Regioselectivity

Regioselectivity is a reaction’s preference for forming one constitutional isomer over other possible isomers. It determines which positions in a molecule become connected or functionalized.

Electrophilic addition: A reaction in which an electrophile and a nucleophile add across a multiple bond. Its competing orientations on an unsymmetrical alkene reveal regioselectivity.
Constitutional isomer: One of two or more compounds with the same molecular formula but different atom-to-atom connectivity. Regioselective products are constitutional isomers that differ in where groups are attached.
Hydroboration–oxidation: A two-step alkene transformation that replaces the double bond with an alcohol, generally placing the hydroxyl group at the less substituted carbon. Its anti-Markovnikov orientation is a standard way to choose an alcohol’s position.
Chemoselectivity: A reaction’s preference for reacting at one functional group rather than another in a molecule. It concerns which functional group reacts, not which position within one pattern reacts.
Markovnikov's rule: A guideline predicting the orientation of additions to unsymmetrical alkenes, often placing hydrogen on the carbon already bearing more hydrogens. It predicts the major orientation in many polar additions to alkenes.
Reaction product: A substance formed by a chemical reaction from its starting materials. Comparing possible products establishes which regioisomer a reaction favors.
Oxymercuration–demercuration: A two-step alkene hydration that generally places a hydroxyl group at the more substituted carbon without carbocation rearrangement. It offers Markovnikov hydration with a different pathway from acid-catalyzed addition.
Stereoselectivity: A reaction’s preference for forming one stereoisomer over other possible stereoisomers. It distinguishes spatial arrangements, whereas regioselectivity distinguishes constitutional connectivity.
Carbocation rearrangement: A structural change in a carbocation, such as a hydride or alkyl shift, that forms a more stable cation. Rearrangement can change the product skeleton after an initially selective addition.
Reaction mechanism: A sequence of elementary steps describing how reactants become products through bond changes and intermediates. Mechanisms connect molecular structure to the observed product ratio.

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