Halogen bonding is a unique type of noncovalent binding phenomenon in which a halogen atom interacts attractively with an electronegative atom such as oxygen or nitrogen. These types of interactions have been the subject of many recent investigations because of their potential in the development of new materials and pharmaceutical compounds. Recently, it was observed that most halogen bonding interactions in biological contexts involve close contacts between a halogen bound to an aromatic ring and a carbonyl oxygen on a protein's backbone structure. In this work we investigate interactions of substituted bromobenzenes and bromopyrimidines with acetone to ascertain the effects of various substituents upon the strengths of these interactions. It was found that replacement of ring hydrogens in these systems has dramatic effects upon the interaction strengths of the resulting complexes, which have interaction energies between -1.80 and -7.11 kcal/mol. Examination of the electrostatic potentials of the substituted bromobenzene and bromopyrimidine monomers indicates that the addition of substituents has a large influence upon the most positive electrostatic potential on the surface of the interacting bromine and thus modulates these halogen bonding interactions. Results obtained using the symmetry-adapted perturbation theory (SAPT) interaction energy decomposition procedure also indicate that electrostatic interactions play the key role in these halogen bonding interactions. These results have important implications in drug design and crystal engineering. Halogen bonds have been a subject of great interest in these fields because of their unique noncovalent bonding characteristics.