Types of Reaction Mechanism in Organic Chemistry

Organic reactions occur through various types of reaction mechanism, defined by how bonds are broken and formed during transformations. Understanding these mechanisms helps predict outcomes and design efficient synthesis pathways.

Note: Each mechanism involves specific reactive species like nucleophiles, electrophiles, radicals, or ions.

1. Nucleophilic Substitution Reaction Mechanism

In this mechanism, a nucleophile replaces a leaving group from a carbon atom.

• SN1 (Unimolecular): Two-step mechanism with carbocation intermediate.
• SN2 (Bimolecular): One-step mechanism with backside attack; inversion of configuration occurs.

Example:

CH₃Br + OH⁻ → CH₃OH + Br⁻ (SN2)

2. Electrophilic Addition Reaction Mechanism

Common in alkenes and alkynes where an electrophile adds to a π bond.

Example:

CH₂=CH₂ + HBr → CH₃-CH₂Br

3. Electrophilic Substitution Reaction Mechanism

Occurs in aromatic compounds where an electrophile replaces a hydrogen atom on the benzene ring.

Example:

C₆H₆ + Cl₂ (AlCl₃) → C₆H₅Cl + HCl

4. Elimination Reaction Mechanism

Two groups are removed from adjacent atoms, forming double or triple bonds.

• E1: Two-step, carbocation intermediate involved.
• E2: One-step, requires a strong base.

Example:

CH₃-CHBr-CH₃ + OH⁻ → CH₃-CH=CH₂ + H₂O + Br⁻

5. Free Radical Reaction Mechanism

Involves homolytic cleavage and the formation of free radicals (species with unpaired electrons).

Example:

CH₄ + Cl₂ → CH₃Cl + HCl (UV light initiation)

6. Rearrangement Reaction Mechanism

Atoms or groups rearrange within a molecule to form an isomeric product, often via carbocation migration.

Example:

Pinacol rearrangement: Pinacol → Pinacolone

Tip: Identifying the reactive species (nucleophile, electrophile, free radical) is the first step in determining the reaction mechanism.

Comparison Table of Reaction Mechanisms

Mechanism Type	Reactive Species	Common Substrates
Nucleophilic Substitution	Nucleophile	Alkyl halides
Electrophilic Addition	Electrophile	Alkenes, alkynes
Electrophilic Substitution	Electrophile	Aromatic compounds
Elimination	Base	Alkyl halides
Free Radical	Free radical	Alkanes, halogens

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Quiz: Types of Reaction Mechanism in Organic Chemistry

1. Which type of reaction involves a nucleophile replacing a leaving group?
2. What type of species is involved in a free radical mechanism?
3. Name a reaction mechanism common in aromatic compounds.
4. In which mechanism does a π bond break to add atoms?
5. Which reaction type often results in a double bond?

Answers:

1. Nucleophilic substitution (SN1 or SN2)
2. Free radicals (unpaired electrons)
3. Electrophilic substitution
4. Electrophilic addition
5. Elimination (E1 or E2)

For deeper reading, check this Organic Chemistry resource.