Free-Radical Reactions: Mechanisms, Polymerization, and Alkane Halogenation

Chapter 10: Free-Radical Reactions

10.1 Free-Radical Addition of Hydrogen Bromide to Alkenes

This section explores the mechanism and consequences of free-radical addition of HBr to alkenes, focusing on the peroxide effect and the role of free radicals in organic reactions.

• Peroxide Effect: The presence of peroxides reverses the regioselectivity of HBr addition to alkenes, causing bromine to add to the less substituted carbon (anti-Markovnikov addition).

• Mechanism: Involves the formation of free radicals—species with at least one unpaired electron.

• Key Example: Addition of HBr to isobutylene in the presence of peroxides yields 1-bromo-2-methylpropane.

Radical Reactions and Electron Movement

• Heterolytic Bond Breaking: Both electrons from a bond go to one atom, forming ions.

• Homolytic Bond Breaking: Each atom takes one electron, forming radicals.

• Fishhook Notation: Curved arrows with a single barb (fishhook) are used to indicate the movement of single electrons in homolytic processes.

Three Steps in Free-Radical Chain Reactions

1. Initiation: Formation of radicals from non-radical species (e.g., decomposition of di-tert-butyl peroxide).

2. Propagation: Radicals react with stable molecules to form new radicals, continuing the chain.

3. Termination: Two radicals combine to form a stable, non-radical product (rare due to low radical concentration).

Propagation vs. Termination

• Propagation steps are more common than termination steps because free-radical intermediates are present in very low concentrations.

Explanation of the Peroxide Effect

• Anti-Markovnikov addition occurs because the formation of a tertiary radical is faster and more stable than a primary radical due to steric effects.

• Steric Effects: The bromine radical adds first, and the transition state is stabilized by minimizing steric repulsions.

Free-Radical Stability

• Stability increases with alkyl substitution at the radical center: 3° > 2° > 1°.

Radical	Structure	ΔHo (kJ/mol)	ΔHo (kcal/mol)
methyl	CH3•	146.6	35.0
ethyl	CH3CH2•	118.3	28.3
propyl	CH3CH2CH2•	100.4	24.0
isopropyl	(CH3)2CH•	96.0	21.5
butyl	CH3CH2CH2CH2•	77.0	18.4
sec-butyl	CH3CH2CH•CH3	77.0	18.4
tert-butyl	(CH3)3C•	64.0	15.3

Electrophilic Addition vs. Radical Addition

• Electrophilic Addition: Proton adds first to give the more stable carbocation.

• Radical Addition: Bromine atom adds first to give the more stable carbon radical.

• Example: Addition of HBr to 1-hexyne yields 1-bromo-1-hexene under radical conditions.

Bond-Dissociation Energies (BDE)

• Defined as the standard enthalpy change for homolytic bond cleavage: $X-Y \rightarrow X\cdot + Y\cdot$

• Measures the intrinsic strength of a chemical bond.

Why Peroxide Effect is Not Observed with HI

• The propagation steps for HI are energetically unfavorable due to bond dissociation energies.

• $\Delta H^\circ = \text{BDE (bonds broken)} - \text{BDE (bonds formed)}$

10.2 Conversion of Internal Alkynes into Trans Alkenes

This section describes the reduction of internal alkynes to trans alkenes using sodium in liquid ammonia, a reaction that proceeds via a radical mechanism.

• Reaction: $\text{R-C} \equiv \text{C-R} + 2\text{Na} + 2\text{NH}_3 \rightarrow \text{trans-RCH=CHR} + 2\text{NaNH}_2$

• Mechanism:

  1. Na metal in liquid ammonia provides solvated electrons: $\text{Na} + n\text{NH}_3 (liq) \rightarrow \text{Na}^+ + e^-(\text{NH}_3)_n$

  2. Electron adds to the alkyne, forming a radical anion.

  3. Radical anion abstracts a proton from ammonia, forming a vinylic radical.

  4. Trans vinylic radical is more stable due to reduced steric repulsion.

  5. Another electron and proton transfer yields the trans alkene.

• Note: Catalytic hydrogenation with Lindlar catalyst gives cis alkenes.

10.3 Polymers; Free-Radical Polymerization of Alkenes

Polymers are large molecules made from repeating monomer units. Free-radical polymerization is a key method for synthesizing polymers from alkenes.

• Polymerization: Monomers react in the presence of initiators or catalysts to form polymers.

• Example: $n\text{H}_2\text{C}=\text{CH}_2 \xrightarrow{\text{initiator}} (\text{CH}_2-\text{CH}_2)_n$ (polyethylene)

• Types: Chain (addition) polymerization, step-growth polymerization.

Free-Radical Polymerization Mechanism

1. Initiation: Formation of initiating radical, which adds to a monomer.

2. Propagation: Radical adds to successive monomers, growing the polymer chain.

3. Termination: Two chains combine or a β-scission occurs, ending the chain growth.

Table: Some Alkene Polymers Produced by Chain Polymerization

Polymer Name (Trade Name)	Structure of Monomer	Properties	Uses
Poly(ethylene)	H2C=CH2	Flexible, semirigid	Containers, films
Poly(styrene)	H2C=CH–C6H5	Heat resistant, transparent, rigid	Packaging, insulation, containers
Poly(vinyl chloride) (PVC)	H2C=CHCl	Rigid, can be plasticized	Pipes, leather, hoses
Poly(tetrafluoroethylene) (Teflon)	F2C=CF2	Chemically inert	Nonstick cookware, gaskets
Poly(methyl methacrylate) (Lucite)	H2C=C(CH3)CO2CH3	Clear, colorless	Lenses, windows, fiber optics
Poly(acrylonitrile) (PAN)	H2C=CH–CN	Crystalline, strong, high luster	Fibers

Polymer Stereochemistry

• Isotactic: All substituents on the same side of the polymer chain.

• Syndiotactic: Substituents alternate sides regularly.

• Atactic: Stereocenters are randomly distributed.

10.4 Free-Radical Substitution: The Halogenation of Hydrocarbons

Alkanes react with Cl2 or Br2 under heat or light to form alkyl halides via a free-radical mechanism.

• Successive Halogenation: Multiple halogenations can occur, producing a mixture of products (e.g., methane to methyl chloride, dichloromethane, chloroform, carbon tetrachloride).

• Isolation: Products can be separated by fractional distillation.

Mechanism of Free-Radical Halogenation

1. Initiation: Halogen molecule splits into two radicals (e.g., Cl2 → 2Cl• under light).

2. Propagation: Radical abstracts hydrogen from alkane, forming alkyl radical, which reacts with halogen molecule to regenerate halogen radical.

3. Termination: Two radicals combine to form a stable molecule.

Regioselectivity of Free-Radical Halogenation

• If the alkane has different types of hydrogens, multiple products are possible. The more stable radical intermediate leads to the major product.

• Example: Bromination of isobutane yields mainly tert-butyl bromide due to the stability of the tert-butyl radical.

Bromination vs. Chlorination of Alkanes

• Bromination is more selective than chlorination because the transition state for bromination is higher in energy and more closely resembles the radical intermediate (Hammond's postulate).

• Chlorination is less selective but faster due to a lower activation energy.

Reactivity–Selectivity Principle

• A more reactive species is less selective, and a less reactive species is more selective.

Additional info: These notes cover the core mechanisms and principles of free-radical reactions, including addition, polymerization, and substitution, as well as the energetic and stereochemical considerations relevant to Organic Chemistry students.