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{{short description|Substance added to a system to cause a chemical reaction}}
{{distinguish|Regent}}
{{redirect-distinguish|Reactants|Reactance (disambiguation){{!}}Reactance}}
[[File:SulfurReagent.jpg|thumb|Reagents, such as [[sulfur]] (pictured), are the starting materials used in chemical reactions.]]

In [[chemistry]], a '''reagent''' ({{IPAc-en|r|i|ˈ|eɪ|dʒ|ən|t}} {{respell|ree|AY|jənt}}) or '''analytical reagent''' is a substance or compound added to a system to cause a [[chemical reaction]], or test if one occurs.<ref name=gold>{{GoldBookRef |title=Reactant |file=R05163 |year=1996}}</ref> The terms ''reactant'' and ''reagent'' are often used interchangeably, but '''reactant''' specifies a substance ''consumed'' in the course of a chemical reaction.<ref name=gold/> ''[[Solvent]]s'', though involved in the [[reaction mechanism]], are usually not called reactants. Similarly, ''[[catalysis|catalysts]]'' are not consumed by the reaction, so they are not reactants. In [[biochemistry]], especially in connection with [[enzyme]]-catalyzed reactions, the reactants are commonly called [[substrate (biochemistry)|substrates]].

==Definitions==

=== Organic chemistry ===
In [[organic chemistry]], the term "reagent" denotes a chemical ingredient (a compound or mixture, typically of inorganic or small organic molecules) introduced to cause the desired transformation of an organic substance. Examples include the [[Collins reagent]], [[Fenton's reagent]], and [[Grignard reagent]]s.{{cn|date=June 2025}}

=== Analytical chemistry ===
In [[analytical chemistry]], a reagent is a compound or mixture used to detect the presence or absence of another substance, e.g. by a color change, or to measure the concentration of a substance, e.g. by [[Colorimetry (chemical method)|colorimetry]]. Examples include [[Fehling's reagent]], [[Millon's reagent]], and [[Tollens' reagent]].{{cn|date=June 2025}}

==Commercial or laboratory preparations==
In commercial or laboratory preparations, {{anchor|Reagent_grade}}'''[[chemical purity|reagent-grade]]''' designates [[chemical substance]]s meeting [[standardization|standards]] of [[purity (gas)|purity]] that ensure the scientific precision and reliability of [[chemical analysis]], chemical reactions or physical testing. Purity standards for reagents are set by organizations such as [[ASTM International]] or the [[American Chemical Society]]. For instance, reagent-quality water must have very low levels of impurities such as [[sodium]] and [[chloride]] ions, [[silica]], and bacteria, as well as a very high [[electrical resistivity]].  Laboratory products which are ''less'' pure, but still useful and economical for undemanding work, may be designated as ''technical'', ''practical'', or ''crude'' grade to distinguish them from reagent versions.{{cn|date=June 2025}}

==Biology==
In the field of biology, the [[biotechnology]] revolution in the 1980s grew from the development of reagents that could be used to identify and manipulate the chemical matter in and on cells.<ref>{{cite journal|last1=Fox|first1=Jeffrey L|title=Antibody reagents revolutionizing immunology|journal=Chemical & Engineering News Archive|date=1 January 1979|volume=57 |pages=15–17|doi=10.1021/cen-v057n001.p015}}</ref><ref name=NIHreport1998>{{cite web |title=Report of the National Institutes of Health (NIH) Working Group on Research Tools|url=http://www.nih.gov/news/researchtools/index.htm|publisher=NIH|archive-url=https://web.archive.org/web/20000816014021/http://www.nih.gov/news/researchtools/index.htm |archive-date=August 16, 2000|date=June 4, 1998}}</ref>  These reagents included [[Antibody#Research applications|antibodies]] ([[Polyclonal antibodies|polyclonal]] and [[Monoclonal antibody|monoclonal]]), [[oligomers]], all sorts of [[model organisms]] and [[immortalised cell line]]s, reagents and methods for [[molecular cloning]] and [[DNA replication]], and many others.<ref name=NIHreport1998/><ref>{{cite journal |last1=Ishino |first1=S|last2=Ishino|first2=Y|title=DNA polymerases as useful reagents for biotechnology: the history of developmental research in the field |journal=Frontiers in Microbiology|date=29 August 2014|volume=5|pages=465|pmid=25221550|pmc=4148896|doi=10.3389/fmicb.2014.00465|doi-access=free}}</ref>

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===Tool compounds===
Tool compounds are an important class of reagent in biology. They are small molecules or biochemicals like [[siRNA]] or antibodies that are known to affect a given biomolecule{{ambiguous|reason=needs punctuating to clarify phrase structure|date=February 2024}}—for example a [[drug target]]—but are unlikely to be useful as drugs themselves, and are often starting points in the [[drug discovery]] process.<ref>{{cite journal |last1=Kenakin|first1=T|last2=Bylund|first2=DB|last3=Toews|first3=ML|last4=Mullane|first4=K|last5=Winquist|first5=RJ|last6=Williams|first6=M|title=Replicated, replicable and relevant-target engagement and pharmacological experimentation in the 21st century |journal=Biochemical Pharmacology|date=1 January 2014 |volume=87 |issue=1 |pages=64–77 |doi=10.1016/j.bcp.2013.10.024|pmid=24269285}}</ref><ref>{{cite journal|last1=Lindsley|first1=CW|title=2013 Philip S. Portoghese Medicinal Chemistry Lectureship: drug discovery targeting allosteric sites|journal=Journal of Medicinal Chemistry|date=25 September 2014 |volume=57 |issue=18 |pages=7485–7498|pmid=25180768|pmc=4174999|doi=10.1021/jm5011786}}</ref>

However, many natural substances are hits in almost any assay in which they are tested, and therefore not useful as tool compounds. Medicinal chemists class them instead as [[pan-assay interference compounds]]. One example is [[curcumin]].<ref>{{cite journal|last1=Baker|first1=Monya|title=Deceptive curcumin offers cautionary tale for chemists |journal=Nature |date=9 January 2017|volume=541|issue=7636|pages=144–145|doi=10.1038/541144a|pmid=28079090|bibcode=2017Natur.541..144B|doi-access=free}}</ref><ref>{{cite journal|last1=Dahlin|first1=JL|last2=Walters|first2=MA|title=The essential roles of chemistry in high-throughput screening triage |journal=[[Future Medicinal Chemistry]]|date=July 2014|volume=6|issue=11|pages=1265–1290|pmid=25163000|pmc=4465542|doi=10.4155/fmc.14.60}}</ref><ref>{{cite journal|last1=Baell|first1=JB|last2=Holloway|first2=GA|title=New substructure filters for removal of pan assay interference compounds (PAINS) from screening libraries and for their exclusion in bioassays.|journal=Journal of Medicinal Chemistry |date=8 April 2010|volume=53|issue=7|pages=2719–2740|doi=10.1021/jm901137j|pmid=20131845|citeseerx=10.1.1.394.9155}}</ref>

==See also==
* [[Limiting reagent]]
* [[List of reagents|Common reagents]]
* [[Product (chemistry)|Product]]
* [[Reagent bottle]]
* [[Substrate (chemistry)|Substrate]]
* {{in title}}

==References==
{{reflist}}

==External links==
* {{Commonscat-inline2|Reagents}}
* {{Wiktionary-inline|reagent|reactant}}

{{Authority control}}

[[Category:Reagents| ]]
[[Category:Reagents for organic chemistry| ]]
[[Category:Biological techniques and tools]]
[[Category:Chemical reactions]]
[[Category:Reagents for biochemistry]]