Key Concepts: Bonds breaking, homolytic and heterolytic cleavage, geometric implications, stability, and resonance.
Bond Cleavage
Heterolytic Cleavage
Induced by: Heat (Delta symbol)
Process: Both electrons in a bond move to one atom, forming ionic species ( + and - charges).
Representation: Double barbed arrow.
Homolytic Cleavage
Induced by: Light
Process: Electrons in a bond split, one electron going to each atom forming radicals.
Representation: Single barbed arrows leading to neutral species with unpaired electrons.
Radical Geometry
Carbocation Geometry (SP2)
Example: Tertiary carbocation (three methyl groups in the same plane)
Hybridization: SP2
Empty P Orbital: Perpendicular to the plane
Anion Geometry (SP3)
P Orbital filled with lone pairs
Shape: Trigonal pyramidal due to lone pairs repelling bond pairs
Radical Characteristics
Radical Species: Neutral with one unpaired electron
Geometry: Between planar (sp2) and trigonal pyramidal (sp3), but often assumed to be planar.
Radical Stability: Follows similar order as carbocations due to hyperconjugation.
Stability Trends
Methyl radical < Primary radical < Secondary radical < Tertiary radical due to hyperconjugation.
Bond Dissociation Energies
Significance: Energy required to break a bond homolytically
Example:
Methane: 435 kJ/mol
Tertiary radical: 381 kJ/mol
Trends: Lower energy indicates more stable radicals
Resonance Structures in Radicals
Allylic Radicals: Stabilized by resonance
Benzylic Radicals: Extremely stabilized by resonance compared to allylic
Special Case: Vinylic Radicals
Energy: 464 kJ/mol, thus very unstable
Radical Reactions
No Rearrangements
Unlike carbocations, radicals don't typically rearrange.
Patterns of Radical Reactions
1. Homolytic Bond Cleavage
Example: X-Y bond cleavage under light or heat to form radicals.
2. Addition to a Pi Bond
Example: Radical adds to a double bond, forming a new radical on the adjacent carbon.
3. Hydrogen Abstraction
Example: Radical abstracts a hydrogen from another molecule, forming a new radical.
4. Halogen Abstraction
Example: Alkyl radical abstracts a halogen atom from a molecule.
5. Elimination
Example: Radical removes itself as a pi bond forms on the adjacent carbons.
6. Coupling
Example: Two radicals combine to form a covalent bond.
Stages of Radical Reactions
Initiation
Description: Formation of radical species to start the reaction.
Example: Homolytic cleavage.
Propagation
Description: Radicals react with molecules to form new radicals and continue the reaction.
Utilizes: Hydrogen and halogen abstraction, addition to a pi bond, and elimination.
Termination
Description: Radicals combine and neutralize each other to end the reaction.
Example: Coupling.
Summary
Radical reactions are critical in chemistry, involving specific steps and patterns. Understanding bond dissociation energies, geometry, and stability trends is essential for mastering the behavior of radicals in various reactions.