When a covalent bond is formed between atoms of different electronegativities, the electron density is 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.
“The permanent dipole, induce in one bond by another, is called the inductive effect”
Let us consider chloroethane (CH3CH2Cl) in which the C–Cl bond is a polar covalent bond. It is polarized so that the carbon-1 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). The shift of electron density is shown by an arrow pointing from δ+ to δ– end of the polar bond.
In turn, carbon-1, which has developed a partial positive charge (δ+), draws some electron density towards it from the adjacent C-C bond. Consequently, some positive charge (δδ+) develops on carbon-2 also, where δδ+ symbolizes a relatively smaller positive charge as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarization of σ-bond caused by the polarization of adjacent σ-bond is referred to as the inductive effect. This effect is passed on to the subsequent bonds also but the effect decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds.
The inductive effect is related to substituent(s) ability to either withdraw or donate electron density to the attached carbon atom. The substituents can be classified as electron-withdrawing or electron-donating groups relative to hydrogen-based on this ability.
Electron-Withdrawing Groups (-I Effect)
Halogens and many other groups such as nitro (- NO2), cyano (- CN), carboxy (- COOH), ester (-COOR), aryloxy (-OAr, e.g., OC6H5), etc. are electron-withdrawing groups. An approximate order of decreasing strength is given as
Electron Donating Groups(+I Effect)
On the other hand, the alkyl groups like methyl (–CH3) and ethyl (–CH2–CH3) are usually considered electron-donating groups. An approximate order of decreasing strength is given as
The inductive effect depends upon the Electronegativity and type of Hybridization, eg., other things being same, -I effect of an atom increase as the hybridization changes as:
Sp< sp2 < sp3
This is because s-orbital is closer to the nucleus than p-orbitals.