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In chemistry, regioselectivity is the preference of one direction of chemical bond making or breaking over all other possible directions.^[1]^[2] It can often apply to which of many possible positions a reagent will affect, such as which proton a strong base will abstract from an organic molecule, or where on a substituted benzene ring a further substituent will add.

A specific example is a halohydrin formation reaction with 2-propenylbenzene:^[3]

Because of the preference for the formation of one product over another, the reaction is selective. This reaction is regioselective because it selectively generates one constitutional isomer rather than the other.

Certain examples of regioselectivity have been formulated as rules for certain classes of compounds under certain conditions, many of which are named. Among the first introduced to chemistry students are Markovnikov's rule for the addition of protic acids to alkenes, and the Fürst-Plattner rule for the addition of nucleophiles to derivatives of cyclohexene, especially epoxide derivatives.^[4]^[5]

Regioselectivity in ring-closure reactions is subject to Baldwin's rules. If there are two or more orientations that can be generated during a reaction, one of them is dominant (e.g., Markovnikov/anti-Markovnikov addition across a double bond)

Regioselectivity can also be applied to specific reactions such as addition to pi ligands.

Selectivity also occurs in carbene insertions, for example in the Baeyer-Villiger reaction. In this reaction an oxygen is regioselectively inserted near an adjacent carbonyl group. In ketones this insertion is directed toward the carbon which is more highly substituted (i.e. according Markovnikov's rule). For example, in a study involving acetophenones this oxygen was preferentially inserted in-between the carbonyl and the aromatic ring to give acetyl aromatic esters, not methyl benzoates.^[6]

References

^ March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.), New York: Wiley, ISBN 0-471-85472-7
^ http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch06/ch6-0-1.html Regioselectivity & Stereoselectivity
^ Regioselectivity in Organic Synthesis: Preparation of the Bromohydrin of alpha-Methylstyrene Brad Andersh, Kathryn N. Kilby, Meghan E. Turnis, and Drew L. Murphy 102 Journal of Chemical Education • Vol. 85 No. 1 January 2008
^ W. Markownikoff (1870). "Ueber die Abhängigkeit der verschiedenen Vertretbarkeit des Radicalwasserstoffs in den isomeren Buttersäuren". Annalen der Pharmacie 153 (1): 228–259.
^ Fürst, A.; Plattner, P. A. Helv. Chim. Acta 1949, 32, 275
^ Palmer, Billy W.; Fry, Arthur (1970). "Variation of carbon-14 isotope effect with substituent and the mechanism of the m-chloroperbenzoic acid oxidation of laeled para-substituted acetophenones". Journal of the American Chemical Society 92 (8): 2580–2581. doi:10.1021/ja00711a079.

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English Journal

Plasmonic properties of regiospecific core-satellite assemblies of gold nanostars and nanospheres.

D S Indrasekara AS1, Thomas R, Fabris L.
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Tyrosine O-prenyltransferases TyrPT and SirD displaying similar behavior toward unnatural alkyl or benzyl diphosphate as their natural prenyl donor dimethylallyl diphosphate.

Yu H1, Liebhold M, Xie X, Li SM.
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PMID 25707862

Ruthenium Hydride Catalyzed Silylvinylation of Internal Alkynes Using Ethylene as an Additive.

Wilson RJ1, Kaminsky L1, Ahmed I1, Clark DA1.
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An efficient synthetic strategy for the regiospecific silylvinylation of internal alkynes is described. This transformation is catalyzed by RuHCl(CO)(SIMes)PPh3 and provides a net 5-exo-dig trans-silylvinylation of internal alkynes. Ethylene was used to decrease reaction times and displayed altered
PMID 26241076

Japanese Journal

A novel synthesis of 1-aryl-1H-benzotriazoles via oxidative C-H amination

Zhou Zhou,Liu Qi-Lun,Li Wen [他]
Heterocycles 83(9), 2057-2065, 2011-09-01
NAID 40018958916

Regiospecific rearrangement of hydroxylamines to secondary amines using diisobutylaluminum hydride

Cho Hidetsura,Sugimoto Kenji,Iwama Yusuke [他]
Heterocycles 82(2), 1633-1644, 2011-03-01
NAID 40018702743

〈2,3〉 Wittig rearrangement of β'-hydroxyethyl bis-allylic ethers: highly regiospecific entry to singly dehydroxylated 19-nor-1(or 3),25-dihydroxyvitamin D3