Chemical routes in the synthesis of large interstellar molecules are quite obscure, but interstellar grains are believed to play a significant role. We suggest that functional molecular groups (e.g., OH, CH, CH2, CH3, NH2, etc.) on grain surfaces may provide the precursor basis for ensuing grain chemistries that produce complex molecules. Herein we report a first attempt to determine the relative abundance ratios of molecular groups toward the Galactic center and the Orion Nebula by using previously reported gas-phase abundances of complex molecules observed with single antennas having arcminute resolutions. Specifically, for SgrB2(OH), which is an extended cloud possessing no concentrated molecular core, we obtain OH : CH3 : CH2 : CH : NH2 : HCO : CN ~ 4190 : 100 : 29 : 71 : 36 : 57 : 178, and for OMC-1 we obtain OH : CH3 : CH : NH2 : HCO ~ [4040-8080] : 100 : 36 : 25 : [83-167], where both sets of relative abundances are normalized to the CH3 group. We suggest that these ratios may represent the relative abundance of the functional molecular groups on grain surfaces where the molecules were formed and subsequently released into the gas phase. We find that the relative functional group ratios in SgrB2 and OMC-1 are similar based on single-antenna beam-averaged abundances. We predict that higher spatial resolution (i.e., better beam-source coupling, as afforded by interferometric arrays) should be able to establish the role of functional molecular groups on grain surfaces in more compact interstellar molecular environments possessing a variety of different physical conditions.