Practice question

Stevenson's rule, also called the even-electron rule for fragmentation pathways, governs charge retention during dissociation of odd-electron molecular ions in mass spectrometry. When a covalent bond cleaves, two fragments form but only one retains the positive charge. Thermodynamic stability decides outcome; the fragment possessing lower ionization energy better stabilizes the charge through resonance or inductive effects. Consequently, the more substituted, conjugated, or heteroatom-containing fragment typically carries the charge and appears with higher abundance. This principle predicts dominant peaks, aids structural elucidation of natural products, and rationalizes why certain cleavages predominate over random placement.