FREE RADICAL REACTIONS

Refer to organic reaction mechanisms

COMMON RADICAL INIATORS:

UV light, Br₂, Cl₂, I₂, SmI₂, O₂(triplet) , etc.

RADICAL INIATORS:

1. UV light and a halogen molecule Br₂, Cl₂, or I₂,

e.g.   Br₂  +  UV light ^____> Br* + Br *

can be written as,

Br-Br + UV light ^_____> Br* + Br *

2. Single electron donors such Samarium iodide [ SmI₂ ]

It is an ionic compound that has samarium with 4 unpaired valence electrons available for donation.

CH₃Cl + SmI₂ ^_____> *CH₃ + SmI₂Cl

CH₃HC=O + SmI₂ ^_______> CH₃HC*-O:SmI₂

Can be written as

CH₃HC=O + SmI₂ ^_______> CH₃HC*-O-SmI₂

Two equivalents of single electron donors will give two electrons to any species to form nucleophilic.

e.g   CH₃Cl + 2SmI₂ ^_____> + SmI₂Cl + I₂Sm:CH₃ 

can be written as

CH₃Cl + 2SmI₂ ^_____> + SmI₂Cl + I₂Sm-CH₃   [methyl nucleophilic agent]

1. Reactions of Sodium or lithium in liquid ammonia involves initial donation of an electron to another specie to form a radical but the specie receives another electron to form an anion.

• Na + A ^______> Na ⁺ + A* [radical]
• Na + A* ^______> Na ⁺ + A** can be written as {anion A^–}

2. Triplet oxygen [ O₂(triplet) *O-O* etc.

It does not readily function as common radical initiator but very reactive with other free radicals.

Common radical reactions

Free radical mechanism usually involves 3 steps: initiation, propagation and termination(combination).

Examples

Long term degradation of methylene chloride to HCl and other gases when exposed to sunlight.

2CH₃Cl + UV light + 4Cl₂ ^________> 2CHCl₃ + 4HCl
2CH₃Cl + UV light + Cl₂ ^________> 2HCl + 2CH₂Cl₂

Mechanism

Initiation 

Cl₂    + UV Light ^_____> Cl *   + Cl *

illustration by half arrow mechanism

Propagation  reactions [illustration only ]

CH₃Cl + Cl * ^________> *CH₃ + Cl₂

CH₃Cl + Cl *^________> HCl + *CH₂Cl

*CH₂Cl + HCl ^________> CH₃Cl + Cl

*CH₃ + Cl₂ ^________> CH₃Cl + Cl

*CH₃ + HCl ^________> CH₄ + Cl

CH₄ + *CH₂Cl^________> *CH₃ + CH₃Cl

CH₃Cl + *CH₂Cl^________> CH₂Cl₂ + *CH₃

*CH₂Cl + Cl₂ ^________> CH₂Cl₂ + Cl*

CH₂Cl₂ + Cl *^________> *CHCl₂ +HCl

Cl₂ + *CHCl₂ ^________> CHCl₃ + Cl *

CHCl₃ + Cl * ^________> *CCl₃ + HCl

Cl₂+ * CCl₃ ^________>  CCl₄ + Cl*

*CH₃ + CCl₄ ^________> *CCl₃ + CH₃Cl

TERMINATION / COMBINATION

Cl* + * CCl₃ ^________> CCl₄

Cl * + *CHCl₂ ^________> CHCl₃

Cl * + *CH₂Cl ^________> CH₂Cl₂

*CH₃ + Cl * ^________> CH₃Cl

*CH₃ + *CH₃ ^________> CH₃CH₃