TEMPO

Other names: 2,2,6,6-Tetramethylpiperidoxyl, (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl

C₉H₁₈NO

1/50 common

TEMPO (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl is a remarkably stable free radical, an unusual trait since most radicals are highly reactive and short-lived under standard conditions. Discovered in the 1960s, TEMPO's stability arises from the steric protection provided by its four bulky methyl groups, which shield a two-center three-electron nitrogen-oxygen bond from quenching reactivity. This exceptional stability allows TEMPO to function as an oxidation catalyst, particularly in the selective oxidation of alcohols to aldehydes and ketones without overoxidation. It is also widely used in polymer chemistry, facilitating controlled radical polymerization, and in biochemistry as a spin probe for electron paramagnetic resonance (EPR) spectroscopy. In its pure crystalline form, TEMPO exhibits a distinctive ruby-red color, a visual hallmark of its persistent radical nature. Its unique combination of stability, reactivity, and striking appearance makes TEMPO an invaluable tool in both industrial and academic research.