$$\ce{N}$$-bromosuccinimide (NBS) is a reagent used for benzylic and allylic radical bromination, among other things. It is commonly found in its reagent jar as a yellow or orange solid (Figure 3.3a), as over time the solid degrades and generates $$\ce{Br_2}$$, the source of the color. Before use, NBS should be crystallized from hot water, and the resulting crystals are pure white (Figure 3.3c). If NBS were used without previous crystallization, yields would undoubtedly suffer as the quantity of active reagent would be unknown, and side produces might form due to the presence of excess bromine.