<![CDATA[Organic chemistry requires students to reason about three-dimensional molecular structures and to represent electron movement through curved-arrow notation. This study examined how spatial ability and representational competence in reaction mechanisms jointly predict organic chemistry achievement among pre-service chemistry teachers, and identified common errors in mechanism drawing. A correlational study involved 128 students of a chemistry education program. Spatial ability was measured with the Purdue Visualization of Rotations Test, representational competence with a 10-item mechanism test scored with an analytic rubric, and achievement with final examination scores, controlling for general chemistry grades. Hierarchical regression showed that prior knowledge explained 20.7% of the variance in achievement, spatial ability added 5.0%, and representational competence added a further 11.8% (total R² = 0.374). In the final model, the effect of spatial ability became non-significant (β = 0.11, p = 0.141), whereas representational competence was the strongest predictor (β = 0.39, p < 0.001). Bootstrap analysis confirmed a significant indirect effect of spatial ability through representational competence (B = 0.357, 95% CI [0.121, 0.634]). The most frequent errors were starting arrows from atoms or charges rather than electron pairs (55.5%) and skipping mechanistic steps (45.3%). These findings suggest that spatial ability supports organic chemistry learning mainly by enabling students to construct meaningful mechanistic representations.]]>
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