One of the important reaction mechanisms in organic chemistry is the radical reaction. Radicals are highly reactive intermediates, and new bonds can be formed by using radical reactions.
Radical reactions are a different reaction mechanism from the general organic synthetic reactions with acids and bases. It is difficult for many people to understand what kind of reaction a radical reaction is, because we have to learn something new with different properties.
However, the conditions under which radical reactions occur are fixed. Also, there are certain substances, such as halogens and peracids, that can produce radicals. What’s more, the types of radical reactions are common. Therefore, if you learn when radical reactions occur, you will be able to understand radical reactions.
In this section, we will explain the basics of radical reactions in an easy to understand manner so that you can understand how radical reactions proceed and obtain the products.
Table of Contents
- 1 The Difference Between Heterolysis (Ion) and Homolysis (Radical)
- 2 Identify the Types of Radical Reactions
- 3 Radical Chain Reaction: Initiation and Propagation Step
- 4 Reacts with Alkyl Chains to Create New Bonds
- 5 Synthetic Reactions with Highly Reactive Unpaired Electrons
The Difference Between Heterolysis (Ion) and Homolysis (Radical)
In general, synthetic reactions in organic chemistry focus on the movement of ions. The relationship between an acid and a base causes electrons to move, which causes a synthetic reaction to occur.
When electrons move, two electrons are transferred in most cases. This transfer of electrons is called heterolysis. Heterolysis results in the formation of ions.
The creation of an ion causes the compound to take on a positive or negative charge. This is the relationship between acids and bases, and heterolysis is involved in many of the synthetic reactions.
In radical reactions, on the other hand, a phenomenon called homolysis. When a bond is broken, two electrons do not move to form an ion. Radicals are created when the bond cleaves so that the electrons move one by one to each atom.
In radical reactions, homolysis is always the first step. Just as most synthetic reactions are carried out by acids and bases, radical reactions start with radical formation. Therefore, if homolysis does not occur, a radical reaction cannot occur.
Radicals That Are Highly Reactive and Give Different Compounds
Among the intermediates of organic compounds, radicals are known to be highly reactive. Because they have a single electron, instead of two, they can easily react chemically with other compounds.
Also, compared to the ions produced by heterolysis, the radicals produced by homolysis can yield different products. For example, in the addition reaction of hydrogen bromide, the products of the acid/base reaction and the radical reaction are different as follows.
Different reaction conditions will result in different compounds. The reason why radical reactions are important in organic chemistry is that they give a different product than reactions with ions.
So why are the products different as described above? In the addition reaction of hydrogen bromide to an alkene, a carbocation is formed, and the synthetic reaction proceeds.
On the other hand, when hydrogen bromide is reacted with the same alkene, radical products can be synthesized in the presence of oxygen or peroxide. Radical reactions mainly use peracids. The peracids undergo homolysis to create radicals, which can then be used to obtain radical products.
-Conditions for Obtaining Radicals
When are radicals produced? In most cases, radicals are formed at high temperatures above 200°C by homolysis. Radicals are created when high energy is applied.
However, there are cases that radicals can be produced even if the temperature is a little higher than room temperature. In compounds with weak bonds, radicals are created when the bonds are broken. We have mentioned that radical reactions use mainly peracids, and peracids are known to produce radicals easily.
Hydrogen peroxide is one of the most famous peracids. Among these peracids, benzoyl peroxide (BPO) is frequently used as a radical initiator. This is because the bonds between the oxygen bonds are weak and easily produce radicals.
Halogens such as chlorine (Cl2) and bromine (Br2) are also frequently given as examples of radical reactions. When the halogen is exposed to heat or light (UV), the bonds are cleaved and radicals are created.
Halogens are also known to bond weakly with each other. Therefore, when halogens such as chlorine and bromine are present, radicals are created.
Identify the Types of Radical Reactions
So what are the different types of radical reactions? We have already discussed homolysis. The first of all radical reactions is homolysis. It is because homolysis occurs that radical reactions proceed.
There is also the reverse of homolysis. When radicals react with each other and form a bond, it is called radical coupling. The following reactions are radical coupling.
Of the radical reactions, homolysis and radical coupling are easy to understand. However, there are three other types of radical reactions that must be learned. These are the following radical reactions.
- Radical hydrogen abstraction
- Radical addition to a double bond
- Alkene formation in β-cleavage
Learning about these reaction mechanisms will help you to understand how radical reactions occur.
Radical Atom Abstraction Causes Hydrogen Atom Abstraction
One of the reactions characteristic of radicals is the withdrawal of hydrogen atoms. In essence, when a radical is present, the hydrogen atoms are pulled out and new radicals are formed.
This reaction is known as radical atom abstraction. The most important part of atom abstraction is the hydrogen atom abstraction. For this reason, radical atom abstraction is also called hydrogen atom abstraction.
Why not one name: hydrogen atom abstraction? Because not only hydrogen, but also halogen atoms such as chlorine and bromine can be pulled out. In other words, radical atom abstraction is not limited to hydrogen atom abstraction.
However, even though atom abstraction occurs, only hydrogen atom abstraction occurs if no halogen atoms are present. Therefore, hydrogen atom abstraciton is the most important reaction of radical atom abstraction.
Radical Addition Reaction to Double Bond is Important
When there is a double bond in a molecule, radicals react with alkenes to cause an addition reaction. Two important aspects of radical addition reactions to double bonds are as follows.
- Reacting with the end of the double bond to create a bond.
- A new radical is created on the opposite side of the added site.
In a normal addition reaction to an alkene, a polysubstituted alkane is synthesized, as mentioned in the hydrogen bromide example earlier. On the other hand, in a radical reaction, a new bond is formed by the reaction of the radical with the end of the alkene.
After the addition reaction, a new radical is formed on the opposite side. For example, it is as follows.
Why do radicals attack the carbon atoms at the terminal sites? This is because the stability of the radical intermediate is involved.
In general addition reactions, the stability of the carbocation is involved. In the same way, radical intermediates have an order of stability. They are as follows.
When new radicals are formed on the other side after a radical reaction, they tend to be more stable if they are tertiary or secondary radicals. Although radicals are highly reactive intermediates, they have an order of stability. Therefore, in the radical addition reaction to the double bond, there is a positional selectivity.
In the case of the previous compound, the intermediates formed after the radical reaction are tertiary radicals. Considering this, we can predict the compounds that will be synthesized.
β-Cleavage Creates a New Double Bond
In radical reactions, a new double bond may be created. This is called beta cleavage (β-cleavage). We have just described the radical addition reaction to a double bond. The opposite reaction is the formation of an alkene by β-cleavage.
Radical cleavage can occur when an atom causing a radical reaction is bound to the beta position of a carbon atom, such as a halogen. In this case, a radical is transferred to the halogen while creating a double bond. The reaction mechanism is as follows.
In radical reactions, double bonds can be created by β-cleavage. In this reaction, the radical is transferred to a chlorine atom.
Radical Chain Reaction: Initiation and Propagation Step
There are several types of radical reactions. Importantly, these radical reactions can occur in a chain of reactions. In other words, the production of radicals through homolysis leads to a number of radical reactions.
There are always the following stages in a radical reaction
- Initiation step
- Propagation step
- Termination step
We will explain how the chain reaction works in the following simple radical reaction.
-A Radical Reaction Begins in the Initiation Step
When chlorine molecules are used in a radical reaction, they are exposed to heat or light (ultraviolet: UV). When this is done, radicals are generated. All radical reactions have an initiation step due to homolysis.
-Obtain Products in the Propagation Step
After the radicals are formed, the radicals attack other molecules, which leads to further reactions. In this reaction, the chlorine radicals produce a radical to methane by hydrogen atom abstraction. It is as follows.
After producing a methyl radical, the radical next attacks the chlorine. This attack yields the desired compound, chloromethane.
It is important to note that even after the target compound is obtained, radicals are still being generated. The radicals generated in this reaction cause radical atom abstraction again, producing methyl radicals. This is called a chain reaction because the reaction occurs one after another.
In this reaction, hydrogen is pulled out by radicals. On the other hand, when an alkene is reacted, for example, an addition reaction to a double bond occurs. The products vary depending on what reagents are used to react the compounds.
Termination Step Creates a Bond and Stops the Reaction
So is the chain reaction permanent? Of course not. Homolysis causes a chain reaction to occur, but at some point the reaction stops. This is called the termination step.
Radicals can cause hydrogen withdrawal, addition to a double bond, beta cleavage, etc. However, radicals often react with each other to form new bonds. The reverse of homolysis occurs. The following reactions are examples of this.
When these reactions occur, the radicals disappear. Since no new homolysis occurs, new bonds are made and the reaction is completed. Radicals make bonds with each other and the chain reaction is stopped by the termination step.
Strictly speaking, the reaction with oxygen in the air and water is also a radical termination reaction. But don’t consider these reactions with air and water, just think of the termination step as a radical reaction between compounds.
-A Certain Amount of Radical Reaction Initiator is Needed
Since the reaction proceeds by a chain reaction, the desired product can be obtained even with a small amount of an initiator such as benzoyl peroxide (BPO).
However, if the amount of initiator is too small, the reaction will not proceed. This is because some of the generated radicals can form new bonds with each other and stop the reaction, as described above. Therefore, the best amount of initiator is not too much or too little.
Reacts with Alkyl Chains to Create New Bonds
So how can we use these radical reactions in organic chemistry? Synthetic reactions using radical reactions are frequently used to create new bonds.
For example, an alkane can be reacted with a halogen (Cl2 or Br2) to synthesize a compound with an added halogen.
New carbon bonds can also be created by reacting a compound with a double bond. There are several ways to make carbon chains in organic chemistry. One of them is the radical reaction. For example, new carbon chains can be made by the following synthetic reactions.
Radical reactions are useful when you want to make new bonds. Radical reactions can be used to add halogens to carbon chains or to create carbon chains.
-Polymers Are Obtained Through Polymerization Reactions
In addition, because the reaction proceeds through a chain reaction, some compounds often undergo a polymerization reaction to yield polymers. Polymers are produced by the formation of carbon chains one after another.
For reference, in compounds that generate radicals by heat or light (UV), radical terminator such as antioxidants are often added. The reason for this is that without the terminator, molecules would cause radical reactions to occur and polymerize to form polymers.
Regioselectivity and Radical Stability
In addition, when using radical reactions, it is important to consider the positional selectivity. You have to understand where in the alkyl chain the radicals react with and what kind of product you get.
We have already discussed regioselectivity. The stability of the radical intermediates that are produced plays a major role in position selectivity. Again, the stability of the radicals can be described as follows.
- Tertiary radicals > Secondary radicals > Primary radicals
Therefore, when radicals are formed, the formation of radicals with more substituents is given priority. For example, the following is an example.
Checking the radical intermediates, the more stable secondary radicals are preferentially produced in this case. Therefore, there is one main compound.
Stability of Allyl and Benzyl Compounds
Allyl and benzyl compounds also exist, depending on the compound. If these compounds have radicals, the structural formula is as follows.
The intermediates of allyl and benzyl radicals are highly stable. Of course, because they are radicals, they are highly reactive. However, allyl and benzyl radicals are known to be more stable than tertiary radicals. The stability of radicals is as follows.
- Benzyl radicals > Allyl radicals > Tertiary radicals > Tertiary radicals > Secondary radicals > Primary radicals
The reason for this order is that the allyl and benzyl radicals can write the following resonance structure.
When you understand these, you will be able to understand what kind of positional selectivity allows the reaction to proceed. For example, in the following reactions, we can consider two types of reactions.
However, there is only one reaction that actually occurs. In the intermediate you can draw a benzyl radical or a secondary radical.
In radicals, the benzyl radicals are more stable. Therefore, the product is 3-bromocyclohexene. Of the compounds shown in the figure above, the upper compound (3-bromocyclohexene) can be obtained selectively. This is the regioselectivity in the radical reaction.
Synthetic Reactions with Highly Reactive Unpaired Electrons
Many people are not familiar with radical reactions because they are not common chemical reactions. Radicals are unstable intermediates with unpaired electrons, and they cause chain reactions.
However, radical reactions are useful for creating new bonds. Nucleophilic substitution, addition, and cleavage reactions are known to be caused by the relationship between acids and bases. Radical reactions can be used to obtain compounds that cannot be synthesized by these chemical reactions.
In radicals, the reaction is started by the presence of a initiator that causes homolysis, such as a halogen or peracids. The reactions that occur, such as radical atom abstraction and addition to a double bond, are also fixed. There is also an order of stability in radical intermediates and a positional selectivity.
Once these properties are understood, it becomes possible to understand what compounds can be obtained by radical reactions.