Concentration in vacuo is used to remove liquid impurities. This technique is especially useful to evaporate volatile liquid impurities since, under a vacuum, vapor pressure doesn’t prevent evaporation. In this reaction, concentration in vacuo was used to purify product 18.

 

University of Michigan Chemistry, http://www.umich.edu/~chemh215/W06HTML/SSG2/ssg2/HTML%20Project/invacuo.htm (accessed April 4, 2014).