Rainer Ludwig Claisen (January 14, 1851 – January 5, 1930) was a famous German chemist best known for his work with condensations of carbonyls and sigmatropic rearrangements. He was born in Cologne as the son of a jurist and studied chemistry at the university of Bonn (1869), where he became a member of K.St.V. Arminia. He served in the army as a male nurse in 1870-1871 and continued his studies at Göttingen University. He returned to the University of Bonn in 1872 and started his academic career at the same university in 1874. He died in 1930 in Godesberg am Rhein (near Bonn).
Discovered (1887) the condensation reaction of an ester with an activated methylene group, now known as the Claisen condensation.
Synthesis of cinnamates by reacting aromatic aldehydes with esters. The reaction is known as the Claisen reaction and was described by Claisen for the first time in 1890.
Discovered the thermally induced rearrangement of allyl phenyl ether in 1912. He details its reaction mechanism in his last scientific publication (1925). In his honor, the reaction has been named the Claisen rearrangement.
Synthesis of isatin via a process known as the Claisen isatin synthesis, described for the first time in 1879.
Designer of a special distillation flask, now known as the Claisen flask.