A multitude of metrics exist to assign scores to synthetic routes within computer-aided synthesis planning (CASP) tools. A quantitative scoring method is necessary to identify the most promising synthetic approaches to a molecule. However, current CASP tools are limited in their capacity to evaluate reaction selectivity and are unable to fully account for the effect of side products on the purification sequences associated with chemical syntheses. We develop a novel quantitative metric called ExtractionScore for evaluating synthetic routes based on the predicted identities of side products as well as the separability of major and side products by liquid-liquid extraction based on chemical property prediction. By comparing industrially practiced routes to a collection of 200 pharmaceutically relevant compounds with routes suggested by state-of-the-art CASP software, we show that ExtractionScore may improve retrosynthetic recommendations by incorporating information about the formation of side products.