Editing Regioselectivity (section)

{{Short description|Preference of chemical bonding or breaking in one direction over others}}
{{Use dmy dates|date=July 2023}}

In [[organic chemistry]], '''regioselectivity''' is the preference of [[chemical bond]]ing or breaking in one direction over all other possible directions.<ref>{{JerryMarch}}</ref><ref>http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch06/ch6-0-1.html Regioselectivity & Stereoselectivity</ref> It can often apply to which of many possible positions a [[reagent]] will affect, such as which [[proton]] a strong [[Base (chemistry)|base]] will [[Hydrogen atom abstraction|abstract]] from an [[Organic compound|organic molecule]], or where on a substituted [[benzene]] ring a further [[substituent]] will be added.

A specific example is a [[halohydrin formation reaction]] with [[2-propenylbenzene]]:<ref>''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'''</ref>

:[[File:RegioselectivityInhalohydrinformation.svg|445px|Regioselectivity in halohydrin formation]]

Because of the preference for the formation of one [[Product (chemistry)|product]] over another, the reaction is selective.  This reaction is regioselective because it selectively generates one [[constitutional isomer]] rather than the other.

Various examples of regioselectivity have been formulated as rules for certain classes of [[Chemical compound|compound]]s under certain conditions, many of which are named. Among the first introduced to chemistry students are [[Markovnikov's rule]] for the addition of protic [[acid]]s to [[alkene]]s, and the [[Fürst-Plattner rule]] for the addition of [[nucleophile]]s to derivatives of [[cyclohexene]], especially [[epoxide]] derivatives.<ref>W. Markownikoff (1870). "Ueber die Abhängigkeit der verschiedenen Vertretbarkeit des Radicalwasserstoffs in den isomeren Buttersäuren". Annalen der Pharmacie 153 (1): 228–259.</ref><ref>Fürst, A.; Plattner, P. A. Helv. Chim. Acta 1949, 32, 275</ref>

Regioselectivity in [[ring-closure reaction]]s 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]] [[Insertion reaction|insertion]]s, for example in the [[Baeyer-Villiger]] reaction. In this reaction, an oxygen is regioselectively inserted near an adjacent [[carbonyl]] group. In [[ketone]]s, this insertion is directed toward the carbon which is more highly substituted (i.e. according to Markovnikov's rule). For example, in a study involving [[acetophenone]]s, this oxygen was preferentially inserted between the carbonyl and the [[aromatic]] ring to give [[acetyl]] aromatic [[ester]]s instead of [[methyl benzoate]]s.<ref>{{cite journal|last1=Palmer|first1=Billy W.|last2=Fry|first2=Arthur|title=Variation of carbon-14 isotope effect with substituent and the mechanism of the m-chloroperbenzoic acid oxidation of laeled para-substituted acetophenones|journal=Journal of the American Chemical Society|date=1970|volume=92|issue=8|pages=2580–2581|doi=10.1021/ja00711a079|bibcode=1970JAChS..92.2580P }}</ref>