Inductive effect

In chemistry and physics, the inductive effect is an experimentally observable effect of the transmission of charge through a chain of atoms in a molecule by electrostatic induction.^[1] The net polar effect exerted by a substituent is a combination of this inductive effect and the mesomeric effect.

The electron cloud in a σ-bond between two unlike atoms is not uniform and is slightly displaced towards the more electronegative of the two atoms. This causes a permanent state of bond polarization, where the more electronegative atom has a slight negative charge (δ–) and the other atom has a slight positive charge (δ+).

If the electronegative atom is then joined to a chain of atoms, usually 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.

Some groups, such as the alkyl group are less electron-withdrawing than hydrogen and are therefore considered as electron-releasing. This is electron releasing character and is indicated by the + I effect.

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. The inductive effect is permanent but feeble, as it involves the shift of strongly held σ-bond electrons, and other stronger factors may overshadow this effect.

The inductive effect may be caused by some molecules also. Relative inductive effects have been experimentally measured with reference to hydrogen.

Inductive effects can be measured through the Hammett equation.

Inductive Effect can also be used to determine whether a molecule is stable or unstable depending on the charge present on the atom under consideration and the type of groups bonded to it. For example, if an atom has a positive charge and is attached to a −I group its charge becomes 'amplified' and the molecule becomes more unstable than if I-effect was not taken into consideration. Similarly, if an atom has a negative charge and is attached to a +I group its charge becomes 'amplified' and the molecule becomes more unstable than if I-effect was not taken into consideration. But, contrary to the above two cases, if an atom has a negative charge and is attached to a −I group its charge becomes 'de-amplified' and the molecule becomes more stable than if I-effect was not taken into consideration. Similarly, if an atom has a positive charge and is attached to a +I group its charge becomes 'de-amplified' and the molecule becomes more stable than if I-effect was not taken into consideration. The explanation for the above is given by the fact that more charge on an atom decreases stability and less charge on an atom increases stability.

Definition

The permanent polarity between a carbon atom and any other atom or group in an organic molecule due to the partial displacement of sigma bond pair of electrons towards more electronegative atom or group is known as inductive effect.

Applications

• Aliphatic carboxylic acids. The strength of a carboxylic acid depends on the extent of its ionization: the more ionized it is, the stronger it is. As an acid becomes stronger, the numerical value of its pK_a drops. In aliphatic acids, the electron-releasing inductive effect of the methyl group increases the electron density on oxygen and thus hinders the breaking of the O-H bond, which consequently reduces the ionization. Greater ionization in formic acid when compared to acetic acid makes formic acid (pK_a=3.75) stronger than acetic acid (pK_a=4.76). Monochloroacetic acid (pK_a=2.82), though, is stronger than formic acid, since the electron-withdrawing effect of chlorine promotes ionization.
• Aromatic carboxylic acids. In benzoic acid, the carbon atoms which are present in the ring are sp₂ hybridised.As a result, benzoic acid(pK_a=4.20) is a stronger acid than cyclohexane carboxylic acid(pK_a=4.87). Also, electron-withdrawing groups substituted at the ortho and para positions, enhance the acid strength.
• Dioic acids. Since the carboxyl group is itself an electron-withdrawing group, the dioic acids are, in general, stronger than their monocarboxyl analogues.
• In the so-called Baker–Nathan effect the observed order in electron-releasing alkyl substituents is apparently reversed.

See also

• Important publications in organic chemistry

References

1. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "inductive effect".