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Inductive effect

Organic chemistry – Inductive effect

An Inductive effect is a permanent effect that arises whenever an electron-withdrawing group is attached to the end of a carbon chain. To understand this, let us consider a chain of a carbon atom having Cl atom at one end:

We know that the chloride is more electronegative than carbon. Due to the larger electronegativity of Cl, the electron pair shared between C1 and Cl will be more displaced towards the Cl atom. As a result of this, Cl acquires a partial negative charge and C acquires a partial positive charge. This displacement is, however not limited to the C1-Cl bond but is transmitted to other carbon atoms along the chain. This is because the small positive charge on C1 will attract σ-electrons of the C1-C2 bond towards it. As a result of this displacement, the positive charge on C1 is partially neutralized while a small positive charge developed on C2. The charge of C2 is less than that on C1. Similarly, C3 will acquire a small positive charge. Such a polarization of σ-bond caused by the polarization of adjacent σ-bond is called inductive effect. Thus, the process of displacement of σ-electrons along the saturated carbon chain due to the presence of a polar covalent bond at one end of the chain is called inductive effect and is denoted as I effect.

This is a permanent effect and is generally represented by an arrow with its head in the middle of the covalent bond pointing in the direction of electron displacement as shown below:

However, it may be noted that this effect decreases sharply as we move away from the atom involved in the initial polar bond and becomes negligible from the fourth atom onwards.

Types of inductive effects

The inductive effect is related to the ability of the substituent to either withdraw or donate electron density to the attached carbon atom. Based on this ability, the substituents may be classified as a electron-withdrawing or electron-donating group relative to hydrogen. For comparing the relative effect, hydrogen is taken as standard.

The substituents having electron attracting power more than hydrogen are called electron-withdrawing substituents. For example, groups such as  are electron-withdrawing groups.

On the other hand, the substituents having electron attracting power less than hydrogen are called electron-donating groups. For example, alkyl groups such as etc, are electron-donating groups.

Based on the substituents, there are two types of inductive effects designated as –I effect and +I effect.

  • If the substituents attached to the end of the carbon chain are electron-withdrawing, the effect is called –I effect. The –I effect of some substituents in the decreasing order w.r.t. hydrogen is: 
  • If the substituents attached to the end of the carbon chain is electron-donating group, the effect is said to be +I effect. The +I effect of some substituents in the decreasing order w.r.t. hydrogen is: 

It is clear that a more substituted alkyl group, stronger is its +I effect.

The inductive effect plays a very significant role in organic chemistry in understanding reactions. The Carbon of methyl chloride, for example, acquires partially positive charge due to a shift of electrons towards chlorine and provides a site for attack by a negatively charged species (say OH^-)

 

 

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