Inductive Effect

The polarization of a σ bond due to electron withdrawing or electron donating effect of adjacent groups or atoms is referred to as inductive effect. 

A sigma bond between two atoms, which differ in their electronegativity, is polarized due to displacement of bond pair towards the more electronegative atom. In this process, the electronegative atom gets partial negative charge while the other atom gets partial positive charge. Thus induced polarity is transmitted through the sigma bonds in the molecule by creating a permanent dipole. This phenomenon is referred to as inductive effect.

It is represented by an arrow pointing towards the more electronegative atom carrying partial negative charge.

Salient features of inductive effect 

* It arises due to electronegativity difference between two atoms forming a sigma bond. 

* It is transmitted through the sigma bonds. No pi bonds are involved.

* The magnitude of inductive effect decreases while moving away from the groups causing it. 

* It is a permanent effect. It may create permanent dipole in the molecule.

* In general, the inductive effect is relatively weak and is overshadowed by other electronic effects like resonance effect, hyperconjugation etc.

* It influences the chemical and physical properties of compounds.

EXAMPLE OF INDUCTIVE EFFECT

The C-Cl bond in the butyl chloride, CH₃-CH₂-CH₂-CH₂-Cl is polarized due to electronegativity difference. The electrons are withdrawn by the chlorine atom. Thus the first carbon atom gets partial positive charge. In turn, this carbon atom drags electron density partially from the next carbon, which also gets partial positive charge. This will continue further and is how the inductive effect is transmitted through the carbon chain.

One must note that the inductive effect weakens away along the chain and is not that much significant beyond the 3rd carbon atom.

Also note that inductive effect is a permanent effect and is inherent to the molecule, while the electromeric effect is a temporary effect is only created in the presence of attacking electrophilic or nucleophilic reagents.

TYPES OF INDUCTIVE EFFECT

The inductive effect is divided into two types based on the electron withdrawing or electron releasing nature of atom/group inducing it. The strength of inductive effect is measured by comparing with that of hydrogen.

1) Negative inductive effect (-I):  

The electron withdrawing nature of groups or atoms is called as negative inductive effect. It is indicated by -I. Following are the examples of groups in the decreasing order of their -I effect: 

NH₃⁺ > NO₂ > CN > SO₃H > CHO > CO > COOH > COCl > CONH₂ > F > Cl > Br > I > OH > OR > NH₂ > C₆H₅ > H 

2) Positive inductive effect (+I):  

It refers to the electron releasing nature of the groups or atoms and is denoted by +I. Following are the examples of groups in the decreasing order of their +I effect. 

C(CH₃)₃ > CH(CH₃)₂ > CH₂CH₃ > CH₃ > H

Why alkyl groups are showing positive inductive effect?

Though the C-H bond is practically considered as non-polar, there is partial positive charge on hydrogen atom and partial negative charge on carbon atom. Therefore each hydrogen atom acts as electron donating group. This cumulative donation turns the alkyl moiety into an electron donating group.