Paul Walden (; ; ; 26 July 1863 – 22 January 1957) was a Russian, Latvian and German chemist known for his work in stereochemistry and history of chemistry. In particular he invented the stereochemical reaction known as Walden inversion and synthesized the first room-temperature ionic liquid, ethylammonium nitrate. Early life and education Walden was born in Rozulas in present-day Stalbe parish, Pārgauja municipality, Latvia in a large Latvian peasant family. At the age of four, he lost his father and later his mother. Thanks to financial support from his two older brothers who lived in Riga (one was a merchant and another served as a lieutenant) Walden managed to complete his education – first graduated with honors from the district school in the town of Cēsis (1876), and then from the Riga Technical High School (1882). In December 1882, he enrolled into the Riga Technical University and became seriously interested in chemistry. In 1886, he published his first scientific study on the color evaluation of the reactions of nitric and nitrous acid with various reagents and establishing the limits of sensitivity of the color method to detection of nitric acid. In April 1887, Walden was appointed a member of the Russian Physico-chemical Society. During this time, Walden started his collaboration with Wilhelm Ostwald (Nobel Prize in Chemistry 1909) which greatly influenced his development as a scientist. Their first work together was published in 1887 and was devoted to the dependence of the electrical conductivity of aqueous solutions of salts on their molecular weight. Work in chemistry In 1888, Walden graduated from the university with a degree in chemical engineering and continued working at the Chemistry Department as an assistant to professor C. Bischof. Under his guidance, Walden began compiling "Handbook of Stereochemistry" which was published in 1894. In preparation of this handbook, Walden had to perform numerous chemical syntheses and characterizations which resulted in 57 journal papers on stereochemistry alone, published between 1889 and 1900 in Russian and foreign journals 57 articles on the stereochemistry. He also continued his research in the field of physical chemistry, establishing in 1889 that the ionizing power of non-aqueous solvent is directly proportional to the dielectric constant. During the summer vacations of 1890 and 1891, Walden was visiting Ostwald at the University of Leipzig and in September 1891 defended there a master thesis on the affinity values of certain organic acids. Ostwald suggested