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Free Radical- Introduction, Structure, Types

Learning Objective: To study the history, nature, examples, and types of free radicals, a reactive intermediate in organic chemistry.

Skill Level - Intermediate

 

Prerequisites: 

 

Chapter: Reactive Intermediates

Sub-topic: Free Radicals

 

Author's Note: A free radical reaction is mentioned widely in several areas of science, such as polymer preparation, petroleum, environmental studies, and also when speaking about aging or oxidative stress.

However, the carbon-center free radical is organic chemistry's most studied reactive intermediate. This section covers a brief history of how the science of free radicals evolved to its present-day status, its nature, types, and examples. 

Free Radical- Introduction, Structure, Types

History of Free Radicals 

In the 1700s, the term radical, Latin for root, was used for any group that remained rooted or unchanged in a chemical reaction. So, the -CH3 in CH3-OH undergoing chemical transformation to CH3-Cl was radical.

Therefore, a substituent or a functional group free from a chemical transformation was called a ‘free radical.’

However, when Moses Gomberg, in 1900, discovered a reactive specie, a triphenylmethyl radical (Ph3C.), that reacted unlike any hydrocarbon or a cation or anion that the meaning of free radical underwent a significant change. The looser definition became - any transient or chemically unstable specie (atom, molecule, or ion).

Today the term radical and free radical are used interchangeably.

 

What is a Free Radical?

Free radicals are highly reactive, electron-deficient species (atom, molecule, or ion) with at least one unpaired electron, denoted officially with a middle dot (.) beside the atom.

 

examples of free radicals

 

Most radicals are unstable and have a very short life span, and to find stability, they quickly dimerize, forming an additive product (adduct) by linking two radical specie.

 

How radicals dimerize

 

In organic chemistry, the reactions involving free radicals are usually carbon centered.

A carbon-centered free radical is a neutral, electron-deficient carbon atom. The electron deficiency is due to its lack of one electron from ideal eight in the valence shell of carbon by covalent bonding.

The carbon radical with its single unpaired electron function as a reactive intermediate in many organic chemistry reactions.

 

Carbon centered free radicals

 

Types of Carbon-Centered Radicals

Based on the number of alkyl substituents on the carbon, carbon-free radicals are of three types: Primary, secondary, and tertiary.

The primary (10) carbon-centered radical has one alkyl substituent, the secondary (20) carbon-centered radical has two alkyl substituents, and the tertiary (3o) carbon-centered radical has three alkyl substituents. The carbon atom with no alkyl substituents is called methyl carbon-centered radical.

 

types of free radicals

 

Subscribe to learn about carbon-centred free radicals, a reactive intermediate in organic chemistry.

Next: Formation of Carbon Centered Free Radicals

Next: Structure of Carbon Centered Free radical

Next: Stability of Carbon Centered Free Radical

Next:  Comparing Free Radical Stability using dissociation energy Values

Next: Fate of Free Radicals

Next: Common Reactions involving Carbon - Centered Free Radicals

 

Reactive Intermediates
Organic Chemistry - Drawing Structures, Concepts, and Examples

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What is Organic Chemistry?

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Atom

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  • Power of Protons
  • Mass Number
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Bonding In Atoms

  • Octet Rule - Introduction and Bonding
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  • Covalent Bond - How it Forms
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Covalent Bond

  • Theories on Covalent Bond Formation
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  • Hybridization- Introduction and Types
  • sp3 Hybridization of Carbon, Nitrogen, and Oxygen
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  • VSEPR Theory- Introduction
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  • Electron Wave Property, LCAO and MOT - Introduction
  • Linear Combination of Atomic Orbitals - Formation of Sigma and Pie bonds using MO Approach
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  • Features of LCAO Theory
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  • Setting up the MO diagram for homonuclear diatomic molecules – Second Period Elements
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  • The Non-bonding Molecular Orbitals
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  • Covalent bond Characteristics - Bond Length
  • Factors affecting Bond Length
  • How does Electron delocalization (Resonance) affect the Bond length?
  • Covalent bond Characteristics- Bond Angle
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  • How Bond Order Corresponds to the Bond Strength and Bond Length
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Electronic Displacement in a Covalent Bond

  • Electronegativity- Introduction
  • Factors Affecting Electronegativity- Atomic number, Atomic size, Shielding effect
  • Factors Affecting Electronegativity-s-orbitals, Oxidation state, Group electronegativity
  • Application of Electronegativity in Organic Chemistry
  • Physical Properties Affected by Electronegativity
  • Inductive effect - Introduction, Types, Classification, and Representation
  • Factors Affecting Inductive Effect- Electronegativity
  • Factors Affecting Inductive Effect- Bonding Order and Charge
  • Factors Affecting Inductive Effect- Bonding Position
  • Application of Inductive Effect- Acidity Enhancement and Stabilization of the counter ion due to -I effect
  • Application of Inductive Effect-Basicity enhancement and stabilization of the counter ion due to +I effect
  • Application of Inductive Effect-Stability of the Transition States
  • Application of Inductive Effect-Elevated Physical Properties of Polar Compounds
  • Is the Inductive Effect the same as Electronegativity?
  • Resonance - Introduction and Electron Delocalization
  • Partial Double Bond Character and Resonance Hybrid
  • Resonance Energy
  • Significance of Planarity and Conjugation in Resonance
  • p-orbital Electron Delocalization in Resonance
  • Sigma Electron Delocalization (Hyperconjugation)
  • Significance of Hyperconjugation
  • Resonance Effect and Types
  • Structure Drawing Rules of Resonance (Includes Summary)
  • Application of Resonance
  • Introduction to Covalent Bond Polarity and Dipole Moment
  • Molecular Dipole Moment
  • Lone Pair in Molecular Dipole Moment
  • Applications of Dipole Moment
  • Formal Charges - Introduction and Basics
  • How to Calculate Formal Charges (With Solved Examples)
  • Difference between Formal charges and Oxidation State

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Common Types of Reactions

  • Classification of common reactions based on mechanisms
  • Addition Reactions
  • Elimination Reactions (E1, E2, E1cb)
  • Substitutions (SN1, SN2, SNAr, Electrophilic, Nucleophilic)
  • Decomposition
  • Rearrangement
  • Oxidation-Reduction

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Drawing Organic Structures

  • Introduction
  • Empirical Formula
  • How to Calculate Empirical Formula from percentage composition and atomic masses
  • Related Numerical Problems - Finding Empirical Formula (Solved)
  • Molecular Formula
  • Numerical Problems related to finding molecular formula  (Solved)
  • How to calculate molecular formula from empirical formula and molecular masses
  • Hill Nomenclature - The Empirical and Molecular Formula Writing Rules
  • E/Z Nomenclature -  Structure Writing Rules for Substituted Alkenes
  • Kekulé
  • Condensed
  • Skeletal or Bond line
  • Polygon formula
  • Lewis Structures- What are Lewis structures and How to Draw
  • Rules to Draw Lewis structures- With Solved Examples
  • Lewis structures- Solved Examples, Neutral molecules, Anions, and Cations
  • Limitation of Lewis structures
  • 3D structure representation- Dash and Wedge line
  • Molecular models for organic structure representation- Stick model, Ball-stick, and Space-filling
  • Newman Projection- Introduction and Importance
  • How to Draw Newman Projections from Bond-Line Formula (5 step-by-step solved examples on alkane, substituted alkane, alkene, ketone, and cycloalkane)
  • Drawing Newman Projections to the Bond line Formula (solved examples)
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Functional Groups in Organic Chemistry

  • What are functional groups?
  • Chemical and Physical Properties affected by the Functional Groups
  • Identifying Functional Groups by name and structure
  • Functional Group Categorization- Exclusively Carbon-containing Functional Groups
  • Functional Group Categorization- Functional Groups with Carbon-Heteroatom Single Bond
  • Functional Group Categorization- Functional Groups with Carbon-Heteroatom Multiple Bonds
  • Rules for IUPAC nomenclature of Polyfunctional Compounds
  • Examples of polyfunctional compounds named according to the priority order
  • Examples of reactions wherein the functional group undergoes transformations

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Structural Isomerism

  • Introduction
  • Chain Isomerism
  • Position Isomerism
  • Functional Isomerism
  • Tautomerism
  • Metamerism
  • Ring-Chain Isomerism

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Intermolecular Forces

  • Ion-Dipole Interactions-Introduction and Occurrence
  • Factors Affecting the Ion-Dipole Strength
  • Importance of Ion-Dipole Interactions
  • Ion-Induced Dipole - Introduction, Strength and Occurrence
  • Factors Affecting the Strength of Ion-Induced Dipole Interactions
  • Ion-Induced Dipole Interactions in Polar Molecules
  • Vander Waals Forces -Introduction
  • Examples of Vander Waals' forces
  • Vander Waals Debye (Polar-Nonpolar) Interactions
  • Factors affecting the Strength of Debye Forces
  • Vander Waals Keesom Force - Introduction, Occurrence and Strength
  • Vander Waals London Force - Introduction, Occurrence, And Importance
  • Factors Affecting the Strength of London Dispersion Forces- Atomic size and Shape
  • Introduction, Occurrence and Donor, Acceptors of Hydrogen Bond
  • Hydrogen bond Strength, Significance and Types
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Physical Properties

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  • Physical State Change-Melting Point
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  • Preliminary Qualitative Analysis of some Organic Compounds | Intensive Physical Property Measurements

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Fundamentals of Organic Reactions

  • Types of Arrows Used in Chemistry
  • Curved Arrows in Organic Chemistry- with Examples
  • Electrophiles - Introduction, Identification and Reaction
  • Formation and Classification of Electrophiles- Neutral and Charged
  • Difference between Electrophiles and Lewis Acids
  • Nucleophiles - Identification and Role in a Reaction
  • Types of Nucleophiles- Lone Pair
  • Types of Nucleophiles- Pie Bond
  • Types of Nucleophiles- Sigma Bond
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  • Nucleophile Reactions- Aliphatic Displacement type - SN1, SN2
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  • Ambident Nucleophiles- Introduction and Formation
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  • Lone Pair - Introduction and Formation
  • Physical Properties Affected by the Lone Pair- Shape and Bond Angle
  • Physical Properties Affected by the Lone Pair- Hydrogen Bonding
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  • Chemical property affected by the Lone pair- Nucleophilicity
  • Leaving Group - Introduction and Nature
  • Good and Bad Leaving Group
  • Factors Determining Stability of the Leaving Groups- Electronegativity, Size, Resonance Stability
  • Using pKa as a Measure of Leaving Group Ability
  • Leaving Groups in Displacement Reactions
  • Leaving Groups in Elimination Reactions

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Reactive Intermediates

  • Carbocation - Introduction, Nature, and Types
  • Formation of Carbocation
  • Stability of Carbocations- Inductive, Resonance, and Hyperconjugation
  • Other Structural Features Increasing Carbocation Stability
  • Structural Feature Decreasing Carbocation Stability
  • Fate of the Carbocation
  • General Carbocation Formation Reactions
  • Carbanion - Introduction, Nature, and Types
  • Formation of Carbanions
  • Carbanion Stabilization
  • Ease of Formation of Carbanion -Acidic proton
  • Fate of the Carbanion
  • Free Radical - Introduction and Types of Carbon-Centred Radicals
  • Structure of Carbon-Centred Free Radical
  • Formation of Radicals
  • Stability of the Carbon-Centred Radicals
  • Other Structural Feature Increasing Free Radical Stability
  • Comparing Free Radical Stability using Dissociation energies (D-H)
  • Fate of Free Radicals
  • Common Reactions Involving Carbon-Free Radicals

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Stereoisomerism - Conformation and Configurational Isomerism

  • Conformations in Organic Chemistry - An Introduction
  • How are Conformational Isomers Depicted
  • Open Chain and Closed Chain Conformations
  • Nomenclature related to sp3-sp3 and sp3-sp2 bond rotations
  • Conformational Analysis
  • Factors affecting the stability of conformers - Stabilizing Interactions |Hyperconjugation
  • Factors affecting the stability of conformers - Stabilizing Interactions | Intramolecular Hydrogen Bonding
  • Factors affecting the stability of conformers - Stabilizing Interactions | Dipole Minimizations
  • Factors affecting the stability of conformers - Destabilizing Interactions | Steric strain
  • Factors affecting the stability of conformers - Destabilizing Interactions | Torsional strain
  • Factors affecting the stability of conformers - Destabilizing Interactions | Angle strain
  • Importance of Conformational Analysis
  • Conformation in Compounds with Lone Pairs
  • Role of Solvents in Conformations
  • An Example of Conformation Dependent Reaction and Product Selectivity
  • Geometrical Isomerism - Introduction
  • Impact of cis-trans isomerism on physical properties
  • Impact of cis-trans isomerism on chemical reactions
  • Scope of Geometrical Isomerism in Biological Systems and Industrial Applications
  • E/Z Nomenclature in Substituted Alkenes
     

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