Free Radicals | Chemistry Optional Notes for UPSC PDF Download

Introduction

Any molecular species that have an unpaired electron in an atomic orbital and is capable of independent existence is referred to as a free radical. Most free radicals exhibit certain similar characteristics when an unpaired electron is present. They are highly reactive and inherently unstable. Hence, they can act as oxidants or reductants by either giving or receiving an electron from other molecules.

In various disease conditions, the hydroxyl radical, superoxide anion radical, hydrogen peroxide, oxygen singlet, hypochlorite, nitric oxide radical, and peroxynitrite radical are the most significant oxygen-containing free radicals causing metabolic problems.

Free Radicals

In Chemistry, a molecule with at least one unpaired electron in its outer shell is referred to as a free radical. The majority of molecules have an even number of electrons, and the covalent chemical bonds that hold the atoms in a molecule together often comprise pairs of electrons shared in a bond.

Most radicals are thought to have formed by the cleavage of typical electron-pair bonds, with each cleavage resulting in two distinct entities that both have one unpaired electron from the broken bond along with the rest of the paired electrons.

Free radicals are produced via the homolytic bond fission process. These are extremely reactive compounds with odd electron configurations. Due to the presence of unpaired electrons, free radicals are paramagnetic which implies that they have a small permanent magnetic moment. The presence of free radicals can be determined using this paramagnetic behaviour of any compound or molecule.

Free radicals are extremely reactive, which allows them to remove electrons from other compounds in order to attain stability. Thus, the attacked molecule loses its electron and transforms into a free radical, starting a cascade of events or chain reactions. Hydroxyl radical (OH^∙)  is the most common example of a free radical. The triplet oxygen and triplet carbene are examples of free radicals with two unpaired electrons.

Types of Free Radicals

There are mainly two types of free radicals that exist in nature:

• Neutral Radicals
• Charged Radicals

Neutral Radicals

These radicals do not contain any charge on the central atom and are found to be less reactive than the other types. For example, methyl radical, butyl radical, hydroxyl radical, superoxide radical, etc.

Charged Radicals

The entire compound is positively or negatively charged in charged free radicals. These are found to be highly reactive that can easily react or combine with other compounds or elements.

Structure of Free Radicals

The structure of free radicals varies with the central atom. For example, the geometry of an alkyl halide shows a planar shape with sp² hybridization. 

A carbon atom with three bonds and one unpaired electron is called an organic free radical.

Free radicals with carbon that has an unpaired electron may also be in an sp² hybridised state, in which case the structure is planar and has an odd electron in the p orbital, or it may be sp³ hybridised, in which case the structure may be pyramidal.

An unpaired single electron in an unhybridized p-orbital with trigonal pyramidal or planar geometry and an angle of 120 degrees is sp² hybridised with a carbon radical.

Free Radical Mechanisms

The free radical reactions are chain reactions. The mechanism of such reactions completes in three simple steps. Let us discuss them by taking the chlorination of methane as an example.

Step 1. Initiation

This step begins with the splitting of one of the reactant molecules. In the example of methane chlorination, homolysis of chlorine takes place at initiation resulting in the formation of a neutral chlorine free radical. The splitting is done in the presence of heat or light.

Step 2. Propagation

The “chain” component of chain reactions is described in the propagation phase. Once a reactive free radical is created, it can combine with other stable molecules to create further reactive free radicals. These fresh free radicals then produce yet more radicals, and so forth. Hydrogen abstraction or the insertion of radicals to double bonds are frequent propagation processes.

In the chlorination of methane, a primary methyl radical is produced once the hydrogen atom is removed from methane. The chlorine atom is then drawn away by the methyl radical, producing chlorine-free radicals.

Step 3. Termination

When two free radical species combine to produce a stable, non-radical adduct, chain termination takes place. The low concentration of radical species and the minimal possibility of two radicals interacting with one another make this occurrence. In other words, the reaction has a very high Gibbs free energy barrier, primarily because of entropic rather than enthalpic factors.

Free Radical Reactions

In organic chemistry, several free radical reactions take place including free radical substitution, addition, elimination, etc. The most common reaction observed is the halogenation of alkanes which results in the formation of a mixture of isomers.

Properties of Free Radicals

Uses of Free Radicals