Electron Displacements in Covalent Bond

For a reaction to take place by breaking of a covalent bond, with the attack of a nucleophile or electrophile, the molecule or bond under attack must develop polarity on some of its carbon atoms. This polarity can only be developed by the displacement (partial or complete) of bonding electrons due to certain effects.

Some of these electronic effects are permanent (inductive) and others are temporary (electromeric) in nature. Such changes or effects involving displacement of electrons in the substrate molecules (molecule under attack of a reagent) are known as electron displacement or electronic effects.

Inductive Effect

In a covalent bond between the two dissimilar atoms, the shared electron pair is attracted more towards the atom having greater electronegativity. For example, in case of a haloalkane (higher than halomethane) the halogen atom (X) being more electronegative than carbon atom, pulls the bonded electrons of the C–X bond.

The carbon atom gets a partial +ve charge (δ+) and halogen atom a partial negative charge (δ–). This positively charged C₁ attracts bonded electrons of C₁–C₂ bond, thus making C₂ atom a little less positive than C₁. Similarly, this +ve charge is shifted to C₃ but to a very less magnitude (nearly zero +ve charge is present after third atom).

The transmission of induced charges along a chain of σ bonded carbon atoms is known as inductive effect. The inductive effect is a permanent polarization in the molecule and it decreases as you move along a chain of carbon atoms, away from the electronegative atom.

Groups with –I Effect (Electron Withdrawing Groups)

Any atom or group of atoms that withdraws electrons more strongly than the H-atom, is said to have –I effect. Following are various groups arranged in the decreasing order of their –I effect.

(CH₃)₃N⁺ > –NO₂ > –CN > –F > –Cl > –Br > –I > –OH > –OCH₃ > –C₆H₅ > –H

Groups with +I Effect (Electron Releasing Groups)

Any atom or group of atoms that repels electrons more strongly than hydrogen, is said to have +I effect. Following are the various groups in the decreasing order of +I effect.

(CH₃)₃C– > (CH₃)₂CH– > CH₃CH₂– > –CH₃ > –H

Electromeric Effect

This type of temporary electron displacement takes place in compounds containing multiple covalent bonds. It involves the complete transfer of electrons resulting into the development of +ve and –ve charges within the molecule. The electromeric effect takes place in the direction of more electronegative atom.

The electromeric effect is represented by the symbol E. It is known as +E effect when displacement of electron pair is away from the atom or group, or –E effect when the displacement is towards the atom or group.