Chapter 11: Radicals and Radical Reactions in Organic Chemistry

Radical Compounds

Introduction to Free Radical Compounds

Free radicals are chemical species that contain an unpaired electron, making them highly reactive and generally unstable. Unlike most stable molecules, free radicals do not follow the octet rule, which contributes to their reactivity in organic reactions.

• Definition: A free radical is an atom or molecule with an unpaired valence electron.

• Instability: The lack of a complete octet makes radicals highly reactive and short-lived.

• Common Radical Reaction: Peroxides often undergo homolytic cleavage to generate radicals.

Example Reaction:

Homolytic cleavage of a peroxide bond:

$\mathrm{R-O-O-R} \longrightarrow \mathrm{R-O^\cdot} + \mathrm{^\cdot O-R}$

• Homolytic Cleavage: The process where a bond breaks evenly, and each atom retains one electron, forming two radicals.

• Radical reactions are less common and less predictable than electron pair (polar) reactions.

Radical Stability

Factors Affecting Radical Stability

Radicals are inherently unstable due to their incomplete octet, but their stability can vary depending on their structure. The stability order of radicals is similar to that of carbocations, largely due to the hybridization and resonance effects.

• Electron Location: The unpaired electron in a radical can occupy a p orbital, which is less stable than a filled sp2 orbital.

• Stability Trend: Allylic > Tertiary > Secondary > Primary > Methyl > Vinylic

• Resonance Stabilization: Allylic radicals are stabilized by resonance, making them more stable than simple alkyl radicals.

Stability and Reactivity Table:

Radical Type	Relative Stability	Relative Reactivity
Vinylic	Least stable	Most reactive
Methyl	Low	High
Primary	Moderate	Moderate
Secondary	High	Low
Tertiary	Very high	Very low
Allylic	Most stable	Least reactive

• Forming more stable radicals results in faster reactions.

Radical Halogenation

Overview of Radical Halogenation

Radical halogenation is a reaction where alkanes react with halogens (such as Cl2 or Br2) to form alkyl halides. This process proceeds via a radical chain mechanism and can yield multiple products due to the reactivity of different C–H bonds.

• Any C–H bond in the alkane can potentially react, leading to a mixture of products.

• Major products often result from the most stable radical intermediates.

• Multiple halogenations can occur, especially with excess halogen.

Example: Chlorination of butane yields both 1-chlorobutane and 2-chlorobutane, with the latter being the major product due to the greater stability of the secondary radical intermediate.

$\mathrm{CH_3CH_2CH_2CH_3} + \mathrm{Cl_2} \xrightarrow{h\nu} \mathrm{CH_3CH_2CH_2CH_2Cl} + \mathrm{CH_3CH_2CHClCH_3} + \mathrm{HCl}$

• Product ratio is determined by the relative stability of the possible radical intermediates.

Mechanism of Radical Halogenation

Radical halogenation follows a three-step chain mechanism:

1. Initiation: Formation of radicals, usually by homolytic cleavage of a halogen molecule under heat or light.

   • $\mathrm{Cl_2} \xrightarrow{h\nu} 2\,\mathrm{Cl^\cdot}$

2. Propagation: Radicals react with substrate to form new radicals and products, continuing the chain.

   • $\mathrm{Cl^\cdot} + \mathrm{RH} \rightarrow \mathrm{R^\cdot} + \mathrm{HCl}$

   • $\mathrm{R^\cdot} + \mathrm{Cl_2} \rightarrow \mathrm{RCl} + \mathrm{Cl^\cdot}$

3. Termination: Two radicals combine to form a stable molecule, ending the chain.

   • $\mathrm{Cl^\cdot} + \mathrm{Cl^\cdot} \rightarrow \mathrm{Cl_2}$

   • $\mathrm{R^\cdot} + \mathrm{Cl^\cdot} \rightarrow \mathrm{RCl}$

Allylic Bromination

Selective Bromination at the Allylic Position

Allylic bromination is a highly selective reaction that introduces a bromine atom at the allylic position (the carbon adjacent to a double bond) using N-bromosuccinimide (NBS) and ultraviolet light.

• Allylic C–H bonds are more reactive due to resonance stabilization of the resulting radical.

• NBS is used to maintain a low concentration of Br2, favoring allylic substitution over addition to the double bond.

• Adding only NBS results in allylic bromination; adding HBr can lead to addition across the double bond.

Example: Bromination of cyclohexene with NBS yields 3-bromocyclohexene.

$\mathrm{Cyclohexene} + \mathrm{NBS} \xrightarrow{h\nu, CCl_4} \mathrm{3\text{-}Bromocyclohexene}$

• Allylic radicals are resonance stabilized, leading to multiple possible products.

Radicals and Polymerization

Polymer Formation via Radical Reactions

Polymers are large molecules composed of repeating structural units (monomers). Many important polymers are synthesized via radical chain reactions, especially those involving alkenes.

• Polymerization: The process of linking monomers to form a polymer.

• Radical Initiators: Small amounts of radical initiators (e.g., peroxides) are used to start the chain reaction.

• Example: Ethylene can be polymerized to polyethylene via a radical mechanism.

$\mathrm{n\,CH_2=CH_2} \xrightarrow{\text{radical initiator}} \mathrm{-(CH_2-CH_2)-}_n$

• Most of the polymer mass comes from the monomer units, not the initiator.

Examples of Naturally Occurring Polymers

Many biological macromolecules are polymers, including:

• Cellulose: A glucose polymer found in plant cell walls.

• Proteins: Polymers of amino acids.

• Nucleic acids: Polymers of nucleotides (DNA and RNA).

These natural polymers are essential for life and are formed by condensation reactions rather than radical mechanisms, but understanding their structure is important in organic chemistry.

Summary Table: Types of Polymers

Polymer	Monomer Unit	Type
Polyethylene	Ethylene	Synthetic, radical polymerization
Cellulose	Glucose	Natural, condensation
Protein	Amino acid	Natural, condensation
Nucleic acid	Nucleotide	Natural, condensation

Additional info: Radical polymerization is a key industrial process for making plastics and synthetic rubbers. The control of radical reactions is crucial for determining polymer properties such as molecular weight and branching.