It is currently assumed that the earliest stages of massive stars are represented by infrared dark clouds (IRDCs). Only recently have molecular surveys, in which many lines were observed for the same object, started to appear in the literature. Based on our multiline survey, we developed chemical models with a simplified internal structure for two representative IRDCs: IRDC013.90-1 and IRDC321.73-1. The two clouds represent two distinct temperature regimes: IRDC013.90-1 is a colder cloud with T = 15 K, whereas IRDC321.73-1 is somewhat warmer with T = 25 K, based on our NH3 measurements. We compared our modeling results with observed molecular abundances for eight species: N2H+, HC3N, HNC, HCO+, HCN, C2H, NH3 and CS, and discovered a limited temperature range of 20 - 30 K, where grain-surface chemistry becomes important for gas-phase abundances of certain species such as CO, HCO+, and N2H+. Hence, those IRDCs that have temperatures in the 20 - 30 K range can be an interesting laboratory to study the impact of surface reactions on the abundances of simple gas-phase species.