Highly carcinogenic bis-chloromethyl byproducts containing this functional group limit industrial use of the Blanc chloromethylation. Potassium iodide can be used to test for explosive peroxides (10[1])in stocks of compounds with this group that have been exposed to air. Cryptands are 3D analogues of a cyclic version of these groups useful for complexing alkali metals and for acting as phase transfer catalysts. This group’s ability to complex Mg2+ ions and general non-reactivity allows molecules with this group to be used to store Grignard reagents. Cyclic (10[1])molecules containing this group include furans and molecules named after crowns. For 10 points, name this functional group consisting of an oxygen flanked by two R groups, whose diethyl variety can be used as an anesthetic. ■END■ (10[1])