Likewise, for the CO2 additions two addition orders are possible (Fig ). The main cycle mechanism involves an external addition to the aldehyde (R8) followed by an internal addition to the C3 position (R15; Path 1) while the alternative involves a CO2 addition to the internal C2 position followed by the aldehyde addition (Path 2). The reaction coordinates for both reaction pathways were localized by the exploration. The internal addition step in Path 2 exhibits a barrier that is 7.2 kcal⸱mol -1 lower than the corresponding external addition step in Path 1. However, the second step in Path 2 encounters a steep barrier that is 54.2 kcal⸱mol -1 higher. The high barrier steps -specifically the second step in Path 2 and the external addition step in Path 1 -involve complete bond cleavage in the transition state while the internal addition in Path 1 proceeds via a concerted mechanism that lowers its barrier (Fig ). These findings underscore that a reduction in the barrier for an initial step does not ensure overall kinetic favorability if a subsequent step is significantly hindered. Overall, the pathway that begins with an external CO2 addition followed by an internal addition is kinetically advantageous for the cycle. These reactivity differences again reveal a kinetic preference in the uncatalyzed system that mirrors the rTCA cycle.