INDUCTIVE AND MESOMERIC EFFECTS. ELECTRONIC EFFECTS OF THE SUBSTITUENTS. IMPORTANCE FOR PROPERTIES OF THE MOLECULES
I. Inductive effect (I). A bond with the bonding electrons shared equally between the two bonded atoms is called a nonpolar bond. Examples of nonpolar bond are the bond in H2 and the C-C bond in ethane.
The electron density of the covalent bond is not equally distributed between different atoms or even between the same atoms, which are differently substituted or are in different hybrid states. The more electronegative atom will acquire the greater share of the electron density and will thus have a partial negative charge; the other atom being a poorer attractor will be charged partially positive. This type of covalent bond is named polar bond. The partial electronic charges of the atoms are marked with the Greek letter \delta.
When carbon atom in sp³-hybrid state is bonded to chlorine, the bonding pair of electrons is attracted more strongly to the chlorine atom. The carbon atom bears a small partial positive charge, and the chlorine atom bears an equal amount of negative charge.
This structure shows the polar carbon-chlorine bond inchloromethane. The symbol “δ-“ means “a small amount of negative charge”.
We often use electronegativities (χ) as a guide in predicting whether a given bond will be polar and the direction of its dipole moment. Elements with higher electronegativities generally have more attraction for the bonding electrons.
The next order shows the electronegativities for some of the important elements in organic compounds:
F > O > Csp³ > N, Cl > Br, Csp² > I > Csp, S >H > B > Mg > Li > Na 4.0 3.5 3.2 3.0 2.8 2.6 2.5 2.2 1.8 1.3 1.0 0.9
Notice that the electronegativity decreases from left to right, that is, it increases from left to right across the periodic table.
The electronegativity of hydrogen is similar to that of carbon in sp³ hybrid state, and we usually consider such kind of σ bond between carbon and hydrogen to be nonpolar.
The presence of a strongly polar bond in a polyatomic molecule causes polarization of the neighboring bonds.