Undergraduate
  1. General chemistry  1.1. Introduction to Chemistry  1.2. Atomic Structure  1.2.1. Subatomic particles  1.2.2. Atomic Model  1.2.3. Quantum numbers  1.2.4. Electron Configuration  1.2.5. Periodic trends  1.2.6. Isotopes and their applications  1.3. Chemical bond  1.3.1. Ionic bond  1.3.2. Covalent bonds  1.3.3. Metal Bond  1.3.4. Hybridization  1.3.5. Molecular geometry and VSEPR theory  1.3.6. Bond polarity and dipole moment  1.4. Stoichiometry  1.4.1. Mole concept  1.4.2. Empirical and molecular formula  1.4.3. Limiting reactant and excess reactant  1.4.4. Percent Yield and Purity  1.4.5. Dilution calculation  1.5. States of matter  1.5.1. Gas Laws  1.5.2. Matter and Intermolecular Forces  1.5.3. Solid and Crystal Structures  1.5.4. Phase diagram  1.5.5. Plasma and supercritical fluid  1.6. Chemical reactions  1.6.1. Types of Chemical Reactions  1.6.2. Balancing Chemical Equations  1.6.3. Thermochemical reactions  1.6.4. Reaction energy profiles  1.7. Solutions and Mixtures  1.7.1. Concentration units  1.7.2. Syndrome properties  1.7.3. Solubility and Solubility Rules  1.7.4. Henry's Law and Raoult's Law  1.7.5. Partition coefficient  1.8. Chemical equilibrium  1.8.1. Law of mass action  1.8.2. Le Chatelier's principle  1.8.3. Reaction quotient  1.8.4. To use this online calculator for Equilibrium Constant, enter Equilibrium Constant (Eq.) and hit the calculate button. Here is how the Equilibrium Constant calculation can be explained with given input values -> 0.05 = 0.05/0.  1.8.5. Acid-base balance  1.9. Acids and bases  1.9.1. Arrhenius, Brønsted–Lowry, and Lewis definitions  1.9.2. pH and pOH  1.9.3. Acid-base titration  1.9.4. Buffer Solution  1.9.5. Acid-base indicator  1.9.6. Polyprotic Acids and Bases  1.10. Electrochemistry  1.10.1. redox reactions  1.10.2. Galvanic and Electrolytic Cells  1.10.3. Nernst equation  1.10.4. Electrolysis  1.10.5. Faraday's laws of electrolysis  1.11. Thermodynamics  1.11.1. Laws of Thermodynamics  1.11.2. Enthalpy, entropy and Gibbs free energy  1.11.3. Spontaneity of reactions  1.11.4. Thermodynamic cycles (Hess's law, Carnot cycle)  1.12. Kinetics  1.12.1. Rate law and reaction order  1.12.2. Activation Energy and Catalyst  1.12.3. Collision theory  1.12.4. Reaction mechanism  1.12.5. Transition state theory
  2. Organic chemistry  2.1. Structure and relationships  2.1.1. Hybridisation in Carbon Compounds  2.1.2. Resonance and aromatization  2.1.3. Molecular orbitals  2.1.4. Inductive and mesomeric effects  2.2. Hydrocarbons  2.2.1. Hydrocarbons  2.2.2. Alkene  2.2.3. Alkynes  2.2.4. Aromatic compounds  2.2.5. Cycloalkanes and Conformation  2.3. Functional Group  2.3.1. Alcohols and phenols  2.3.2. Ethers and epoxides  2.3.3. Aldehyde and ketone  2.3.4. Carboxylic acids and derivatives  2.3.5. Amin  2.3.6. Esters and amides  2.3.7. Thiols and sulfides  2.4. Stereoscopic  2.4.1. Isomerism in Stereochemistry  2.4.2. Chirality and optical activity  2.4.3. Structural analysis  2.4.4. Diastereomers and Enantiomers  2.5.1. Substitution reactions  2.5.2. Elimination reactions  2.5.3. Addition reactions  2.5.4. Radical reactions  2.5.5. Rearrangement reactions  2.5.6. Pericyclic Reactions  2.6. Spectroscopy and structural analysis  2.6.1. Infrared Spectroscopy  2.6.2. Nuclear magnetic resonance spectroscopy  2.6.3. Mass Spectrometry  2.6.4. UV-visible spectroscopy  2.6.5. X-ray crystallography  2.7. Polymer chemistry  2.7.1. Addition and Condensation Polymers  2.7.2. Polymerization mechanism  2.7.3. Biodegradable Polymer  2.7.4. Conducting and smart polymers
  3. Inorganic chemistry  3.1. Coordination chemistry  3.1.1. Ligands and coordination compounds  3.1.2. Crystal field theory  3.1.3. Molecular Orbital Theory in Coordination Compounds  3.1.4. Jahn–Taylor distortion  3.2. Main Group Chemistry  3.2.1. Alkali and alkaline earth metals  3.2.2. Halogens and Noble Gases  3.2.3. Transition metals and their complexes  3.2.4. Lanthanides and Actinides  3.3. Solid state chemistry  3.3.1. Crystal lattices and unit cells  3.3.2. Defects in solids  3.3.3. Electrical and magnetic properties  3.3.4. Superconductivity  3.4. Bio-inorganic chemistry  3.4.1. Metal ions in biology  3.4.2. Enzymatic reactions with metals  3.4.3. Metal poisoning and detoxification  3.4.4. Metalloproteins and Metalloenzymes
  4. Physical chemistry  4.1. Quantum Chemistry  4.1.1. Wave–particle duality  4.1.2. Schrödinger Equation  4.1.3. Quantum Numbers and Orbitals  4.1.4. Atomic and molecular spectroscopy  4.2. Statistical mechanics  4.2.1. Maxwell–Boltzmann distribution  4.2.2. Partitioning functions  4.2.3. Bose–Einstein and Fermi–Dirac statistics  4.3. Chemical thermodynamics  4.3.1. Gibbs Free Energy  4.3.2. Phase transition  4.3.3. Thermodynamic efficiency  4.4. Surface chemistry  4.4.1. Absorption and Catalysis  4.4.2. Colloids and Emulsions
  5. Analytical Chemistry  5.1. Classical methods  5.1.1. Gravimetric Analysis  5.1.2. titrimeter  5.2. Instrumental methods  5.2.1. Chromatography  5.2.2. Spectroscopy  5.2.3. Electroanalytical methods  5.2.4. X-ray diffraction
  6. Industrial Chemistry  6.1. Petrochemistry  6.2. Chemical engineering principles  6.3. Green Chemistry  6.4. Pharmaceuticals and Drug Synthesis
  7. Biochemistry  7.1. Carbohydrates  7.2. Proteins and enzymes  7.3. Lipids and membranes  7.4. Nucleic acids  7.5. Metabolism and Bioenergetics  7.6. Enzyme kinetics and mechanism

Undergraduate → Organic chemistry ↓


Radical reactions


Organic chemistry is replete with reaction mechanisms that describe how molecules change and react with each other. Of these, radical reactions are a fascinating type because of the unique nature of radicals. Radicals are species that have unpaired electrons, which makes them highly reactive. Understanding radical reactions involves understanding the behavior, formation, and types of reactions that radicals can undergo.

What are radicals?

A radical is a molecule or atom that has an unpaired electron. Since electrons prefer to be paired, these unpaired electrons result in high reactivity. The presence of an unpaired electron is usually indicated by a dot, as in Cl• for the chlorine radical.

Formation of radicals

There are several ways for radicals to form, the most common being homolytic bond cleavage. This process involves breaking a covalent bond so that each atom involved in the bond takes one electron from the shared pair, resulting in the formation of two radicals.

a—b → a• + b•

The energy required for this process is usually provided by heat or light. For example, either high temperatures or ultraviolet light are needed to split chlorine gas into radicals:

2Cl2 → 2Cl•

Types of radical reactions

Radical reactions can be classified into three main types: initiation, propagation, and termination. Each type of step plays an important role in the overall radical reaction mechanism.

Initiation

This is the step where radicals are first formed. As mentioned earlier, initiation usually involves homolytic cleavage. This step is important because it begins the entire radical chain reaction.

Cl2 + energy (light or heat) → 2 Cl•

Propagation

In the propagation phase, radicals react with stable molecules to form new radicals. This can maintain the reaction sequence once it has begun without the continued need for external energy. Propagation phases are usually exothermic and increase the number of radicals.

For example, in the halogenation of alkenes, methane reacts with the chlorine radical:

CH4 + Cl• → CH3• + HCl
CH3• + Cl2 → CH3Cl + Cl•

Each step produces a new radical, carrying the chain reaction forward.

Termination

The radicals can combine with each other to form a stable product, thus terminating the chain reaction. In this step, two radicals collide and bond, thereby eliminating their reactive potential. Termination is necessary to control the extent of the radical reaction.

CH3• + Cl• → CH3Cl
2 Cl• → Cl2

Applications of radical reactions

Radical reactions have many important applications in industrial and laboratory chemistry.

Halogenation of alkenes

Alkane halogenation is one of the most common applications of radical chemistry. For example, the chlorination of methane is a radical organic reaction:

CH4 + Cl2 → CH3Cl + HCl

This reaction is used to produce a variety of chlorinated compounds, which serve as important intermediates in manufacturing.

Polymerization

Radical polymerization is an important process used to make polymers such as polyethylene and polystyrene. In this context, radical initiators are used to start the reaction, forming long polymer chains from monomers:

initiator → chain start
monomer + radical → propagating radical
propagating radical + monomer → polymer chain

Biological systems

Radicals are also prevalent in biological systems, playing a role in processes such as DNA synthesis and repair. For example, radical enzymes are involved in converting ribonucleotides to deoxyribonucleotides:

Ribonucleotide• → Deoxyribonucleotide

Factors affecting radical reactions

Several factors can affect the rate and outcome of radical reactions:

Temperature

High temperatures provide the energy needed to break bonds and facilitate radical formation. However, they can also increase the rate of side reactions.

Stability of radicals

Radical stability is affected by the nature of the substituents surrounding the radical center. Generally, the stability of radicals increases in the following order:

Methyl < Primary < Secondary < Tertiary

Stability is often enhanced by resonance effects, where radicals are displaced over several atoms.

Concentration of reactants

The concentration of the reactants can affect the probability of radical collisions, affecting both the propagation and termination steps.

Solvent effect

Solvents can stabilize or destabilize radicals and intermediates, affecting the rate and pathway of the reaction. Polar solvents can stabilize charge-separated intermediates, affecting the kinetics of the reaction.

Conclusion

Radical reactions are the cornerstone of organic chemistry, serving as vital tools for both synthetic and biological processes. Their unique mechanisms, which involve steps such as initiation, propagation, and termination, allow for a versatile range of applications ranging from industrial polymer production to complex biological reactions. Understanding the full scope of radical reactions provides insight into their enormous practical and theoretical importance in the field of organic chemistry.


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Radical reactions

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