In this article, you are going to learn about the inductive effect, including definition, examples, order, and applications.
What is Inductive Effect?
In a covalent σ-bond between two atoms of dissimilar electronegativity, the more electronegative atom drags the bonding electron pair towards itself due to its greater affinity for electrons. As a consequence, a partial negative charge (δ–) develops on the more electronegative atom and a partial positive charge (δ+) is originated to the other. Thus, the covalent bond can be polarized due to the unequal sharing of the bonding electron, leading to a permanent dipole in a bond. This permanent state of bond polarization is referred to as the inductive effect. The inductive effect is usually shown by the symbol (–>–), where the arrow is always pointed towards the more electronegative atom.
In a long chain compound, the inductive effect is relayed from one atom to the next, but the effect quickly falls off and becomes negligible. Thus, in a long chain carbon compound, if the terminal carbon atom is linked up with an electronegative halogen atom (electronic-withdrawing group), then the positive charge is relayed along the carbon chain, and the inductive effect may be represented as in the following diagram,
Similarly, when an electron-donating group or atom is attached to the terminal carbon atom of a carbon chain, the negative charge is relayed along the carbon chain, and the inductive effect may be represented as in the following diagram,
Types of Inductive Effect
When an atom or group donates electrons towards the bond and acquires a partial positive charge, the atom or group is called +I group, and the corresponding effect is called the positive Inductive Effect or the +I effect.
Example of +I groups: – O –, – COO –, –CR3, –CHR2, –CH2R, –CH3, –D
When an atom or group attracts the bonding electrons towards itself and acquires a partial negative charge, the atom or group is called -I group and the corresponding effect is called the electron-withdrawing inductive effect, or the -I effect.
Example of -I groups: –NO2, – SO2R, –CN, –SO2Ar, –COOH, –F, – Cl, – Br, – I, –OAr, –COOR, –OR, –COR, –SH, –SR, –OH, –Ar, – CH = CR2
Inductive Effect Order
The following groups are listed approximately in order of decreasing strength for both – I and + I effects.
Inductive Effect Order for +I Groups
– O –> – COO –> –CR3 > –CHR2 > –CH2R > –CH3 > –D
Inductive Effect Order for -I Groups
-NR3+ > -SR2+ > -NH3+ > –NO2 > – SO2R > –CN > –SO2Ar > –COOH > –F > – Cl > – Br > – I > –OAr > –COOR > –OR > –COR > –SH > –SR > –OH > –Ar > – CH = CR2
Applications of Inductive Effect
The inductive effect affects some important following properties of organic compounds and these are may be
- Acidic strength of aliphatic carboxylic acids.
- The relative strength of organic bases.
- The effect on dipole moment and bond length.
- Stability of alkyl carbocation, carbanions, and carbon-free radicals.