Inductive Effect and it's Applications
Inductive Effect (I- Effect)
The polarization of a σ bond due to electron withdrawing or electron donating effect of adjacent atoms or group of atoms is called inductive effect.
A σ bond between two different electronegative atoms is polarized due to displacement of bond pair towards the more electronegative atom. So, the more electronegative atom gets partial negative charge while the other atom gets partial positive charge.
The induced polarity is transmitted through the σ bonds in the molecule by creating a permanent dipole. This phenomenon is known as inductive effect.
Inductive effect is represented by an arrow (i.e.→) pointing towards the more electronegative atom.
Important feature of Inductive Effect
1. It arises due to electronegativity difference between two atoms forming a sigma bond.
2. Bonding electron is transmitted towards the more electronegative atom.
3. It is transmitted or operated through the σ bonds.
4. It is a permanent effect in the molecule or ion.
5. The magnitude of inductive effect decreases (generally diminished after 3rd carbon) while moving away from the groups causing it.
6. inductive effect is relatively weaker than other electronic effects like resonance, hyperconjugation etc.
7. It influences the chemical and physical properties of compounds.
Types of Inductive Effect
Inductive Effect is of two types-Positive inductive effect (+I Effect)
The electron donating nature of atoms or group of atoms is called positive inductive effect. It is denoted by +I.
Examples-
-C(CH3)3, -CH(CH3)3, -CH2CH3, -CH3 etc.
Negative inductive effect (-I Effect)
The electron withdrawing nature of atoms or group of atoms is called negative inductive effect. It is denoted by -I.
Examples-
-NH3+, -NO2, -CN, -SO3H, -CHO, -CO, -COOH, -COCl, -CONH2, -X, -OH, -OR, -C6H5 etc.
Applications of Inductive Effect
Stability of carbocations (carbonium ions)
The +I groups increases the stability of carbocations. The +I groups reduce the positive charge on the carbon by donating electrons. This results in greater stability of carbocation.
Whereas, the -I groups destabilize the carbocations as they increase the positive charge by withdrawing electron density.
So, the stability order of carbonium ions of methyl, primary, secondary and tertiary carbocations is-
methyl < primary < secondary < tertiary
Stability of carbanions
The +I groups decrease the stability of carbanions by donating electrons to negatively charged carbon atom while the -I groups increases their stability by withdrawing electrons.
Thus the order of stability of carbanions is reversed of stability of carbonium ion. So, the stability order of carbanions of methyl, primary, secondary and tertiary carbanions is-
methyl > primary > secondary > tertiary
Stability of free radicals
Stability of free radicals increases with increase in the number of alkyl groups like carbonium ions. So, the stability order of free radicals of methyl, primary, secondary and tertiary carbocations is-
methyl < primary < secondary < tertiary
Acidic strength of carboxylic acids
The electron withdrawing groups (i.e. negative inductive effect) reduce the negative charge on the conjugate base of acid by pulling the electron density and stabilizing it. Hence the acidic strength increases when electron withdrawing groups are present.
CH3COOH < CH2FCOOH
The electron donating groups (i.e. positive inductive effect) decrease the acidic strength as they destabilize the conjugate base of acid by donating electron density which increases the negative charge on the anion and destabilizing conjugate base.
HCOOH > CH3COOH > C2H5COOH > C3H7COOH
Basic strength of amines
The electron donating groups increase the basic strength of amines by donating electrons to nitrogen atom of amines whereas the electron withdrawing groups decrease the basic strength by withdrawing electrons from nitrogen atom of amines. Therefore alkyl amines are stronger Lewis bases than ammonia, whereas aryl amines are weaker than ammonia.
Thus the order of basic strength of alkyl and aryl amines with respect to ammonia is
CH3NH2 > NH3 > C6H5NH2
Reactivity of carbonyl compounds
The +I groups increase the electron density at carbonyl carbon. Hence their reactivity towards nucleophiles decreases. So, HCHO is more reactive than CH3CHO and CH3COCH3 towards nucleophilic addition reactions.
