Understanding alkyl carbanion stability in a theoretical window: revisiting the role of substituent effects

Alkyl carbanion stability is frequently taught by analogy to carbocations, leading students to assume that alkyl groups are electron-donating and destabilize a negatively charged carbon, so that stability decreases from primary to tertiary. This study had two aims: to re-examine the intrinsic stability order of alkyl carbanions using quantum-chemical analysis, and to evaluate students’ conceptions of this order and whether targeted theoretical evidence can revise them. The computational component analyzed ethyl, isopropyl, and tert -Butyl carbanions using DFT (B3LYP-D3/aug-cc-pVTZ), natural resonance theory, second-order NBO perturbation analysis, and isodesmic reactions. The educational component used a two-phase online survey of undergraduate ( n =31; 29 in Phase 2) and postgraduate ( n =17; 13 in Phase 2) students from several countries. Theory indicated that gas-phase stability increases from primary to tertiary, driven by negative hyperconjugation, where the carbanionic lone pair delocalizes into neighbouring σ * Cα–H antibonding orbitals; methyl groups thus act as effectively electron-withdrawing substituents. After exposure to this evidence, the proportion selecting the supported order (primary < secondary < tertiary) rose from 16.1 % to 86.2 % among undergraduates and from 29.4 % to 92.3 % among postgraduates. These findings show that concise, evidence-based orbital reasoning can shift the students’ conceptions and offer a transferable pedagogical strategy for teaching substituent effects.