Inductive effect

A covalent bond formed between two atoms of genuinely DIFFERING electronegativity is, by definition, a polar covalent bond -- the C-Cl bond is the chapter's running example, since chlorine is considerably more electronegative than carbon. Whenever such a polar bond exists somewhere within an organic molecule, it turns out to induce a degree of polarity in the NEIGHBOURING carbon-carbon single bonds as well -- even though those neighbouring bonds are, in themselves, ordinary bonds between two chemically identical carbon atoms, and so would not normally be expected to show any polarity at all. This chain-transmitted polarisation effect is called the INDUCTIVE EFFECT, and it is illustrated using chloroethane, $CH_3{-}CH_2{-}Cl$ (labelling the chlorine-bearing carbon C1 and the methyl carbon C2): the C1-Cl bond itself is polar, carrying a partial negative charge on chlorine and an equal partial positive charge on C1; but because C1 is ALSO bonded onward to C2, that same partial positive charge on C1 goes on to pull, in turn, on the shared electron pair of the C1-C2 bond too -- developing a further, smaller partial positive charge on C2 as well, even though C2 is not itself directly bonded to the chlorine atom. In other words, electron density is pulled toward the chlorine not only along the direct C1-Cl bond, but also, to a lesser extent, along the further C2-C1 bond. This transmitted displacement is conventionally drawn as a small arrowhead placed directly in the MIDDLE of the relevant bond, pointing in the direction that the permanent, ground-state electron displacement is actually occurring. Critically, the inductive effect's strength FALLS OFF rapidly as it is transmitted along a chain of C-C bonds, and becomes negligibly small once it has to cross more than about three bonds -- illustrated by $CH_3{-}CH_2{-}CH_2{-}CH_2{-}Cl$, where the resulting partial positive charges satisfy $\delta_3^{\oplus} < \delta_2^{\oplus} < \delta_1^{\oplus}$ moving outward, away from the chlorine. Depending on which direction a given attached group pulls electron density, substituent groups are classified into two categories relative to hydrogen: ELECTRON-WITHDRAWING (or 'accepting') groups, which pull electron density AWAY from the carbon chain and so are said to exert a NEGATIVE inductive effect (a '-I effect') -- besides the halogens, other co …