Editing Substituent (section)

== Notation ==
{{Further information|Pseudoelement symbol}}
In a chemical [[structural formula]], an organic substituent such as [[methyl]], [[ethyl group|ethyl]], or [[aryl]] can be written as ''R'' (or R<sup>1</sup>, R<sup>2</sup>, etc.) It is a generic placeholder, the ''R'' derived from ''[[Radical (chemistry)|radical]]'' or ''[[Rest (physics)|rest]]'', which may replace any portion of the formula as the author finds convenient. The first to use this symbol was [[Charles Frédéric Gerhardt]] in 1844.<ref>See:
*  Charles Gerhardt, ''Précis de chimie organique'' (Summary of organic chemistry), vol. 1 (Paris, France:  Fortin et Masson, 1844), page 29. From [https://books.google.com/books?id=sjlBAAAAYAAJ&pg=PA29 page 29:]  "''En désignant, par conséquent, les éléments combustibles par R, sans tenir comptes des proportions atomiques de carbone et d'hydrogène, on peut exprimer d'une manière générale:  Par R. — Les hydrogènes carbonés.''"  (Consequently, by designating combustible components by ''R'', without considering the atomic proportions of carbon and hydrogen, one can express in a general way:  By ''R'' — hydrocarbons.)
*  [[William B. Jensen]] (2010) "Ask the Historian:  Why is R Used for Hydrocarbon Substituents?," ''Journal of Chemical Education'', '''87''':  360–361.  Available at:  [http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/163.%20Origin%20of%20R.pdf University of Cincinnati.]</ref>

The symbol ''X'' is often used to denote electronegative substituents such as the [[halides]].<ref>{{Cite journal| doi = 10.1021/ed800139p| title = Why Is "R" Used To Symbolize Hydrocarbon Substituents?| year = 2010| last1 = Jensen | first1 = W. B.| journal = Journal of Chemical Education| volume = 87| issue = 4| pages = 360–361 | bibcode = 2010JChEd..87..360J}}</ref><ref>The first use of the letter ''X'' to denote univalent electronegative groups appeared in:
*  Stanislao Cannizzaro (1858) [https://books.google.com/books?id=VQOrAAAAIAAJ&pg=PA321 "Sunto di un corso di filosofia chimica, fatto nella R. Universita di Genova"] (Sketch of a course of chemical philosophy, offered at the Royal University of Genoa), ''Il Nouvo Cimento'' (The New Experiment), '''7''' :  321–366.  [https://books.google.com/books?id=VQOrAAAAIAAJ&pg=PA355 From page 355:]  " ... ''X indica tutto ciò che vi è nella molecola, oltre l'idrogeno metallico ... " ( ... X stands for all that is in the molecule, apart from metallic hydrogen ... ).''</ref>