Highly correlated ab-initio wavefunctions within the MRCI and UCCSD(T)-F12 formalisms are used to study the long range interactions and the reactive path between C2 and the three hydrogen halides HX (X = F, Cl, Br). The asymptotic regions are investigated for different relative orientations of the two moieties. It is shown that the three van der Waals systems C2 + HX are bound through nucleophilic interactions between C2 and HX for approaches perpendicular to the C-C axis, with decreasing interaction energies from HF to HBr. For HX approaching C2 along its axis, the van der Waals interactions, governed by the electrophilic character of C2 are weaker, with decreasing interaction energies from HBr to HF. Activation barriers towards the molecular systems HCCX or CCHX (0.64 eV and more) are calculated at short distances, making difficult the reaction towards the corresponding tetra-atomic systems.