We have quantum chemically analyzed how substituents R affect the carboncarbon bond strength in 1,2‐disubstituted ethane, RH 2 CCH 2 R (R = H, CH 3 , NH 2 , OH, F), using density functional theory. Our quantitative MO bonding analyses reveal that the CC bond between the two RH 2 C • radicals progressively weakens as the electron‐donating ability of the R group increases because of the occurrence of a lone‐pair shielded radical on RH 2 C • . The lone‐pair type orbital amplitude on the carbon center arises from the interaction with a lone pair on the substituent R. The shielding of the carbon radical center and, thus, CC bond weakening is more pronounced as the substituent becomes less electronegative. Contrary to the commonly accepted rationale that electron‐donating groups R weaken the CC bond by stabilizing the resulting radical RH 2 C • , our analyses show that all substituents, except R = F, destabilize the radical. The present findings demonstrate that the lone‐pair shielded radical features in a broader variety of organic radicals than previously assumed and can be used as a handle for tuning the bond strengths.
Beutick et al. (Sat,) studied this question.
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