance hybrid.

The energy possessed by the actual structure of the molecule (the resonance hybrid) is lower than that of any of its canonical or resonating structures. The difference in energy between the actual structure and the least energetic resonating structure is called the resonance energy or resonance stabilisation energy. The resonance energy increases with the increase in the number of contributing structures.

The resonance effect is defined as ‘the polarity induced in a molecule by the interaction of two \(π-\)bonds or between a π-bond and lone pair of electrons present on an adjacent atom’. The effect is relayed and transmitted through the chain of carbon atoms. 

There are two types of resonance effects designated by \({\rm{ + R}}\) or \({\rm{ – R}}.\)

Positive Resonance Effect (+R)- In this effect, the displacement of electrons takes place away from an atom or substituent group attached to the conjugated system. This results in certain positions in the molecule that possess high electron density. This effect in aniline is shown as :

\({\rm{ + R}}\) effect:-halogen,\({\rm{ – OH, – OR, – OCOR, – N}}{{\rm{H}}_{{{\rm{2}}^{\rm{‘}}}}}{\rm{ – NHR, – N}}{{\rm{R}}_{{{\rm{2}}^{\rm{‘}}}}}{\rm{ – NHCOR}}\)

Negative Resonance Effect(-R)- In this effect, electrons’ displacement occurs towards an atom or substituent group attached to the conjugated system. This results in certain positions in the molecule that possess a positive charge. For example-

\({\rm{ – R}}\,{\rm{effect: – COOH, – CHO, > C = 0, – CN, – N}}{{\rm{O}}_{\rm{2}}}\)

Electromeric Effect

The electromeric effect takes place only in the presence of multiple bonds and attacking reagents. It is an intramolecular movement of electrons in which electrons from a pi bond shift to another atom in the molecule when the compound is subjected to an attacking reagent. It is temporary and is annulled as soon as the attacking reagent is removed from the domain of the reaction. This effect is observed only in organic compounds that contain at least one multiple bond. 

When the atoms constituting multiple bonds is subjected to an attacking reagent, one pi bonding pair of electrons is completely transferred to any one of the two atoms.

Based on the direction in which the electron pair is transferred, the electromeric effect is classified into two-

+E Effect or Positive Electromeric Effect

The \({\rm{ + E}}\) effect is generally observed when the attacking reagent is an electrophile. The electrophile gets attached to that atom of the multiple bonds to which the pi electrons have been transferred.

-E Effect or Negative Electromeric Effect

The \({\rm{ – E}}\) effect is generally observed when the attacking reagent is a nucleophile. The electrophile gets attached to that atom of the multiple bonds from which the pi electrons have been transferred away to another atom. In simple words, the attacking reagent does not get attached to the atom to which the pi-electron transfer has taken place; instead gets attached to the positively charged atom in the molecule, i.e., the atom which lost the electron pair in the transfer.

When inductive and electromeric effects operate in opposite directions, the electomeric effect predominates.

Hyperconjugation

Hyperconjugation or No bond resonance is very similar to resonance and is a permanent effect. In resonance, the lone pair of electrons and pi bonds (double/triple bonds) are involved in delocalisation; however, in hyperconjugation, sigma electrons of \({\rm{C – H}}\) single bonds are involved in the electron delocalisation. It helps explain the stability of alkyl radicals. 

Hyperconjugation involves the delocalisation of \(σ\) electrons of the \({\rm{C – H}}\) bond of an alkyl group directly attached to an atom of unsaturated system or to an atom with an unshared \({\rm{p}}\) orbital. The \(σ\) electrons of the \({\rm{C – H}}\) bond of the alkyl group enter into partial conjugation with the attached unsaturated system or with the unshared \({\rm{p}}\) orbital.

For example,  in ethyl cation \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CH}}_2^ + } \right),\) the positively charged carbon atom has an empty \(p\) orbital. One of the \({\rm{C – H}}\) bonds of the methyl group can align in the plane of this empty \({\rm{p}}\) orbital, and the electrons constituting the \({\rm{C – H}}\) bond can then delocalise into the empty \({\rm{p}}\) orbital.

The hyperconjugation effect stabilises the carbocation as it helps in the dispersal of positive charges. Hence, the greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilisation of the carbocation. The relative stability on the basis of hyperconjugation is given as,

Hyperconjugation is also possible in alkenes and alkylarenes. Delocalisation of electrons by hyperconjugation in the case of alkenes is shown below:

Steric Effects

The steric effect is manifested when two or more groups approach each other’s van der Waals radii that result in a mutual repulsion and a lot of destabilisation. Steric effect affects different properties of molecules, like general reactivity, acidity, basicity but is not relayed in the chain of carbon atoms. 

For example, a substitution reaction does not occur between a halide and a hydroxide ion due to steric hindrance.

Summary

Chemical reactions take place as a result of the give and take and/or sharing of electrons. Different electronic effects influence the distribution of electrons in a covalent bond of an organic molecule. These effects tend to stabilise the unstable species that occur due to electron displacements. This article taught us the different electronic effects such as inductive effect, resonance effect electromeric effect, and hyperconjugation. We also learned how these effects minimise the energy of charged species and impart stability.

Frequently Asked Questions

Q.1. What are the types of electronic effects?
Ans: Electronic Effects are of the following types-
1. The Inductive Effect (\({\rm{ + I}}\) and \({\rm{ – I}}\))
2. Resonance or Mesomeric effect (\({\rm{ + R}}\) or \({\rm{ – R, + M}}\) or \({\rm{ – M}}\))
3. Electromeric Effect (\({\rm{ + E}}\) or \({\rm{ – E}}\))
4. Hyperconjugation.

Q.2. What is the E effect?
Ans: \({\rm{E}}\) effect is the electromeric effect that involves the complete transfer of electrons of multiple bonds to one of the bonded atoms in the presence of an attacking reagent. This effect is temporary and is annulled as soon as the attacking reagent is withdrawn from the vicinity of the reaction.

Q.3. Why is hyperconjugation known as no bond resonance?
Ans: Hyperconjugation involves delocalised sigma electrons from a \({\rm{C – H}}\) bond of an alkyl group to some other atom in the molecule. As the electrons are delocalised, there exists no bond between the hydrogen and the carbon atom; hence hyperconjugation is also known as no bond resonance.

Q.4. What are the conditions necessary for the electromeric effect to occur?
Ans: There are two necessary conditions for the electromeric effect to take place. These are-
Presence of multiple bonds
Attacking reagent

Q.5. What is the mesomeric effect?
Ans: The mesomeric effect is used in a qualitative way to describe the electron-withdrawing or to release properties of substituents based on relevant resonance structures and is symbolised by the letter \({\rm{M}}.\)

Now you are provided with all the necessary information on the electronic effects and we hope this detailed article is helpful to you. If you have any queries regarding this article, please ping us through the comment section below and we will get back to you as soon as possible.