Organic Reactions & Mechanisms

Organic Reactions & Their Mechanisms

• This subject covers organic reactions and their mechanisms.
• It examines factors influencing chemical reactions, including inductive effect, mesomeric effect, electromeric effect, and hyperconjugation.
• Different types of reagents are explored.
• Substitution reactions are also detailed.
• Various reaction mechanisms are analyzed.
• The presentation includes carbocation and carbanion stability.

Lesson Plan

• Factors influencing Chemical Reactions: Inductive effect, mesomeric effect, electromeric effect, hyperconjugation are key elements.
• Reagents: Electrophilic and nucleophilic reagents are discussed.
• Substitution Reactions: SN1, SN2, and free radical substitution reactions are detailed.
• Other Topics: Free radicals, resonance are addressed.

Inductive Effect

• Inductive effect is the process of electron displacement through a chain of carbon atoms.
• It's a permanent effect, appearing as dipole moments in molecules.
• It doesn't rely on the presence of a reagent.
• Electron-withdrawing effect (-I): Atoms or groups attracting electrons more strongly than hydrogen (e.g., Cl, NO2, COOH).
• Electron-releasing effect (+I): Atoms or groups donating electrons less strongly than hydrogen (e.g., alkyl groups).
• The effect diminishes as you move away from the initial atom.

Inductive Effect (-I effect)

• Electrons are displaced towards electron-withdrawing groups.
• These groups obtain a slight negative charge (δ-), while carbon atoms gain a slight positive charge (δ+).
• The effect weakens significantly with distance from the electron-withdrawing atom.

Inductive Effect (+I effect)

• Electrons are repelled by electron-releasing groups.
• These groups acquire a slight positive charge (δ+), while the carbon atoms acquire a slight negative charge (δ-).
• The effect weakens significantly with distance from the electron-donating atom.

Relative Stabilities of Carbocations

• Carbocations' stability: Tertiary > Secondary > Primary > Methyl.
• Factors influencing stability are electron donating/withdrawing groups, resonance stabilization, and inductive effects.

Relative Stabilities of Carbanions

• Carbanions' stability: Tertiary > Secondary > Primary > Methyl.
• Factors influencing stability include electron donating groups, negative charge delocalization, and resonance effects.

Mesomeric Effect (M effect)

• This effect pertains to polarity produced in molecules due to interactions between pi bonds or a pi bond and a lone electron pair.
• It impacts conjugated systems.
• Pi electrons get delocalized, leading to resonance structures.
• In carbonyl groups, pi electrons are drawn towards the more electronegative oxygen atom.
• It is a permanent effect and is independent of the reagent present.

Electromeric Effect

• This effect is apparent in compounds with double or triple bonds.
• Pi electrons are shifted toward the attacking reagent.
• The effect is temporary, disappearing once the reagent is removed.
• Two types of electromeric effects are observed: +E (electrons move toward the reagent) and -E (electrons move away from the reagent).

Hyperconjugation

• Sigma electrons (usually C–H or C–C) interact with adjacent empty or partially filled p-orbitals.
• This interaction creates an extended molecular orbital, leading to increased stability.
• Carbocations and substituted alkenes benefit from hyperconjugation.

Reagents

• Reagents are substances used to initiate or observe reactions.
• Some reagents are elements, most are compounds.

Electrophile / Nucleophile

• Electrophile: Accepts an electron pair in a reaction.
• Nucleophile: Donates an electron pair in a reaction.
• Electrophilies attack regions of high electron density.
• Nucleophiles attack regions of low electron density.

Substitution Reactions

• Involves replacing one or more atoms or groups with other atoms or groups.
• Subtypes classified based on reaction initiation: electrophilic, nucleophilic, and free radical.

SN1 Reaction

• Two-step mechanism.
• First step: Formation of a carbocation (rate-determining step).
• Second step: Nucleophile attack on the planar carbocation.

SN2 Reaction

• One-step, second-order reaction.
• Nucleophile attacks the carbon atom from the back side.
• The reaction proceeds through a transition state.
• Inversion of configuration at the carbon where the substitution occurs (Walden inversion).

ESR (Electrophilic Substitution Reaction)

• A three-step reaction:
• Generation of electrophile
• Formation of carbocation
• Removal of proton from intermediate

Free Radical Substitution Reaction

• Example: Chlorination of methane
• Three steps: initiation, propagation, and termination.
• Initiation: Chlorine molecules are cleaved, generating free radicals.
• Propagation: Free radicals attack molecule to form new free radicals.
• Termination: Combining free radicals form stable molecules.

Resonance

• Molecules are often represented by multiple structures (resonance structures).
• Contributing structures differ only by electron position, not atomic positions.
• The actual structure is an intermediate of resonance forms, named resonance hybrid.
• Resonance delocalises pi electrons throughout the molecule.

Tutorial Questions

• Differences between inductive and mesomeric effects
• Differences between inductive and electromeric effects
• SN1 vs SN2 reactions
• Examples of +M,+I groups