According to Stevenson’s rule, fragmentation favors:
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 do
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.