OF FREE RADICAL
Embedded Files
TABLE OF CONTENT
Introduction
Free radical
Stability of free radical
Hyperconjucation
Inductive effect
Resonance
Steric hindrance
Steric hindrance and resonance(With reference video)
Captodative effect
Nptel video
Conclusion
MCQ With answers and explanation
Glossary
Reference
INTRODUCTION
Free radicals are chemical species with an unpaired electron, making them highly reactive. Their stability depends on several electronic effects such as inductive, hyperconjugative, resonance, steric, and captodative effects. Factors like the geometry of the radical carbon (sp² and planar) and delocalization of electrons play major roles in determining stability. Topics like resonance vs steric preference, triphenylmethyl radical will be discussed to understand how structure influences radical behavior.
Free radical
The Species formed as an intermediate in a chemical reaction and have one or more unpaired electrons
Life of a radical depends its stability they are extremly short lived in solution, long lived in crystals
Persistant Free radical have a long lifetime and are resistant to dimerization and dispropotionation
Stability of Free radical
Inductive effect
Hyperconjugative effect
Resonance effect
Steric hindrance
Captodative effect
HYPERCONJUCATION
HYPERCONJUCATION
The odd electron delocalized onto the B-Hydrogens, through hyperconjugation, which gives Stability to the radical.
Greater the hyperconjugative structures more the stability
RESONANCE
The free radical in the carbon centre will be in conjugation to a double bond
More resonating structures more stability
Inductive effect
Inductive effect
Greater the number of alkyl groups attached to free radical carbon centre,Greater the stability of the radical
electron donating alkyl group increases the electron deficiency of the radical decreases order of stability 3°>2°>1°
Push-Pull effect/Captodative effect
The stability of free radicals is enhanced by the presence at the radical center of both an electron-donating and an electron-withdrawing group.This is called the push-pull or captodative effect.The effect makes increased resonance
Steric hindrance
They are twisted ~30° like a propellar. So delocalization of the radical electron into the rings is not perfect.
Yet, triphenylmethyl radical is much stable due to steric Protection
3 twisted phenyl groups shield the central radical carbon
Sterically hidered and very hard to react
If dimerization does occur, it happens at the least hindered position [edge Carbons of rings. Not the central Carbon]
VID-20251009-WA0000.mp4Some one got a story to tell you.....Meet who he is!"
Resonance and Steric hindrance
(A)
Planar
Resonance occurs
Dimerisation also occurs
Less stable
(B)
Planar
Resonance occurs
Dimerisation also occurs
Less stable
(B) is more sterically hindered than (A)
This is due to steric hindrance in structure (B)
Steric hindeance is the major cause for stabilily of Free radical than resonance.
Therefore, (B) is stable and does not dimerize where (A) is Compariterly less Stable and occurs dimerization.
steric hindrance and resosnace.mp4Here is a intresting video on STABLE FREE RADICALS
VID-20251008-WA0004.mp4A complete video presentation on the topic "STABILITY OF FREE RADICALS"
2025-10-11 09-15-30.mp4QUESTIONS BASED ON STABILITY
QUESTIONS BASED ON STABILITY
Arrange the following radicals in decreasing order of stability:
•C(CH₃)₃ , •CH₂CH₃ , •CH₃
Options:
(a) •C(CH₃)₃ > •CH₂CH₃ > •CH₃
(b) •CH₂CH₃ > •C(CH₃)₃ > •CH₃
(c) •CH₃ > •CH₂CH₃ > •C(CH₃)₃
(d) •CH₂CH₃ > •CH₃ > •C(CH₃)₃
Answer: (a) •C(CH₃)₃ > •CH₂CH₃ > •CH₃
Reasoning:
Tertiary radicals are more stable than secondary and primary radicals because of:
+I effect of three alkyl groups (electron-donating)
Hyperconjugation from adjacent C–H bonds
Thus, stability increases as:
Tertiary > Secondary > Primary > Methyl.
Arrange the following radicals in decreasing order of stability:
(i) (CH₃)₂C=CH–CH₂•
(ii) C₆H₅–CH₂–C•H(CH₃)₂
(iii) •CH₂–CH=CH–C(CH₃)₃
(iv) (CH₃)₃C•
Answer:
(iii) > (ii) > (i) > (iv)
Reasoning:
(iii) has allylic + tertiary substitution, giving both resonance and strong hyperconjugation.
(ii) is benzylic + secondary — resonance + moderate hyperconjugation.
(i) has a double bond proximity (allylic), but less substitution.
(iv) is tertiary only — high hyperconjugation, but lacks resonance.
Video lecture on Free radical by a proffesional
Glossary
Glossary
Free Radical : Atom/molecule with an unpaired electron •CH₃ (Methyl radical)
Inductive Effect: (+I) Electron-donating effect through sigma bonds Alkyl groups stabilize radicals
Hyperconjugation : Delocalization of electrons from adjacent C-H bonds Tertiary radicals more stable than primary
Resonance : Electron delocalization over π bonds Benzyl radical
REFERENCE
Smith, Michael B., and Jerry March. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. 6th ed., Wiley-Interscience, 2007.
Jain, M. K., and S. C. Sharma. Modern Organic Chemistry. Vishal Publishing Co., latest edition.
Bakhshi, A. K., Diwan S. Rawat, Beena Negi, and Kamal K. Kapoor. “Generation, Structure, Stability and Reactivity of Free Radicals.” Organic Chemistry-II (Reaction Mechanism-1), Module 8, ePathshala/NPTEL, University of Delhi, coordinated by SGTB Khalsa College
Clayden, Jonathan, Nick Greeves, Stuart Warren, and Peter Wothers. Organic Chemistry. Oxford University Press, 2001.
Page updated
Report abuse