Rainer Ludwig Claisen (German pronunciation: ['?a??n? 'kla??sn?]; 14 January 1851 – 5 January 1930) was a 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 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).