Wallach, Otto (1847-1931) was a German chemist who won the 1910 Nobel Prize in chemistry for determining the nature of the complex mixtures found in the ethereal, or essential, oils of plants. His work was a vital contribution to the fragrance industry.

Wallach was born in 1847 in önigsberg, Prussia (now Kaliningrad, Russia). He earned his doctorate in chemistry from the University of Göttingen in 1869 with a dissertation on isomers of toluene. Isomers are substances made of molecules containing the same atoms, but with different molecular arrangements. These different arrangements give the substances different physical and chemical properties. Toluene is a product of coal distillation, and is used as a solvent in preparing fragrances.

Wallach became an assistant to German chemist Friedrich August Kekule von Strad-onitz at the University of Bonn in 1870, and he remained at the university for 19 years. He was appointed a chemistry professor in 1876 and chair of the pharmacology department in 1879.

In 1879, Wallach discovered several bottles of essential oils that had been abandoned. Previously, no one had successfully isolated pure compounds from these oils. Intrigued, Wallach began to analyze them, distilling them several times until he succeeded in isolating and identifying eight pure substances that he called terpenes. He continued studying how these terpenes were related, and by 1887 he found that they all were composed of five carbon atom fragments called isoprene units. He further demonstrated that in many cases one terpene could be converted into another by applying strong acids or high temperatures. Naturally occurring terpenes include bayberry, rose oil, peppermint, and turpentine.

Wallach's discovery enabled precise and consistent scientific production of perfumes, flavorings, and medicines that use terpenes. He was director of the Chemical Institute at the University of Göttingen from 1889 until 1915. By about 1900, terpene chemistry was an important branch of organic chemistry, and eventually expanded to the study of carotenoids and steroids.

He was awarded the Davy Medal in 1912.