Electronic Displacement Effects (GOC)

The Electron displacement effects in covalent bonds are fundamental concepts in organic chemistry  (GOC) because they provide a deep understanding of how and why organic molecules behave the way they do in various chemical reactions and environments. 

1.0What is Electronic Displacement Effect?

The Electronic Displacement Effect in organic chemistry refers to the movement or shifting of electrons within a molecule, affecting the molecule's reactivity and stability. This concept is very important for understanding reaction mechanisms and the behavior of organic compounds. Here are the key types of electronic displacement effects:

1. Inductive Effect (I-effect): This is a permanent effect where electrons are displaced along a chain of atoms due to the electronegativity difference between the atoms. It results in a permanent polarization of the bond, influencing the molecule's chemical properties.

2. Resonance Effect (R-effect): Also known as mesomeric effect, it involves the delocalization of π-electrons or lone pairs of electrons across adjacent atoms, creating structures known as resonating or canonical structures. This effect stabilizes the molecule and is significant in determining the molecule's structure and reactivity.

3. Hyperconjugation (H-effect): This is the interaction of σ-bonds (usually C-H bonds) with adjacent π-bonds or empty p-orbitals, leading to delocalization of electrons. It affects the stability and reactivity of the molecule, playing a crucial role in the stability of carbocations, for instance.

4. Electromeric Effect (E-effect): This is a temporary effect where electron pairs shift from one atom to another in the presence of a reagent. It is observed during a chemical reaction and disappears once the reaction is complete.

5. Inductomeric Effect (In-Effect): which is often considered a subset of the inductive effect. It refers to the temporary shift of electrons along the sigma bonds due to the presence of an attacking reagent, typically during a chemical reaction.

2.0Inductive Effect 

The inductive effect is a concept in organic chemistry that describes the electronic effect transmitted through σ bonds. It originates from the electronegativity differences between atoms, causing a shift in electron density along the chain of atoms in a molecule. This effect can influence the molecular structure, reactivity, and the acidity or basicity of compounds.

As the chain of atoms extends in a molecule, the inductive effect spreads from one atom to the next, gradually diminishing in significance. For instance, if a carbon atom at the end of a chain is bonded to an electronegative halogen atom, such as fluorine or chlorine, which acts as an electron-withdrawing group, the positive charge is transmitted along the carbon chain due to the inductive effect. This phenomenon is depicted in the accompanying diagram.

δ = δ₁+δ₂+δ₃+δ₄

Where: δ > δ₁> δ₂ > δ₃ > δ₄

It is a permanent effect, present in saturated carbon chains due to the difference in electronegativities of the atoms or groups attached to the chain. It does not involve the π electrons but is instead transmitted through σ bonds.

Types of Inductive Effect

Application of Inductive Effect

1. Acidity and Basicity: 

The inductive effect significantly impacts the acidity of molecules. 

For instance, in carboxylic acids, electron-withdrawing groups (showing a -I effect) increase the acid strength by stabilizing the carboxylate anion through electron withdrawal. 

Conversely, electron-donating groups (showing a +I effect) decrease the acidity by destabilizing the anion.

2. Stability of Reactive Intermediates: The inductive effect impacts the stability of carbocations, anions, and free radicals. 

Electron-withdrawing groups stabilize anions by dispersing their negative charge. 

Stability of carbanion ∝  EWG (–I)

In contrast, they destabilize carbocations by withdrawing electron density from an already electron-deficient center. Electron-donating groups have the opposite effect.

Stability of carbocation ∝  EWG (+I)

3.0Mesomeric Effect or Resonance Effect

The mesomeric effect, also known as resonance effect, is a phenomenon in organic chemistry where a compound with conjugated double bonds or lone pair electrons exhibits electron delocalization. This effect influences the distribution of electrons in molecules and plays a vital role in determining the chemical properties of compounds.

Types of Mesomeric Effects

Applications of Resonance Effect

Stability of Intermediates: In organic reactions, intermediates like carbocations, carbanions, and radicals are stabilized by resonance. For example, a carbocation adjacent to a double bond is more stable due to the delocalization of electrons.

Stability of carbocation ∝ EDG (+R / +H / +I)

Stability of carbon free radical ∝ EDG (+R / +I)  

Stability of carbanion ∝ EWG (–R / –I)

Acidity and Basicity: The mesomeric effect influences the strength of acids and bases. For instance, the acidity of phenol is higher than that of alcohols due to the resonance stabilization of the phenoxide ion.

Resonating structures of phenoxide ion are : 

Note- Groups which are –I, –M increase acidic character of phenol by effectively dispersing negative charge of phenoxide ion. However, +I and + m groups decrease acidic strength.

4.0Hyperconjugation Effect (H-Effect)

The hyperconjugation effect, often referred to as "no-bond resonance" or "Baker-Nathan effect," is a concept in organic chemistry that describes the delocalization of electrons within a molecule, which can influence its stability and reactivity. This effect is particularly notable in alkenes, carbocations, and other systems with adjacent σ (sigma) bonds and π (pi) or empty p orbitals.

Conditions:

1. α carbon (alpha carbon) must be sp³ hybridized.

2. α-H (alpha hydrogen)must be present.

Number of hyperconjugative structures  ∝  number of α-H.

For example, Hyperconjugation in Carbocation-

Important Points-

• Hyperconjugation is a distance independent effect.
• It is a permanent effect.
• Generally, it is more dominant than inductive effect but less dominant than mesomeric/ resonance effect.
• Inductive effect order: -CT₃ > -CD₃ > -CH₃
• But H-effect order: -CH₃ > -CD₃ > -CT₃

Application of Hyperconjugation

1. Stability of Alkenes :- More is the number of hyperconjugative structures, more stable is the alkene. So  "More alkylated alkenes are more stable". 

                                         Stability in decreasing order

2. Heat of hydrogenation : Greater the number of hydrogen results in greater stability of alkene. Thus greater extent of hyperconjugation results in lower value of heat of hydrogenation. So order for heat of hydrogenation of some alkene can be given as

CH₂ = CH₂ >CH₃ –CH =CH₂ > CH₃ – CH = CH – CH₃

3. Dipole moment :Since hyperconjugation causes the development of charge, it also affects the dipole moment of the molecule.

Example : CH₂ = CH₂  <   CH₃ – CH = CH₂  (Dipole moment)

5.0Electromeric Effect

This type of electron displacement effect in organic chemistry refers to a temporary electron displacement within a molecule, typically occurring in response to an external reagent. This effect involves a complete transfer of electrons from a double bond or a lone pair to an adjacent atom in the molecule, significantly influencing the molecule's reactivity during chemical reactions.

1. Positive Electromeric Effect (+E): Occurs when electrons are transferred from an atom or a double bond to an attacking reagent. For instance, in the addition of a proton to an alkene, the electrons from the double bond move towards the proton, showcasing a +E effect.

2. Negative Electromeric Effect (-E): This happens when electrons are shifted towards an atom within the molecule from the attacking reagent.

6.0Inductomeric Effect

Unlike the permanent inductive effect, the inductomeric effect is temporary and occurs in the presence of a reagent during a reaction. Once the reaction is over, the electrons revert to their original positions.

• Mechanism: The effect is seen when a functional group influences the electron density along the sigma bonds in a molecule. When a molecule is subjected to an electrophilic attack, the electron density in the sigma bonds might shift temporarily towards or away from the point of attack, altering the reactivity of the molecule.

7.0Applications of Electronic Displacement Effect

The electron displacement effect in organic chemistry influences various properties and reactions of molecules:

• Acidity/Basicity: Affects strength based on electron-withdrawing or donating groups.
• Stability: Influences the stability of intermediates like carbocations and free radicals.
• Aromatic Reactivity: Determines reactivity in aromatic substitution reactions.
• Pericyclic Reactions: Guides outcomes in reactions involving electron delocalization.
• Bond Properties: Affects bond lengths and strengths in molecules.
• Stereochemistry: Influences molecular conformations and reaction stereochemistry.
• Nucleophilicity/Electrophilicity: Determines the reactive character of molecules.
• Color: Affects the color of compounds, particularly in conjugated systems.