Free Radical: Formation of Free Radicals, Characteristics of Free Radicals, Classification of Free Radicals, Stability of Free Radicals, Properties, Reactions Involving Free Radicals, Practice Problems & FAQs

Two children, one at each end of the seesaw, sit on it. The child who is down pushes the ground with his feet, causing him to rise while his friend falls. The friend then pushes up, causing the first child to fall. They continue to swing back and forth on the seesaw until they are ready to stop.

In this game two children are required to play, similarly for a bond formation two electrons are required.

Do you think it would be possible for a single kid to play the see-saw game?I'm sure you know the answer already. The game cant be played. 

Similarly if only one electron is present on a chemical species would it be in a state of stability? 

What do we call such species consisting of only one electron?

Would they exhibit any kind of stability?

Would they be easy for us to prepare?

Let's talk about those species in a bit more detail.

To begin, such species are called radicals (free radicals).

Table of Contents

• Free Radical
• Formation of Free Radicals
• Characteristics of Free Radicals
• Classification of Free Radicals
• Stability of Free Radicals
• Properties of Free Radicals
• Reactions involving Free radicals
• Practice Problems
• Frequently Asked Questions

Free Radical 

A free radical is defined as an atom or group of atoms having an odd or unpaired electron. These results on account of homolytic fission of a covalent bond and are denoted by putting a dot () against the symbol of an atom or group of atoms. 

Examples:

1. Methyl free radical()
2. Ethyl free radical()
3. Chlorine free radical()
4. Hydrogen free radical()

Molecules often have have bonding electron pairs and lone pairs, which are non-bonding or un-shared unshared electron pairs. Based on Pauli's exclusion principle, each bonding or non-bonding electron pair contains two electrons in opposite spin orientation, and in one orbital, whereas an unpaired electron is a single electron, alone in one orbital. A free radical, which is a molecule with an unpaired electron, is a paramagnetic species.

Molecules frequently contain lone pairs, or non-bonding or unshared electron pairs, in addition to bonding electron pairs. Each bonding or nonbonding electron pair, according to Pauli's exclusion principle, consists of two electrons with opposing spin orientations, +1/2 and -1/2 in the same orbital, as opposed to an unpaired electron, which is a single electron by itself in the same orbital. A free radical is a paramagnetic entity and is a molecule with an unpaired electron.

Formation of Free Radicals

By using heat, light or catalysts, free radicals are formed.

Free radical reactions are those reactions which take place in the presence of heat, electricity, light, peroxide, and radicals.

Characteristics of Free Radicals

1. Free radicals are neutral species.
2. In carbon free radicals, the total number of valence electrons is 7.
3. Shape of the free radical is a trigonal planar.
4. Due to incomplete octet, free radicals are highly unstable and because of the strong tendency of the carbon atom carrying the odd electron to acquire one more electron to complete its octet, and free radicals are also very short-lived highly reactive chemical species.
5. Due to the presence of odd electron free radicals, they are paramagnetic in nature.
6. Free radicals are usually formed in the presence of heat, light, electricity or in the presence of peroxides.

Classification of Free Radicals

Free radicals are further classified as primary (1°), secondary (2°), and tertiary (3°) depending on how many carbon atoms are attached to the carbon carrying the unpaired electron.

1. Primary Free radical: In this radical, only one carbon is attached to the carbon atom which contains odd electron.
2. Secondary Free radical: In this radical, two carbon atoms are attached to the carbon atom which contains odd electron.
3. Tertiary Free radical: In this radical, three carbon atoms are attached to the carbon atom which contains odd electron.

Structure of Alkyl Free radical

The hybridisation of carbon atom of alkyl free radical is . Alkyl free radical is having planar structure with having odd electron in vacant p orbital of carbon which is at to the hybrid orbitals.

As the octet of the carbon is not completed in the free radical, hence the radical structure is an extremely unstable and reactive species. The carbon atom in the free radical generally undergoes hybridization, and the structure is triangular planar. 

Stability of Free Radicals

Free radicals have the same order of stability as carbocations, namely 3° >2° >1°.

This stability order can be explained using hyperconjugation.

The number of alkyl groups attached to the carbon atom carrying the odd electrons increases the delocalization of the odd electrons, making the alkyl free radical more stable.

Resonance stabilizes allyl and benzyl free radicals.

The bond dissociation energies of various free radicals can be used to compare their stability (the energy required for the homolytic cleavage of the covalent bond in the molecule to form two radicals). In general, the less energy required for bond breaking, the more stable the radical formed.

Properties of Free Radicals

• Free radicals are a distinct and rare special species that exist only in very specific and limited circumstances. However, some free radicals are common in our daily lives.
• Free radicals are a distinct and rare species that can only be found in very limited and unique environments. On the other hand, we are all too familiar with some free radicals.
• Free radicals and bi radicals are both types of molecular oxygen. According to Hund's rule, standard and stable molecular oxygen is in triplet state, and the two unpaired electrons have the same spin orientation in two orbitals with the same orbital energy.
• An illustration of a free radical and a biradical species is molecular oxygen. Standard and stable molecular oxygen is in a triplet state in accordance with Hund's rule, and the two unpaired electrons have the same spin orientation in both of their orbitals, which results in their having the same orbital energy.
• Nitrogen monoxide and nitrogen dioxide are both stable free radical species. In addition, oxygen free radicals such as superoxide anion radicals and singlet molecular oxygen play a role in immunity.
• Both nitrogen dioxide and nitrogen monoxide are stable free radical species. Additionally, oxygen-free radicals that are involved in immunity include singlet molecular oxygen and superoxide anion radicals.
• Free radicals are extremely reactive and unstable. They can donate or accept electrons from other molecules, allowing them to act as oxidants or reactants.
• Free radicals are very unstable and reactive. They can behave as oxidants or reactants by giving or taking electrons from other molecules.

Types of Reactions usually takes place in Free Radicals

There are three types of reactions which usually take place with free radicals.

1. To form neutral molecules, free radicals mutually combine with each other.

1. Formation of a new radical when a free radical reacts with a neutral molecule.

1. A free radical loses a neutral molecule to form a new free radical.

Reactions involving Free radicals

1. Wurtz Reaction

Wurtz reaction is a coupling reaction in organic chemistry named after Charles Adolphe Wurtz, who is also known for the discovery of aldol reaction. The Wurtz reaction leads to the preparation of higher alkanes. It is also beneficial in preparing alkanes with an even number of carbon atoms.

This reaction is named after Charles Adolphe Wurtz, a French chemist who also discovered the aldol reaction.

Metals such as silver, indium, activated and iron, in addition to sodium, can be employed in the Wurtz reaction to produce alkanes.

Charles Adolphe Wurtz, a French scientist who also discovered the aldol reaction, is honoured by having his reaction named in his honour. Along with sodium, other metals can be used in the Wurtz reaction to create alkanes, including silver, indium, activated copper, zinc, and iron.

The general equation of Wurtz reaction is:

1. Substitution reaction of alkanes

Halogenation, nitration and sulphonation of alkanes involves free radical mechanisms. Some examples of substitution reactions of alkanes are:

1. Kolbe’s reaction

An aqueous solution of the sodium/potassium salt of a saturated carboxylic acid on electrolysis gives an alkane containing an even number of C atoms at the anode. The general chemical equation is given as follows:

Mechanism:

The reaction occur in two steps:

1. At anode (Oxidation)

1. At the cathode (Reduction)

1. Anti-Markovnikov's addition

When alkenes are treated with HBr in the presence of peroxides, anti-markovnikov’s addition occurs such that the H-atom of HBr gets attached to the C-atom with the fewer H-atoms. It is also known as peroxide or Kharasch effect. This addition to an alkene in the presence of peroxide takes place via free radical mechanism.

Example:

Mechanism

The mechanism of addition of hydrogen halide in the presence of peroxide is given as follows:

Chain initiation

Step 1 : The peroxide bond of RCO–O–O–CO–R breaks into radicals (i.e., ) in the presence of sunlight, which is then followed by the removal of carbon dioxide and the formation of alkyl radical (i.e., ).

Step 2: Now, the alkyl free radical ( ) (i.e., phenyl radical) attacks on HBr and results in the formation of R−H (i.e., benzene in benzoyl peroxide) and Br radicals.

Chain propagation

Step 3: The free radical attacks the double bond of propene. The free radical can attack either the first carbon of propene and give a free radical intermediate or can attack the second carbon of propene and give a free radical intermediate. The two intermediates formed are given in the illustration and we know that a free radical is more stable than a free radical.

Step 4: The alkyl radical formed will react with HBr, forming alkyl bromide in major as well as minor amounts.

The hydrogen radical formed from HBr will attack the free radical to give the major product. Hence, in the major product, will be attached to the carbon with more number of hydrogen atoms, which is in contradiction to Markovnikov’s rule. Hence, it is an anti-Markovnikov addition.

Chain Termination

In the termination step, any two radicals combine together in order to terminate the reaction. As radicals are comparatively very less in number, the products obtained will be very less in quantity.

Uses of Free Radicals

• These highly reactive structures are found in the membranes of cells that contain potentially harmful biologically relevant molecules such as DNA, lipids, proteins, and carbohydrates.
• The membranes of cells, which house a variety of potentially hazardous physiologically significant chemicals, including DNA, lipids, proteins, and carbohydrates, are home to these highly reactive structures.
• Free radicals attack important macromolecules, causing cell damage and disrupting homeostasis, such as proteins and nucleic acids.
• Proteins and nucleic acids are two examples of crucial macromolecules that free radicals attack, harming cells and upsetting homeostases.
• In general, alkyl or aryl halides are used as radical precursors for R or Ar, but halogenation of sugars and nucleosides with many OH groups and other delicate functional groups is difficult.
• Alkyl or aryl halides are typically used as precursors for the radicals R or Ar, albeit it can be difficult to halogenate sugars and nucleosides that contain a lot of OH groups or other delicate functional groups.
• The Barton Mccombie reaction is extremely useful in radical reactions involving sugars, nucleosides, and peptides.
• When it comes to radical reactions involving sugars, nucleosides, and peptides, the Barton Mccombie reaction is quite helpful.

Practice Problem

Q. 1 What are the characteristic features of a free radical?

1. Presence of a negative or positive charge
2. Presence of an unpaired electron
3. Presence of an odd number of electrons
4. Produced by homolytic fission of a bond

Solution:

Since a free radical does not have any charge, So A is an incorrect option.

Since a free radical has an unpaired or odd electron, So B is a correct option.

Since a free radical has an unpaired electron, the total number of electrons must be odd. So, C is a

correct option.

Since radicals are produced when a covalent bond is cleaved homolytically, So D is a correct option.

Hence, options (B), (C), and (D) are the correct answers.

Q. 2. Write the decreasing order of stabilities of the following?

1. iii > ii > i
2. ii > iii > i 
3. iii > i > ii
4. i > iii > ii

Solution:

The correct answer is option(C).

(i), (ii) and (iii) are secondary, primary and tertiary free radicals respectively. The electron donating groups increase the stability of free radicals. So the alkyl group being the electron donating group increases the stability of free radicals.

In tertiary free radicals there are 3 alkyl groups present, so it is the most stable among the three. In secondary free radicals there are two alkyl groups present, so it is more stable than primary free radical.

Q. 3. Which of the following free radicals is most stable?

1. Primary
2. Secondary
3. Tertiary
4. Methyl

Solution: 

The stability of free radicals is the same as that of carbocations. The tertiary radical is more stable than secondary and primary radical. This can be explained by the +I effect of the alkyl groups attached to carbon radicals. The more is the number of alkyl groups attached to the carbon radical, the more is the availability of electrons and more is the stabilization.

So tertiary free radicals are more stabilized than secondary and primary.

Q. 4. Which of the free radicals is more stable?

Solution: Since free radicals are electron deficient species, therefore in general the electron donating group increases the stability of free radicals and the electron withdrawing group destabilizes the stability of free radicals.

Since in A there is an electron donating group attached to the para position, it will stabilise the free radical by +M effect and in B at para position electron withdrawing group destabilize the free radical by -M effect.

So A is more stable than B.

Frequently Asked Questions

Q. 1. Name a disease which is caused by free radicals.

Answer: There is mounting evidence that most of humanity's degenerative diseases are caused by harmful free radical reactions. Atherosclerosis is an examples of such diseases.

Q. 2. What are the effects of free radicals on the body?

Answer: Free radicals in the body cause similar degradation by killing cell membranes and making cells vulnerable to disease and pathogens. These free radicals degrade DNA and mitochondria, the fundamental building blocks of all tissues, and cause a slew of health issues in their wake.

Q. 3. Name a reaction which involves a free radical.

Answer: Anti-Markovnikov's addition or peroxide effect or kharasch effect involves a free radical mechanism.

Q. 4.What are the sources of free radicals?

Answer: Free radicals are produced naturally by the human body's essential metabolic processes or by external sources such as X-ray exposure, ozone, cigarette smoking, air pollution, and industrial chemical substances.

Related Topics