The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarlisation of the bond. Inductive effect and resonance effects are examples of this type of electron displacements. Temporary electron displacement effects are seen in a molecule when a reagent approaches to attack it.
This type of electron displacement is called electromeric effect or polarisability effect. In the following sections we will learn about these types of electronic displacements. . .
Inductive Effect When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds. Let us consider cholorethane (CH CH Cl) in which the C–Cl bond is a polar covalent bond.
It is polarised in such a way that the carbon- gains some positive charge ( δ + ) and the chlorine some negative charge ( δ – ). The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol δ (delta) and the shift of electron density is shown by an arrow that points from δ + to δ – end of the polar bond. δδ + δ + δ − CH → CH → Cl In turn carbon- , which has developed partial positive charge ( δ + ) draws some electron density towards it from the adjacent C-C bond. Consequently, some positive charge ( δδ + ) develops on carbon- also, where δδ + symbolises relatively smaller positive charge as compared to that on carbon – .
In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of σ -bond caused by the polarisation of adjacent σ -bond is referred to as the