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Organic Chemistry
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Organic Chemistry
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General Course
Organic Chemistry
University Study Guides
CHEM 351
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Radical Stability
A radical is an atom or molecule with an unpaired electron. When drawing these reactions, it is important to use a single headed arrow (that looks like a fishhook) to denote a single electron moving.
Factors that Influence Radical Stability
Radicals are highly reactive. That means that in order to predict where a radical reaction will occur, you need to be able to predict where the radical is most stable. There are three things to consider.
1. Hyperconjugation: Tertiary (3°) radicals are more stable than secondary (2°) radicals and primary (1°) radicals are the least stabilized.
2. Resonance: delocalizing a radical by resonance stabilized the radical. This can be done with a double bond or a heteroatom.
3. Hybridization: the less s-character there is on a carbon atom, the more stable the radical.
Wize Tip
The great news about radical stability is it follows the same trends as the carbocation! So everything you know about carbocations, you observe here!
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Example: Radical Stability
Which of the following C-H bonds is weakest (e.g. most likely to undergo homolytic bond cleavage and form radical)?
Remember: Radical stability - (least stable) sp < sp2 < sp3 (primary) < sp3 (secondary) < sp3 (tertiary) < resonance (most stable)
Answer: C-Hd - the resulting radical is on an sp3 tertiary carbon atom and is resonance stabilized. C-Hb also gives a resonance stabilized radical, but it's two contributing structures put radicals on sp3 secondary carbon atoms.
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Example: Radical Stability
Rank the following radical species in order of stability.
Remember: Radical stability - (least stable) sp < sp2 < sp3 (primary) < sp3 (secondary) < sp3 (tertiary) < resonance (most stable)
Answer: (least stable) D (sp carbon), A (sp2 carbon), B (sp3 tertiary carbon), C (sp3 tertiary carbon and resonance!)
Which of the following radical species is most stable?
