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In Organic chemistry, the inductive effect in a molecule is a local change in the electron density due to electron-withdrawing or electron-donating groups elsewhere in the molecule, resulting in a permanent dipole in a bond. It is present in a sigma bond, unlike the electromeric effect which is present in a pi bond.
The halogen atoms in an alkyl halide are electron withdrawing while the alkyl groups have electron donating tendencies. If the electronegative atom (missing an electron, thus having a positive charge) is then joined to a Catenation, typically carbon, the positive charge is relayed to the other atoms in the chain. This is the electron-withdrawing inductive effect, also known as the − I effect. In short, alkyl groups tend to donate electrons, leading to the + I effect. Its experimental basis is the ionization constant. It is distinct from and often opposite to the mesomeric effect.
For example, the water molecule has an electronegative oxygen atom that attracts a negative charge. This is indicated by δ− in the water molecule in the vicinity of the O atom, as well as by a δ+ next to each of the two H atoms. The vector addition of the individual bond dipole moments results in a net dipole moment for the molecule. A polar bond is a covalent bond in which there is a separation of charge between one end and the other - in other words in which one end is slightly positive and the other slightly negative. Examples include most covalent bonds. The hydrogen-chlorine bond in HCl or the hydrogen-oxygen bonds in water are typical.
The inductive effect of alkyl group, has long been a source of misunderstanding. Due to early experimentation, before an understanding of hyperconjugation, results such as the more rapid nitration of toluene compared to benzene, were deduced as being due to an inductively donating effect of alkyl groups. Effects such as the lower acidity of alcohols and higher basicity of substituted amines further deepened the misunderstanding, despite this being due to solvent or polarisability effects.
As the induced change in polarity is less than the original polarity, the inductive effect rapidly dies out and is significant only over a short distance. Moreover, the inductive effect is permanent but feeble since it involves the shift of strongly held σ-bond electrons and other stronger factors may overshadow this effect.
Recent research combining wave functional theory calculations with experiment results (gas phase acidities, ion-specific effects in thermoresponsive polymers, and NMR spectroscopy) has re-examined haloacetic acids and salts. The study found that in trihaloacetates, the trichloro group—despite being less electronegative than fluoro groups—reduces the carboxylate oxygen charge density the most. This inversion of the traditional electronegativity–charge density relationship suggests that other factors beyond the simple inductive effect (such as hyperconjugation) may significantly influence acidity trends.
The strength of inductive effect is also dependent on the distance between the substituent group and the main group that react; the longer the distance, the weaker the effect.
Inductive effects can be expressed quantitatively through the Hammett equation, which describes the relationship between reaction rates and equilibrium constants with respect to substituent.
In acids, the electron-releasing inductive effect of the alkyl group increases the electron density on oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization. Due to its greater ionization, formic acid () is stronger than acetic acid (). Monochloroacetic acid (), though, is stronger than formic acid, due to the electron-withdrawing effect of chlorine promoting ionization.
In benzoic acid, the carbon atoms which are present in the ring are sp2 hybridised. As a result, benzoic acid () is a stronger acid than cyclohexanecarboxylic acid (). Also, in aromatic carboxylic acids, electron-withdrawing groups substituted at the ortho and para positions can enhance the acid strength.
Since the carboxyl group is itself an electron-withdrawing group, dicarboxylic acids are, in general, stronger acids than their monocarboxyl analogues. The Inductive effect will also help in polarization of a bond making certain carbon atom or other atom positions.
| Transfer of shared π-bond electron pairs to one atom under the influence of a strong external electric field. |
| Temporary effect. |
| Only observed in the presence of an electrophilic reagent. |
| Induced charges are integers () |
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