The period 1935-1945 in derivatives established a functional framework in which substituent electronics, reaction type, and molecular architecture guided the design of derivatives. The Hammett equation emerged as a central quantitative tool that linked substituent effects to rates and equilibria, turning qualitative structure-reactivity insights into actionable design rules for later catalysis, medicinal chemistry, and derivative synthesis. The work on radical additions to olefins expanded the scope of derivate formation, enabling new strategies for hydrofunctionalization and polymer modification, and broadening the accessible library of functionalized products. Orientation tendencies in heterocycles and regiochemical substitution patterns in cellulose derivatives demonstrated that precise control of substitution could be achieved and leveraged to tailor properties such as solubility, processability, and performance. The discovery and early exploration of para-amino-benzenesulfonamides and their derivatives highlighted structure-activity relationships that would underpin early medicinal chemistry and antibiotic development. Contemporary research during this period thus fused quantitative understanding with synthetic ingenuity to elevate derivatives from qualitative curiosities to reliable design targets.