Editing Substituent (section)

{{short description|Atom set which has replaced hydrogen atoms on a hydrocarbon's parent chain}}
{{about|the term in chemistry|other meanings|Substitution (disambiguation)}}
{{Use dmy dates|date=February 2020}}

In [[organic chemistry]], a '''substituent''' is one or a group of [[atom]]s that replaces (one or more) atoms, thereby becoming a [[moiety (chemistry)|moiety]] in the resultant (new) [[molecule]].<ref>{{Cite web |title=Definition of SUBSTITUENT |url=https://www.merriam-webster.com/dictionary/substituent |access-date=2022-06-04 |website=www.merriam-webster.com |language=en}}</ref>{{Notetag|In [[organic chemistry]] and [[biochemistry]], the terms ''substituent'' and ''[[functional group]]'', as well as ''[[side chain]]'' and ''[[pendant group]]'', are used almost interchangeably to describe those branches from the parent structure,<ref>{{cite book|title=Organic Chemistry Demystified|url=https://books.google.com/books?id=yVPcSIn5xjAC&pg=PT88|author=D.R. Bloch|year=2006| publisher=Mcgraw-hill |isbn=978-0-07-145920-4}}</ref> though certain distinctions are made in [[polymer]] chemistry.<ref>{{cite journal|url=http://goldbook.iupac.org/src_PAC1996682287.html|title=PAC, 1996, 68, 2287. Glossary of basic terms in polymer science (IUPAC Recommendations 1996)|journal=Pure and Applied Chemistry|volume=68|issue=12|pages=2287–2311|doi=10.1351/pac199668122287|year=1996|last1=Jenkins|first1=A. D.|last2=Kratochvíl|first2=P.|last3=Stepto|first3=R. F. T.|last4=Suter|first4=U. W.|doi-access=free}}  This distinguishes a ''pendant group'' as neither oligomeric nor polymeric, whereas a ''pendant chain'' must be oligomeric or polymeric.</ref> In [[polymers]], side chains extend from the [[Backbone chain|backbone]] structure. In [[protein]]s, side chains are attached to the [[alpha carbon]] atoms of the [[amino acid]] backbone.}}

The suffix ''-yl'' is used when naming [[organic compound]]s that contain a [[single bond]] replacing one hydrogen; ''-ylidene'' and ''-ylidyne'' are used with [[double bond]]s and [[triple bond]]s, respectively.  In addition, when naming hydrocarbons that contain a substituent, positional numbers are used to indicate which carbon atom the substituent attaches to when such information is needed to distinguish between [[isomer]]s. Substituents can be a combination of the [[inductive effect]] and the [[mesomeric effect]].  Such effects are also described as [[electron-rich]] and [[electron withdrawing]]. Additional [[steric effects]] result from the volume occupied by a substituent.

The phrases ''most-substituted'' and ''least-substituted'' are frequently used to describe or compare molecules that are [[Product (chemistry)|product]]s of a [[chemical reaction]]. In this terminology, [[methane]] is used as a reference of comparison. Using methane as a reference, for each hydrogen atom that is replaced or "substituted" by something else, the molecule can be said to be more highly substituted. For example:
*  [[Markovnikov's rule]] predicts that the hydrogen atom is added to the carbon of the [[alkene]] functional group which has the greater number of hydrogen atoms (fewer [[alkyl]] substituents).
*  [[Zaitsev's rule]] predicts that the major reaction product is the alkene with the more highly substituted (more stable) double bond.