In Hatch and Slack pathway, the primary CO_2 acceptor is

In Hatch and Slack pathway, the primary $\mathrm{CO}_2$ acceptor is phosphoenol pyruvate. This occurs in $\mathrm{C}_4$-plants. Phosphoenol pyruvate, a 3 -carbon compound, accepts $\mathrm{CO}_2$ and forms oxaloacetic acid which is a 4-carbon compound.