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Radical Stability Paradox: Substituent Effects versus Heats of Formation
Daniela Rodrigues Silva1, Pascal Vermeeren1, J Martijn van der Schuur2
1Department of Chemistry and Pharmaceutical Sciences, Amsterdam Institute of Molecular and Life Sciences (AIMMS), Vrije Universiteit Amsterdam, Amsterdam, The Netherlands.
Chemistry (Weinheim an Der Bergstrasse, Germany)
|December 29, 2025
Summary
Alkyl groups appear to destabilize organic radicals, yet heats of formation suggest increased stability with more alkyl groups. This study resolves this paradox by separating substituent and intrinsic stability effects.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Thermochemistry
Background:
- Alkyl substituents were previously shown to destabilize the carbon radical center in organic radicals.
- Heats of formation (∆Hf) for n-butyl, s-butyl, and t-butyl radicals appear contradictory, showing increased stability with more alkyl groups.
Purpose of the Study:
- To resolve the apparent paradox between alkyl substituent effects and observed heats of formation for organic radicals.
- To develop a generalizable framework for extracting intrinsic substituent effects from thermochemical data.
Main Methods:
- Analysis of experimental and computational data for isomeric butyl radicals.
- Deconvolution of heat of formation into radical center effects and substituent intrinsic stability.
Main Results:
- The heat of formation (∆Hf) is influenced by both the radical center's stability and the substituents' intrinsic stability.
- A method was developed to isolate the intrinsic electronic effects of alkyl substituents on radical stability.
Conclusions:
- The apparent contradiction is resolved by accounting for the inherent stability of alkyl groups themselves.
- The developed framework offers broad applicability for analyzing substituent effects in radical chemistry and thermochemistry.
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