20.9
Radical Reactivity: Steric Effects
The presence of electron-donating, electron-withdrawing, or conjugating groups adjacent to a radical center, imparts electronic stabilization to the r…
Recall that the presence of electron-donating, electron-withdrawing, and conjugating groups adjacent to a radical center results in electronically-stabilized radicals.
Some of these electronically-stabilized radicals are known as persistent radicals. These radicals are remarkably stable; some can even be isolated and purified.
In addition to the electronic factors, steric factors also contribute to the stability of these persistent radicals.
For example, consider the triphenylmethyl radical. The exceptionally high stability of this radical is due to the presence of three phenyl rings.
The three surrounding rings are twisted out of the plane by 30° in a propeller-like conformation.
As a result, the central carbon bearing the radical character is sterically shielded by the twisted phenyl rings, making the radical highly stable and unavailable for reactions.
Overall, steric effects make a radical more stable and less reactive.
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Q1: What role do steric effects play in radical stability?
Steric effects significantly enhance radical stability by shielding the radical center from molecular attack. Bulky groups surrounding the radical site create steric hindrance, making the radical less accessible and therefore less reactive. This protection reduces the likelihood of unwanted side reactions and contributes to the formation of persistent radicals that can be isolated and purified.
Q2: How does the triphenylmethyl radical demonstrate steric stabilization?
The triphenylmethyl radical achieves exceptional stability through three phenyl rings twisted 30° out of plane in a propeller conformation. These twisted rings sterically shield the central carbon bearing the radical character, making it highly unavailable for reactions. This steric protection is the primary reason for the triphenylmethyl radical's remarkable stability and persistence.
Q3: What are persistent radicals and how do they form?
Persistent radicals are remarkably stable radicals that result from combined electronic and steric stabilization. Electronic stabilization comes from electron-donating, electron-withdrawing, or conjugating groups adjacent to the radical center. Examples include triphenylmethyl, tetramethylpiperidine-N-oxide, and 2,2-diphenyl-1-picrylhydrazyl radicals, which can be isolated and purified due to their exceptional stability.
Q4: Why does steric shielding decrease radical reactivity?
Steric shielding decreases radical reactivity by physically blocking access to the radical center. When bulky groups surround the radical site, they prevent other molecules from approaching and reacting with it. This reduced accessibility makes sterically hindered radicals less likely to participate in radical substitution or addition reactions, effectively stabilizing them against further transformation.
Q5: How do electronic factors contribute to persistent radical stability?
Electronic factors stabilize radicals through electron-donating groups that increase electron density at the radical center, electron-withdrawing groups that delocalize radical character, or conjugating groups that spread unpaired electron density across multiple atoms. These electronic effects work alongside steric factors to create persistent radicals with exceptional stability and reduced reactivity.
Q6: What is the propeller conformation in the triphenylmethyl radical?
The propeller conformation describes the three-dimensional arrangement where the three phenyl rings of the triphenylmethyl radical twist 30° out of the molecular plane. This twisted geometry resembles a propeller and positions the phenyl rings to maximally shield the central carbon atom. The propeller arrangement is crucial for the radical's exceptional stability and resistance to further reactions.
Q7: How do steric and electronic factors work together in radical stabilization?
Steric and electronic factors combine synergistically to create highly stable persistent radicals. Electronic factors stabilize the radical through conjugation and group effects, while steric factors provide physical protection by shielding the radical center from molecular attack. Together, these factors reduce radical reactivity and enable isolation and purification of certain radicals for study using electron paramagnetic resonance epr spectroscopy.