Inductive Effect

The Inductive Effect is an electronic effect observed in organic molecules where the distribution of electrons is shifted along a chain of atoms due to the presence of a more electronegative atom or group. This shift in electron density occurs through sigma bonds (single bonds) and results in the polarization of the molecule.

1.0Characteristics of Inductive Effect

Here is an image representing how the inductive effect works in an organic molecule-

• Permanent Effect: The inductive effect is a permanent shift of electrons along a covalent bond, unlike other effects like resonance which involves electron delocalization.
• Transmission Through Sigma Bonds: The effect is transmitted through sigma bonds, and it diminishes with increasing distance from the source of the effect. Typically, the inductive effect becomes negligible after three carbon atoms from the influencing group. Here are more details- 

2.0Types of Inductive Effect

1. Negative Inductive Effect (-I effect):

The Inductive Effect refers to the permanent displacement of sigma (σ) electrons in a covalent bond due to the electronegativity of an atom or group attached to the chain. If the electron-withdrawing ability of the group is stronger than that of hydrogen, such groups are referred to as –I groups. Key Features are- 

• Groups or atoms that withdraw electron density from the rest of the molecule.
• Example groups: -NO₂, -F, -Cl, -Br, -I, -OH, -COOH.
• These groups pull electrons towards themselves, creating partial positive charges on adjacent atoms.

Example: 1-Chloro-4-fluorobutane is showing an inductive effect where fluorine (F), due to its higher electronegativity, exerts a stronger electron-withdrawing effect (-I effect) than chlorine (Cl). This results in a greater partial negative charge (δ⁻) on F, which influences the electron density along the carbon chain. The magnitude of the inductive effect decreases as the distance from F and Cl increases.

Magnitude : δ₅ > δ₆ > δ₁ > δ₄ > δ₂ >δ₃

Decreasing Order of Electron Withdrawing Tendency (–I Effect):

The following series lists groups in order of their electron-withdrawing power through induction:

–I Series: –NO₂ > –CN > –SO₃H > –CHO > –COOH > –F > –Cl > –Br > –I > –OH > –NH₂ > –C≡CH > –Ph > –CH=CH₂ > (H)

2. Positive Inductive Effect (+I effect):

The +I Effect refers to the ability of certain groups or atoms to donate electron density through sigma bonds towards the rest of the molecule. If a group's electron-donating ability is stronger than that of hydrogen, it is classified as a +I group. These groups tend to push electron density towards the rest of the molecule, making the surrounding atoms more electron-rich. Key features are-

• Groups that donate electron density to the rest of the molecule.
• Example groups: Alkyl groups (CH₃, C₂H₅), -O⁻, -N⁻.
• These groups push electron density towards the rest of the molecule, slightly increasing the electron density on nearby atoms.

+I Series: –CH₂–CH₃ > –C(CH₃)₃ > –CD₃ > –CH₃ > –T (Tritium) > –D (Deuterium) > –H (Hydrogen)

3.0Applications of Inductive Effect

1. Acidity and Basicity:

• The inductive effect can influence the acidity of carboxylic acids or the basicity of amines. Electron-withdrawing groups (-I effect) increase the acidity by stabilizing the negative charge on the conjugate base, while electron-donating groups (+I effect) decrease acidity.
• Example: If we compare the effect of +I (electron-donating) and -I (electron-withdrawing) groups on acidity in given below diagram, the methyl group (+I effect) destabilizes the anion, making the acid less acidic, while the chlorine (-I effect) stabilizes the anion, making the acid more acidic

2. Stability of Carbocations:

• Alkyl groups (+I effect) stabilize carbocations by donating electron density towards the positively charged carbon, thus reducing the overall positive charge.
• Example: A tert-butyl carbocation ((CH₃)₃C⁺) is more stable than an ethyl carbocation (CH₃CH₂⁺) because of the greater +I effect of the additional methyl groups.

3. Reactivity in Substitution Reactions:

• In nucleophilic substitution reactions, electron-withdrawing groups make the electrophilic center more reactive by enhancing its positive character.
• Example: In haloalkanes (like CH₃Cl), the presence of a halogen (especially fluorine or chlorine) makes the carbon more reactive towards nucleophiles due to the strong -I effect of the halogen.

4. Effect on Bond Strength:

• The inductive effect can also influence bond strength by shifting electron density, affecting bond lengths and reactivity.

4.0Summary of Inductive Effect

5.0Solved Examples

Q. Why is Ethyl Amine More Basic than Aniline?

Solution:

• Ethyl amine (CH₃CH₂NH₂) is more basic than aniline (C₆H₅NH₂) because of the +I effect of the ethyl group.
• The ethyl group is an electron-donating group, which increases the electron density on the nitrogen atom in ethyl amine, making it more available to donate a lone pair of electrons to accept a proton (H⁺).
• In aniline, the lone pair of electrons on the nitrogen is delocalized into the benzene ring through resonance, reducing its availability for protonation. This makes aniline less basic compared to ethyl amine.

Q. Why is Cl–NH₂ Less Basic than Methyl Amine?

Solution:

• Chloroamine (Cl–NH₂) is less basic than methyl amine (CH₃NH₂) because of the –I effect of the chlorine group and pπ–dπ conjugation.
• Chlorine is an electron-withdrawing group due to its high electronegativity, which pulls electron density away from the nitrogen atom, reducing its ability to donate electrons for protonation.
• Additionally, the pπ–dπ conjugation between the nitrogen and chlorine atoms allows for further electron withdrawal, weakening the basicity of Cl–NH₂.
• In contrast, methyl amine has a +I effect due to the electron-donating nature of the methyl group, which enhances the electron density on nitrogen, making it more basic.

Q. Which is most basic among the following :

(A) CH₃NH₂                       

(B) CH₃CH₂NH₂                    

(C) NH₃                           

(D) (CH₃)₂CHNH₂

To determine the most basic compound, we need to consider the +I effect (electron-donating effect) of the alkyl groups attached to the nitrogen atom. Alkyl groups increase the electron density on nitrogen, making it more available for protonation and thereby increasing basicity.

Comparison:

(A) CH₃NH₂ (Methylamine): Has a +I effect from the single methyl group, increasing basicity compared to ammonia.

(B) CH₃CH₂NH₂ (Ethylamine): Has a stronger +I effect due to the ethyl group, which donates more electron density than the methyl group, making ethylamine more basic than methylamine.

(C) NH₃ (Ammonia): Has no alkyl group to donate electron density, so it is less basic compared to compounds with alkyl groups.

(D) (CH₃)₂CHNH₂ (Isopropylamine): Has an even stronger +I effect due to the presence of two methyl groups on the isopropyl chain, increasing the electron density on nitrogen, making it more basic than ethylamine and methylamine.

Conclusion:

(D) (CH₃)₂CHNH₂ (Isopropylamine) is the most basic among the given options due to the strong +I effect from the isopropyl group.