Free radical halogenation

1. Initiation

Initiation (chemistry)

In chemistry, initiation is a chemical reaction that triggers one or more secondary reactions. Often the initiation reaction generates a reactive intermediate from a stable molecule which is then involved in secondary reactions....
: Splitting or homolysis

Homolysis

In chemistry, homolysis or homolytic fission is chemical bond dissociation of a neutral molecule generating two free radicals. That is, two electrons that are involved in the bond are distributed one by one to the two species....
 of a chlorine molecule to form two chlorine atoms, initiated by ultraviolet radiation.

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Encyclopedia

In organic chemistry

Organic chemistry

Organic chemistry is a discipline within chemistry which involves the science study of the structure, properties, composition, chemical reaction, and preparation of chemical compounds that contain carbon....
, free radical halogenation is a type of halogenation

Halogenation

Halogenation is a chemical reaction that incorporates a halogen atom into a molecule. More specific descriptions exist that specify the type of halogen: fluorination, chlorination, bromination, and iodination....
. This chemical reaction

Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
 is typical of alkane

Alkane

Alkanes, also known as paraffins, are chemical compounds that consist only of the elements carbon and hydrogen , wherein these atoms are linked together exclusively by single bonds without any cyclic structure ....
s and alkyl

Alkyl

An alkyl is a univalent Radical consisting of carbon and hydrogen atoms, arranged in a chain. The Alkyls form homologous series with the general formula CnH2n+1....
-substituted aromatics under application of heat or UV light. The reaction is used for the industrial synthesis of chloroform

Chloroform

Chloroform, also known as trichloromethane and methyl trichloride, is a chemical compound with chemical formula CarbonHydrogenChlorine3....
 (CHCl₃), dichloromethane

Dichloromethane

Dichloromethane or methylene chloride is the chemical compound with the chemical formula CarbonHydrogen2Chlorine2....
 (CH₂Cl₂) and hexachlorobutadiene

Hexachlorobutadiene

Hexachlorobutadiene, Cl2C=CC=CCl2, is a colorless liquid at room temperature that has an odor similar to that of turpentine....
. It proceeds by a free radical chain mechanism.

General mechanism

The chain mechanism is as follows, using the chlorination of methane

Methane

Methane is a chemical compound with the molecular formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees....
 as a typical example:

1. Initiation

Initiation (chemistry)

In chemistry, initiation is a chemical reaction that triggers one or more secondary reactions. Often the initiation reaction generates a reactive intermediate from a stable molecule which is then involved in secondary reactions....
: Splitting or homolysis

Homolysis

In chemistry, homolysis or homolytic fission is chemical bond dissociation of a neutral molecule generating two free radicals. That is, two electrons that are involved in the bond are distributed one by one to the two species....
 of a chlorine molecule to form two chlorine atoms, initiated by ultraviolet radiation. A chlorine atom has an unpaired electron and acts as a free radical.

2. chain propagation

Chain propagation

Chain propagation is a process in which a reactive intermediate is continuously regenerated during the course of a chemical reaction. In polymerization reaction, the reactive end-groups of a polymer chain react in each propagation step with a new monomer molecule transferring the reactive group to that last unit....
 (two steps): a hydrogen atom is pulled off from methane. The methyl radical then pulls a Cl· from Cl₂.

This results in the desired product plus another chlorine radical. This radical will then go on to take part in another propagation reaction causing a chain reaction

Chain reaction

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
. If there is sufficient chlorine, other products such as CH₂Cl₂ may be formed.

3. chain termination

Chain termination

Chain termination is any chemical reaction leading to the destruction of a reactive intermediate in a chain propagation step in the course of a polymerization, effectively bringing it to a halt....
: recombination of two free radicals:

The last possibility in the termination step will result in an impurity in the final mixture; notably this results in an organic molecule with a longer carbon chain than the reactants.

Control of halogenation

• Halogenation often does not stop at monosubstititution, depending on reaction conditions the chlorination of methane yields dichloromethane
  Dichloromethane
  
  Dichloromethane or methylene chloride is the chemical compound with the chemical formula CarbonHydrogen2Chlorine2....
  , chloroform
  Chloroform
  
  Chloroform, also known as trichloromethane and methyl trichloride, is a chemical compound with chemical formula CarbonHydrogenChlorine3....
   and carbon tetrachloride
  Carbon tetrachloride
  
  Carbon tetrachloride, also known by many other names is the organic compound with the chemical formula CCl4. It is a reagent in organic synthesis chemistry and was formerly widely used in fire extinguishers, as a precursor to refrigerations, and a cleaning agent....
  .
• In most hydrocarbon
  Hydrocarbon
  
  In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "pure" hydrocarbons, whereas other hydrocarbons with bonded com...
  s more than one possible product exists depending on which hydrogen is replaced. Butane
  Butane
  
  Butane, also called n-butane, is the unbranched alkane with four carbon atoms, CH3CH2CH2CH3....
   (CH₃-CH₂-CH₂-CH₃), for example, can be chlorinated at the "1" position to give 1-chlorobutane (CH₃-CH₂-CH₂-CH₂Cl) or at the "2" position to give 2-chlorobutane (CH₃-CH₂-CHCl-CH₃). The product distribution depends on relative reaction rates: in this case the "2" position of butane reacts faster and 2-chlorobutane is the major product.
• Chlorination is generally less selective than bromination. Fluorination is not only even less selective than chlorination, but also highly exothermic
  Exothermic
  
  File:Explosion1.JPG In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in form of light , electricity , or sound....
   and care must be taken to prevent an explosion or a runaway reaction. This relationship is often used as a demonstration of the reactivity–selectivity principle
  Reactivity–selectivity principle
  
  In chemistry the reactivity?selectivity principle or RSP states that a more reactive chemical compound or reactive intermediate is less selective in chemical reactions....
   and can be explained with the aid of the Hammond postulate. A bromine radical is not very reactive and the transition state
  Transition state
  
  The transition state of a chemical reaction is a particular configuration along the reaction coordinate. It is defined as the state corresponding to the highest energy along this reaction coordinate....
   for proton abstraction has much radical character and is reached late. The reactive chlorine radical develops a transition state resembling the reactant with little radical character. When the alkyl radical is fully formed in the transition state it can benefit fully from any resonance stabilization present thereby maximizing selectivity.
• We can also look at the bond dissociation energies
  Bond dissociation energy
  
  In chemistry, bond dissociation energy, D0 or BDE, is one measure of the bond strength in a chemical bond. It is defined as the enthalpy when a bond is cleaved by homolysis, with reactants and products of the homolysis reaction at 0 K ....
   (BDE's) to understand the selectivity of bromination. The BDE of a bond is the energy required to break it by homolytic cleavage, and these values can be used to determine if a reaction or step in a reaction is exothermic
  Exothermic
  
  File:Explosion1.JPG In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in form of light , electricity , or sound....
   or endothermic
  Endothermic
  
  In thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning ?inside? and the Greek suffix ?thermic, meaning ?to heat?....
  . In a chain reaction step of a Br radical reacting with a hydrogen on a secondary carbon to cleave the H-C bond requires 397 kJ/mol and an H-Br is formed. We can look at the BDE of H-Br (=366 kJ/mol) and subtract this value from 397 kJ/mol to get +31 kJ/mol. This positive value tells us that this step in the reaction requires energy (endothermic) and the reactants are more stable than the products. Comparing this with the same situation of a Cl radical we see that this step is exothermic (397 kJ/mol - 431 kJ/mol = -20 kJ/mol). From these values we can conclude that in bromination it is more important that the most stable radical (tertiary) be formed during this step and thus it is more selective than chlorination. This is because less energy is required to form the H-Br and tertiary radical (380 kJ/mol - 366 kJ/mol = +14 kJ/mol). This value is 17 kJ/mol less than the secondary radical formation. Iodine does not even participate in free radical halogenation because the entire reaction is endothermic.
• It is possible to predict the product distribution of different monochloro derivatives resulting from the chlorination of an alkane with non-equivalent hydrogens . From experimentation it has been determined that the relative rates of chlorination to the primary: secondary: tertiary positions are 1: 3.8: 5 respectively. This makes sense from our understanding of radical stability: tertiary>secondary>primary We can use this information to determine the percentage of each product formed. Lets look at 2-methyl butane:

-We will label the unique hydrogens:

a=(CH₃)₂ b=CH c=CH₂ d=CH₃

-Now it is a simple problem of ratios: #H(Rate Factor)/Added Total

a:6 X 1 = 6 a= 6/21.4 = 28%

b:1 X 5 = 5 b= 5/21.4 = 23%

c:2 X 3.7 = 7.4 c= 7.4/21.4 = 35%

d:3 X 1 = 3 d= 3/21.4 = 14%

Total= 6 + 5 + 7.4 + 3= 21.4

• Free radical iodination is usually not possible because iodine is less reactive than the other halogens. Free radical halogenation generally proceeds in the following order:
• Fastest
• Carbons with one or more aryl
  Aryl
  
  In the context of organic molecules, aryl refers to any functional group or substituent derived from a simple aromatic ring, may it be phenyl, thiophene, indole, etc ....
   substituents (benzylic positions)
• Carbons with three alkyl
  Alkyl
  
  An alkyl is a univalent Radical consisting of carbon and hydrogen atoms, arranged in a chain. The Alkyls form homologous series with the general formula CnH2n+1....
   substituents (tertiary positions)
• Carbons with two alkyl substituents (secondary positions)
• Carbons with one or zero substituents (primary positions)
• Slowest
• oxygen
  Oxygen
  
  Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
   is a halogenation inhibitor
  Reaction inhibitor
  
  A reaction inhibitor is a chemical substance that decreases the rate of, or prevents, a chemical reaction....