基
) William B. Jensen (2010) "Ask the Historian: Why is R Used for Hydrocarbon Substituents?," Journal of Chemical Education, 87: 360-361. Available at: University of Cincinnati.
↑ Jensen, W. B. (2010). “Why Is "R" Used To Symbolize Hydrocarbon Substituents?”. Journal of Chemical Education 87 (4): 360–361. doi:10.1021/ed800139p.
↑ The first use of the letter X to denote univalent electronegative groups appeared in: Stanislao Cannizzaro (1858) "Sunto di un corso di filosofia chimica, fatto nella R.