Inductive Effect

Inductive Effect

Consider a carbon chain in which one terminal carbon atom is joined to a chlorine atom : -C3-C2-C1-Cl. Chlorine has a greater electronegativity than carbon; therefore the electron pair forming the covalent bond between the chlorine atom and C1 will be displaced towards the chlorine atom. This causes the chlorine atom to acquire a small negative charge, and C1 a small positive charge. Since C1 is positively charged. It will attract towards itself the electron pair forming the covalent bond between C1 and C2. This will cause C2 to acquire a small positive charge, but the charge will be smaller than that on C1 because the effect of the chlorine atom has been transmitted through C1 to C2. Similarly, C1 acquires a positive charge which will be smaller than that on C2. This type of electron displacement along a chain is known as the inductive effect; it is permanent and decreases rapidly as the distance from the source increases. Form the practical point of view, it may be ignored after the second carbon atom. It is important to note that the electron pairs, although permanently displaced, remain in the same valency shells.

N.B. – This inductive effect is sometimes referred to as a transmission effect, since it takes place by a displacement of the intervening electrons in the molecule. There is also another effect possible, the direct or field effect, which results from the electrostatic interaction across space or through a solvent of two charged centres in the same molecule, i.e., the direct effect takes place independently of the electronic system in the molecule. Apparently it has not been possible to separate these two modes of inductive effect in practice.

The inductive effect may be represented in several ways. 

 The example is : -C→C→C→Cl.

Inductive effects may be due o atoms or groups, and the following is the order of decreasing inductive effects :

For measurement of relative inductive effects, Hydrogen is chosen as reference in the molecule CR3-H  as standard. If, when the H atom in this molecule is replaced by Z (an atom or group), the electron density in the CR3 part of the molecule is less in this part than in CR3-H, than Z is said to have a –I effect (electron-attracting or electron-withdrawing). If the electron density in the CR3 part is greater than in CR3-H then Z is said to have a +I effect (electron-repelling or electron-releasing) e.g., Br is –I ; C2H5 is +I. This terminology is due to Ingold (1926); Robinson suggests the opposite signs for I, i.e. Br is +I; C2H5, -I. the inductive effect may be measured with respect to hydrogen by the effect of substituents on the change in acid strength of substitute acetic acids.