Science, Tech, Math › Science Substituent Definition Share Flipboard Email Print Benzene with two identical substituents. Jü / Wikimedia Commons / CC0 1.0 Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated October 04, 2019 Definition: A substituent is an atom or functional group that replaces a hydrogen atom on a hydrocarbon.In chemical structures, generic substituents are denoted by a capital R. If the substituent is a halide, a capital X.