Lone‐Pair Shielded Radicals: Beyond the Captodative Substitution Pattern

We have quantum chemically analyzed how substituents R affect the carboncarbon bond strength in 1,2‐disubstituted ethane, RH 2 CCH 2 R (R = H, CH 3 , NH 2 , OH, F), using density functional theory. Our quantitative MO bonding analyses reveal that the CC 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, CC bond weakening is more pronounced as the substituent becomes less electronegative. Contrary to the commonly accepted rationale that electron‐donating groups R weaken the CC 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.