In substitution reactions between bromine and hexane, the use of an organic solvent is crucial for facilitating the reaction. Although bromine is a liquid, it requires an organic solvent to promote homolysis, which is essential for generating bromine radicals. Without an organic solvent, the bromine radicals may not dissolve properly, hindering the reaction. If bromine is not added to an organic solvent, the reaction may not occur effectively, potentially leading to incomplete reactions or undesirable side effects. The solubility of bromine in hexane allows for better interaction and reactivity in the substitution process.