Diagnosing Misconceptions in SN1 and SN2 Mechanisms Using a Five-Tier Diagnostic Test (FTDAT): Implications for Curriculum Improvement

Abstract

This study diagnosed undergraduate students’ misconceptions about nucleophilic substitution mechanisms and evaluated a diagnostic-informed teaching intervention. The scope focused on SN1 (unimolecular nucleophilic substitution) and SN2 (bimolecular nucleophilic substitution), emphasizing substrate effects, solvent effects, nucleophile role, and reaction energy profiles. Using a one-group pretest–posttest quasi-experimental design, 30 third-year chemistry students completed a Five-Tier Diagnostic Test (FTDAT) in the diagnostic phase. The FTDAT measured answer choice, justification, confidence, and reported knowledge sources. Pretest results indicated limited conceptual mastery, with scientific understanding observed in 34.5% of item responses (80/232), alongside substantial false-positive understanding and misconception patterns. The intervention applied Diagram-Supported Conceptual Contrast Instruction (DSC2I) using mechanistic visualization, supported by guided practice and digital learning tools. Post-instruction understanding was assessed using a teacher-made open-ended test aligned with the same conceptual indicators. Results showed marked improvement: the mean score increased from 8.6 to 17.4 out of 20, with a normalized gain of g = 0.61 and a large effect size (Cohen’s d = 1.42). These findings support the value of a five-tier diagnosis to guide targeted instruction for improving mechanistic reasoning in organic chemistry