Inductive Effect and Electronic Displacement Principles in Organic Chemistry

Electronic Displacement Effects Overview

Electronic displacement effects describe the movement or shifting of electrons within a molecule, which influences its reactivity and stability. These effects are categorized into two main types:

• Permanent Effects: These effects are inherent to the molecule's structure and exist even in the ground state.
  • Inductive Effect ($I.E.$)
  • Mesomeric Effect (also known as Resonance)
  • Hyperconjugation
• Temporary Effects: These effects only occur in the presence of an attacking reagent and disappear once the reagent is removed.
  • Inductumeric Effect
  • Electromeric Effect

Fundamental Principles of the Inductive Effect ($I.E.$)

• Definition: The partial displacement of $\sigma$-bond electrons towards a more electronegative ($E.N.$) atom or group within a carbon chain. This results in the development of partial charges ($\delta+$ and $\delta-$).
• Cause: The primary cause of the Inductive Effect is the difference in Electronegativity ($E.N.$) between two bonded atoms ($E.N.\, \text{difference} \neq 0$).
• Transmission: The effect is transmitted through the $\sigma$-bond network of the carbon chain.
• Permanence: It is a permanent effect inherently present in the molecule.
• Strength: It is considered a weak effect because it involves only the partial shifting of electron density, not a complete transfer of electrons.
• Atomic Orbitals: During the Inductive Effect, the involved atoms do not change their atomic orbitals.
• Distance Dependency:
  • The Inductive Effect is highly dependent on distance.
  • As the distance from the source (the electronegative atom) increases, the magnitude of the effect decreases drastically.
  • Formulaic representation of magnitude over distance: $\delta_{1}+ > \delta_{2}+ > \delta_{3}+ > \delta_{4}+ \approx 0$.
  • Generally, the Inductive Effect is considered negligible after the third carbon atom ($3C$) in a chain.
• Reference Point: The Inductive Effect of Hydrogen ($H$) is arbitrarily considered to be zero ($I.E. = 0$).
• Additive Nature: The Inductive Effect is additive; multiple groups showing the same effect will increase the overall magnitude of the displacement.

Types of Inductive Effect

The Inductive Effect is classified into two types based on the electron-withdrawing or electron-donating nature of the group compared to Hydrogen:

$-I$ Effect (Electron Withdrawing Group - EWG)

• Mechanism: Groups that withdraw electron density through the $\sigma$-bond towards themselves.
• Characteristic: These groups are more electronegative than Carbon.
• Relationship to E.N.: As the Electronegativity of the atom increases, the magnitude of the $-I$ effect increases ($E.N. \uparrow \implies -I \uparrow$).

$+I$ Effect (Electron Donating Group - EDG)

• Mechanism: Groups that donate or push electron density through the $\sigma$-bond away from themselves.
• Characteristic: These groups are typically less electronegative than Carbon (in a relative sense) or carry a negative charge.
• General Rule: Alkyl groups ($R$ groups) are generally considered to have a $+I$ effect.

Experimental Series for Inductive Effects

$-I$ Power Series (Descending Order)

$-NF_{3}^{+} > -PR_{3}^{+} > -NH_{3}^{+} > -NO_{2} > -C \equiv N > -CHO > -COOH > -F > -Cl > -Br > -I > -OR > -OH > -C \equiv CH > -NH_{2} > -C_{6}H_{5} > -CH=CH_{2} > -H$

$+I$ Power Series (Descending Order)

$-CH_{2}^{-} > -NH^{-} > -O^{-} > -COO^{-} > 3^{\circ} \text{ alkyl} > 2^{\circ} \text{ alkyl} > 1^{\circ} \text{ alkyl} > -CH_{3} > -T > -D > -H$

Factors Affecting Inductive Strength

Hybridization and Electronegativity

• Electronegativity is directly proportional to the percentage of s-character ($E.N. \propto \% \text{ s-character}$).
  • $Csp$: $50\% \text{ s}$, Bond Angle $180^{\circ}$, Highest E.N., Strongest $-I$
  • $Csp^{2}$: $33.33\% \text{ s}$, Bond Angle $120^{\circ}$, Moderate E.N.
  • $Csp^{3}$: $25\% \text{ s}$, Bond Angle $109^{\circ} 28'$, Lowest E.N., Strongest $+I$ (relative to other carbons)
• Electronegativity Trend for Nitrogen: $Nsp > Nsp^{2} > Nsp^{3}$.
• General Comparison: $E.N. \text{ of } Csp > E.N. \text{ of } Nsp^{3}$.

Bond Angle Repulsion

• Due to repulsion, as the bond angle ($B.A.$) increases, the s-character increases ($B.A. \uparrow \implies \% s \text{ character} \uparrow \implies E.N. \uparrow$).

Isotopic Effect (Hydrogen, Deuterium, Tritium)

• The $+I$ order is $T > D > H$.
• Mechanism: Higher atomic mass ($T > D > H$) leads to lower vibration. Lower vibration results in a more effective collision, leading to higher bond strength. High bond strength correlates with a shorter bond length, which increases the Inductive Effect ($I.E. \uparrow$).

Reaction Intermediates

An intermediate is a species that is the product of one reaction step and the reactant for another step in an overall chemical process.

Carbocation ($C^{+}$)

• Formation: Heterolytic fission.
• Octet: Incomplete ($6 e^{-}$); it is an electron-deficient species (electrophile).
• Hybridization: $sp^{2}$.
• Geometry: Trigonal Planar.
• Stability:
  • Increased by Electron Donating Groups ($EDG$/$+I$).
  • Decreased by Electron Withdrawing Groups ($EWG$/$-I$).
  • $\text{Stability} \propto +I \propto \frac{1}{-I}$.

Carbon Free Radical ($C^{\bullet}$)

• Formation: Homolytic fission.
• Octet: Incomplete ($7 e^{-}$); electron-deficient species.
• Hybridization: $sp^{2}$.
• Geometry: Trigonal Planar.
• Stability:
  • Increased by $EDG$ ($+I$).
  • Decreased by $EWG$ ($-I$).
  • $\text{Stability} \propto +I \propto \frac{1}{-I}$.

Carbanion ($C^{-}$)

• Formation: Heterolytic fission.
• Octet: Complete ($8 e^{-}$); electron-rich species (nucleophile).
• Hybridization: Generally $sp^{3}$.
• Geometry: Trigonal Pyramidal.
• Stability:
  • Decreased by $EDG$ ($+I$).
  • Increased by $EWG$ ($-I$).
  • $\text{Stability} \propto -I \propto \frac{1}{+I}$.

Application Rules for Stability: The DNP Rule

When comparing the inductive effects of multiple groups, use the DNP priority sequence:

1. D (Distance): Inductive effect is most sensitive to distance. A group closer to the charge has a much stronger impact.
2. N (Number): If distances are equal, the group with more substituents showing the effect prevails.
3. P (Power): If distance and number are equal, refer to the $-I$ or $+I$ power series to determine stability.

Questions & Homework

Practice Problems on Stability and Effect Identification

• Ring Current: Large rings or cyclic systems can influence E.N. through ring currents; however, I.E. focuses on $\sigma$ bonds.
• Comparing Stability Examples:
  • In a comparison between $\text{CH}_{3}-\text{CH}_{2}^{+}$ and $\text{CH}_{3}-\text{CH}^{+}-\text{CH}_{3}$, the latter is more stable due to two $+I$ groups.
  • Fluorine ($-I$) decreases carbocation stability. If Fluorine is further away ($3C$ distance), the destabilization is less than if it is at $2C$ distance.

Objective Questions

Question 1: Number of compounds having $-I$ group directly attached to benzene in the list: {$SO_{3}H$, $CN$, $NO_{2}$, $CH_{3}$, $CF_{3}$, $OH$, $COOH$}.

• Answer: 6 (all except $CH_{3}$, which is $+I$).

Question 2: Correct decreasing order of carbocation stability.

• Options: (A) $3^{\circ}$ (B) $2^{\circ}$ (C) $1^{\circ}$ (D) Methyl.
• Answer: $A > B > C > D$ due to increasing $+I$ effect from alkyl groups.

Question 3: Which side chain attached to benzene has a $+I$ effect?

• Options: (A) $-OH$ (B) $-CH_{3}$ (C) $-SO_{3}H$ (D) $-COOH$.
• Answer: (B) $-CH_{3}$.

Question 4: Correct Inductive effect strength comparison.

• Correct Order: $-T > -D > -H$ ($+I$) or isotopic comparison.
• Answer: (D) $-NH_{3}^{+} < -PH_{3}^{+} < -AsH_{3}^{+}$ (based on periodic trends and orbital size).

Question 5: Number of groups showing $-I$ effect among: (A) $-NH_{3}^{+}$, (B) $-NH_{2}$, (C) $-COOH$, (D) $-OCH_{2}CH_{3}$, (e) $-CH=CH_{2}$, (f) $-O-CO-CH_{3}$, (g) $-SO_{2}H$, (h) Benzene ring, (i) $-O^{-}$?

• Answer: 8 (All except $-O^{-}$ which is $+I$).

Question 6: Correct statement regarding Inductive Effect.

• Fact: It implies the shifting of $\sigma$ electrons from a less electronegative atom to a more electronegative atom.
• Answer: (C).

Question 10: Incorrect statement about Inductive Effect.

• Fact: It is transmitted through $\sigma$ bonds, not $\pi$ bonds.
• Answer: (C).

Question 12: In the molecule $\text{CH}_{3}-\text{CH}_{2}-\text{CH}_{2}-\text{Br}$, in which $C-C$ bond is the inductive effect least?

• Answer: $C_{2}-C_{3}$ (Bond 2), because it is furthest from the Bromine atom.

Group Identification Task (Question 13)

Identify the type of effect (+I or -I) for the following attached to a cyclic system:

1. $-CH_{3}$: $+I$
2. $-Cl$: $-I$
3. $-OH$: $-I$
4. $-CH_{3}$: $+I$
5. $-CH_{3}$: $+I$
6. $-CHO$: $-I$
7. $-OH$: $-I$
8. $-C=O$: $-I$
9. $-C_{2}H_{5}$: $+I$
10. $-Cl$: $-I$
11. $-NH_{2}$: $-I$
12. $-CH_{3}$: $+I$
13. $-F$: $-I$
14. $-O^{-}$: $+I$
15. $-CN$: $-I$
16. $-OH$: $-I$
17. $-NO_{2}$: $-I$
18. $-NH_{3}^{+}$: $-I$