Electronic Displacement in a Covalent Bond: Inductive Effect

When two atoms of different electronegativity are joined by a covalent bond, the shared electron pair is not shared equally -- it lies closer to the more electronegative atom, which develops a small negative charge (written $\delta-$) while the other atom develops a small positive charge ($\delta+$). If this polarised bond is part of a longer carbon chain, its influence does not stop there: each carbon along the chain pulls a little electron density from its neighbour to partly compensate, so the polarity is relayed, weakening with each successive bond, all the way along the chain. This permanent, distance-weakened relay of electron density through a chain of sigma bonds is called the inductive effect. In practice the effect is strong for the first bond, noticeably weaker for the second, and essentially negligible beyond the third bond from the source.

Groups that pull electron density towards themselves, away from the rest of the chain (relative to hydrogen), are said to have a $-\text{I}$ (negative inductive) effect -- halogens, $-\text{NO}_2$, $-\text{CN}$ and $-\text{COOH}$ are common examples, roughly in decreasing order of strength. Groups that push electron density towards the rest of the chain (again relative to hydrogen) are said to have a $+\text{I}$ (positive inductive) effect -- alkyl groups such as $-\text{CH}_3$ and $-\text{C}_2\text{H}_5$ are the standard examples, and this $+\text{I}$ donation is one reason alkyl-substituted carbocations are more stable than less-substituted ones (Section 11.9). …