Radical Reactions: Key Terms and Definitions
Terms for Chapter 10: Radical Reactions
Radical
(Sec 3.1A, 10.1, 10.6, 10.7): An uncharged, electron-deficient chemical species in which one or more of the valence shell orbitals contains an unpaired electron, such as the methyl radical, •CH3. Also called a free radical.
Homolysis
(Sec 3.1A, 10.1): The cleavage of a covalent bond so that each fragment departs with one of the electrons of the bond that joined them, which usually produces radicals. Also called homolytic cleavage.
Peroxide
(Sec 10.lA): Compound with the general formula ROOR, which contains an oxygen-oxygen single bond that can undergo homolytic cleavage to produce two alkoxyl radicals: ROOR → 2 RO•
Radical Reaction
(Sec 10.1B): A reaction involving homolysis of covalent bonds that produces radicals.
Hydrogen Abstraction
(Sec 10.1B): A reaction in which a radical removes a H• atom from a molecule, which is converted into a new radical.
Homolytic Bond Dissociation Energy
(Sec 10.2): The enthalpy change that accompanies the homolytic cleavage of a covalent bond to produce two radicals.
State Function
(Sec 10.2): A property of a system that depends only on its present state, which is determined by variables such as temperature and pressure, and is independent of any previous history of the system.
Hess’s Law
(Sec 10.2): For a chemical equation that can be written as the sum of two or more steps, the enthalpy changes, ΔH, for the overall equation equals the sum of the enthalpy changes for the individual steps.
Hyperconjugation
(Sec 4.8, 6.11B, 10.2B): The phenomenon of electron delocalization (via orbital overlap) from a filled bonding orbital to an adjacent unfilled orbital.
Substitution Reaction
(Sec 3.1, 6.2, 10.3, 13.2): A reaction in which an atom or group is replaced by a different atom or group.
Radical Halogenation
(Sec 10.3 – 10.6, 10.8, 15.3): Incorporation of one or more halogen atoms into a molecule through a radical chain reaction mechanism.
Chain Reaction
(Sec 10.4 – 10.6, 10.10, 10.11): A reaction that proceeds by a sequential, stepwise mechanism, in which each step generates the reactive intermediate that causes the next step to occur; the radical mechanism involves chain initiating steps, chain propagating steps, and chain terminating steps.
Activation Energy
(Sec 10.5B): The difference in enthalpy, ΔH, of the transition state and the reactants; for reactions with very small entropy changes, ΔS ≈ 0 and ΔG ≈ ΔH, so the activation energy is almost the same as the free energy of activation ( Eact ≈ ΔG‡ ). Also called energy of activation.
Radical Addition to Alkenes
(Sec 8.2D, 8.19, 10.9, 10.10): An anti-Markovnikov addition reaction between hydrogen bromide, HBr, and an unsymmetrical alkene in the presence of peroxides, which produces the less substituted alkyl bromide; the mechanism involves addition of a bromine radical to the less substituted carbon, which produces the more substituted carbon radical that subsequently adds hydrogen.
Monomer
(Sec 10.10): The subunit (simple starting compound) from which a polymer is made.
Polymer
(Sec 10.10): A large molecule made up of many repeating subunits. For example, the polymer polyethylene is made up of repeating ethylene subunits, -(CH2 CH2 )n –
Polymerization
(Sec 10.10): Reaction that joins monomer molecules together to form a polymer.
Radical Polymerization
(Sec 10.10): An alkene polymerization reaction that proceeds via radical intermediates. Also called free-radical polymerization.
Chain-Growth Polymer
(Sec 10.10): A polymer that results from the stepwise addition of monomers to a growing chain (usually through a chain reaction), with no loss of other atoms or molecules in the process. Also called an addition polymer.
Autoxidation
(Sec 10.11D): Reaction of an organic compound with oxygen to form a hydroperoxide, ROOH.
Antioxidant
(Sec 10.11D): A compound that inhibits autoxidation and traps free radicals.
Freon
(Sec 10.11D): A chlorofluorocarbon (CFC), such as CFCl3 or CF2Cl2.
Retrosynthetic Analysis
(Sec 7.16B, 8.21A): A method for planning a synthesis that involves reasoning backward from the target molecule through various levels of precursors, and finally to the starting materials.