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Methyl Substitution Destabilizes Alkyl Radicals.
Eva Blokker1, Willem-Jan van Zeist1, Xiaobo Sun1,2
1Department of Theoretical Chemistry, Amsterdam Institute of Molecular and Life Sciences (AIMMS), Amsterdam Center for Multiscale Modeling (ACMM), Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV, Amsterdam, The Netherlands.
Angewandte Chemie (International Ed. in English)
|July 12, 2022
Summary
Methyl substituents destabilize alkyl radicals, contrary to established chemical models. Increased substitution weakens C-H bonds due to greater molecular destabilization, not radical stabilization.
Area of Science:
- Physical Organic Chemistry
- Quantum Chemistry
- Computational Chemistry
Background:
- Established models suggest alkyl radical stability increases with methyl substitution (methyl < primary < secondary < tertiary).
- This perceived radical stabilization is thought to cause decreasing C-H bond strength in the same series.
Purpose of the Study:
- To quantum chemically investigate the effect of methyl substituents on alkyl radical stability.
- To re-evaluate the relationship between alkyl radical stability and C-H bond strength.
Main Methods:
- Density functional theory (DFT) calculations were performed.
- The M06-2X/TZ2P level of theory was employed for all investigations.
Main Results:
- Alkyl radicals (MemH3-mC⋅) are destabilized with increasing methyl substitution.
- C-H bonds weaken along the substitution series (methyl to tertiary).
- Increased molecular destabilization of sterically congested molecules, rather than radical stabilization, explains the observed C-H bond weakening.
Conclusions:
- The prevailing model explaining C-H bond strength trends based on alkyl radical stability is falsified.
- Steric effects play a crucial role in destabilizing molecules, leading to weaker C-H bonds despite radical destabilization.
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