Electron Displacement Effects in Organic Chemistry

Electron Displacement Effects in Organic Chemistry

• Crucial for grasping reaction mechanisms, though often a source of confusion for students.
• Includes inductive, electromeric, and resonance effects, along with hyperconjugation.

Understanding Electron Movement

• Essential for understanding reaction mechanisms.
• Curved arrows symbolize the movement of electrons.
• The electron source is indicated by the tail of the arrow.
• The electron destination is shown by the head of the arrow.
• Double-headed arrows represent the movement of electron pairs.
• Single-headed arrows (fishhooks) symbolize the movement of a single electron, such as in homolytic cleavage.
• Oxygen with a negative charge attacking carbon is exemplified in a reaction.
• Bromine leaves, taking its bonding electrons, resulting in a bromide ion (Br−).
• Carbon, now electron-deficient, forms a bond with the oxygen from OH.
• Chemical reactions are impacted by electron movement.

Why Do Electrons Move?

• Stability is the driving force behind electron movement.
• Electron displacement occurs due to the Inductive Effect and Electromeric Effect.
• Permanent effects are always present.
• Temporary effects occur only when needed, like the electromeric effect.

Inductive Effect: A Permanent Displacement

• Permanent effect
• Atoms attracting electrons are more electronegative.
• Electronegativity increases across a period (left to right) and decreases down a group in the periodic table.
• The electronegativity concept uses the pizza analogy.
• The more electronegative atom becomes partially negative (δ−).
• The less electronegative atom becomes partially positive (δ+).
• Electronegativity order: sp > sp2 > sp3 hybridized carbons because of increasing s-character.
• Higher s-character means electrons are closer to the nucleus, increasing electronegativity.
• Displacement of sigma (σ) electrons in a molecule.
• The effect decreases as the distance from the electronegative atom increases.

Characteristics of Inductive Effects

• The effect weakens with distance, and δ+ decreases with each carbon atom away from the electronegative atom.
• The strength of the electron-withdrawing group determines the strength of the Inductive effect.
• Groups are classified as electron-donating or electron-withdrawing relative to hydrogen.
• Electron-withdrawing groups are more electronegative than hydrogen.
• Electron-donating groups are less electronegative than hydrogen.

Classifying Groups Based on Inductive Effect

• Electron-Withdrawing Groups (EWG) exhibit a -I effect.
• Electron-Donating Groups (EDG) exhibit a +I effect.

Trends to remember

• Mnemonics for remembering order: “Nuri Noria is a selfish rude, so the acidity is awesome!”
• The acidity may be key here.
• The group attached influences acidity by the number of NO2 molecules on the acidity.

Application of Inductive Effect: Acidity of Carboxylic Acids

• Carboxylic acids (R-COOH) donate H+ ions, where stronger acids donate H+ more readily.
• Electron-withdrawing groups (-I effect) increase acidity.
• Electron-donating groups (+I effect) decrease acidity.
• Stability of the conjugate base (carboxylate ion) determines acid strength.
• EWGs stabilize the carboxylate ion by dispersing the negative charge.
• EDGs destabilize the carboxylate ion by intensifying the negative charge.

Examples of Acidity Trends

• Comparing halogenated acetic acids: FCH2COOH > ClCH2COOH > CH3COOH because of the higher -I effect of fluorine.
• Hybridization affects acidity.
• Example 2 explained: CarbAnalyst
• Another example: ch3 ch2 oh greater CnH2N oh.
• Proximity effect described.
• Difference of electro negativity described.

Resonance

• Some compounds are not accurately represented by a single Lewis structure (e.g., benzene).
• Benzene's actual structure is a hybrid of multiple contributing structures (resonance structures).
• Resonance involves the delocalization of electrons.
• Real molecules are more stable than what would be predicted from resonance structures.

Conditions for Resonance

• The molecule must be planar.
• Only electrons move, not atoms.
• Single and double bond differences
• Single bonds= 154
• double bons= 134
• benzene =139
• Several Organic molecules dont define single obstrcutive sense

Key Features of Resonance

• Atoms retain their positions.
• The number of paired/unpaired electrons remains constant.
• Contributing structures have similar energy levels.
• Resonance hybrid is more stable than any contributing structure.

Factors Affecting Stability of Resonance Structures

• More covalent bonds increase stability.
• Complete octets on all atoms enhance stability.
• Less charge separation is preferred.
• Negative charge on more electronegative atoms increases stability.
• Similar charges located far apart increase stability.

Rules for Drawing Resonance Structures

• Follow the octet rule.
• Only move electrons.
• Keep the same number of electrons.

Electron Movement Patterns

• Lone pair to adjacent π bond
• π bond to adjacent positive charge.
• π bond to adjacent lone pair.
• π bond to adjacent π bond (conjugated system).

Resonance Hybrid Representation

• Dashed lines indicate partial bond character.
• Charges are delocalized over multiple atoms.
• Resonance hybrid better represents actual electron distribution.

Resonance Effect

• Resonance involves electron displacement.

Electrometric Effect vs. Resonance

• Similar outcome, electrons are relocated in the molecule.
• Resonance is permanent, always redistributing the molecules.
• Electrometric is is on the action of a sudden event, so they go under these rules.

Types of Resonance Effects

• Positive and negative types
• Positive: electrons move in the molecules, a transfer of electrons from substitute group to main system.
• Negative: electrons withdraw in the molecules, a transfer of electrons from main system to substitute group.

Positive Resonance Effect (+R or +M)

• Electron donation from substituent group to the main system.
• Enhances electron density in the system.
• Substituents with lone pairs or negative charge exhibit +R effect
• Example: O−

Negative Resonance Effect (-R or -M)

• Electron withdrawal from the main system by substituent group.
• Decreases electron density in the system.
• Substituents with π bonds to electronegative atoms exhibit -R effect.

Comparing Carboxylic Acid and Carboxylate Ion Stability

• Carboxylate has resonance structures with more stability.
• Carboxylic acids with more easily release of hydrogen ions are more acidic,.
• Stability is impacted by whatever is attached to the structures.