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 (CH3CH2Cl) in which the C–Cl bond is a polar covalent bond.
It is polarised in such a way 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) and the shift of electron density is shown by an arrow that points from δ+ to δ– end of the polar bond.
In turn carbon-1, 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-2 also, where + symbolises 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 polarisation of - bond caused by the polarisation of adjacent -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 the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom. Based on this ability, the substitutents can be classified as electron-withdrawing or electron donating groups relative to hydrogen.
Halogens and many other groups such as nitro (- NO2), cyano (- CN), carboxy (- COOH), ester (COOR), aryloxy (-OAr, e.g. – OC6H5), etc. are electronwithdrawing groups. On the other hand, the alkyl groups like methyl (–CH3) and ethyl (–CH2–CH3) are usually considered as electron donating groups.