Inductive Effect - Carbocation & Electron Withdrawing

Inductive Effect – We all know about the polarity of covalent bonds. When two different atoms having a great difference in their electronegativity values come together to form a covalent bond, the electron density is not found to be spread evenly throughout the bond. The electron density is greater towards the more electronegative atom of the bond. Such unevenly distributed electron density leads to a polarity which finally results in the formation of the polar covalent bond (Inductive effect). For example: in the case of chloroethane (CH3CH2Cl) the C–Cl bond is a polar covalent bond as electronegativity difference between carbon and chlorine is very high. It is polarised in such a way that the carbon atom adjacent to the chlorine atom gains some positive charge (δ+) while the chlorine gains some negative charge (δ–). δ(delta) denotes the fractional electronic charges on the two atoms involved in a polar covalent bond. The shift of electron density is shown by an arrow as shown below in the figure.

Inductive Effect

As the carbon atom adjacent to the chlorine atom develops a partial positive charge (δ+) it starts to draw some electron density towards it from the adjacent carbon atom. This results in the development of some positive charge (δδ+) on the second carbon atom. The symbolδδ+ symbolizes that the second carbon atom has developed a lesser positive charge in comparison to the first carbon atom. Thus, we can say that the C-Cl bond has induced some polarity on the adjacent atoms. This polarization of σ-bond due to the polarization of adjacent σ-bond is termed as the inductive effect. It depends on the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom.

Inductive effect

Dependence of the inductive effect of the substituents attached to carbon atom:

Inductive Effect

Inductive effect of the substituents attached to the carbon atom

The substituents attached to the carbon atom are mainly classified into two categories:

  1. Electron withdrawing groups:

    Electron withdrawing groups decreases the electron density on the adjacent carbon atom. These stabilize the carbanion formed through the inductive effect. For example nitro (-NO2), carboxy (- COOH), cyano (- CN), ester (-COOR) etc.

  1. Electron donating groups:

    Electron donating groups increases the electron density on the adjacent carbon atom. These stabilize the carbocation formed through the inductive effect. For example, methyl (–CH3) and ethyl (–CH2–CH3) etc.

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Practise This Question

Considering the basic strength of amines in aqueous solution, which one has the smallest pKb value ?