Regiochemical

Regiochemical refers to regioselectivity in chemical reactions—the tendency for a reaction to occur at one location in a molecule over other possible positions, yielding a preferred constitutional isomer. The term emphasizes where a new bond or substituent is formed, rather than how the atoms are arranged in three-dimensional space (that latter aspect is covered by stereochemistry). Regiochemical outcomes are influenced by electronic effects, steric factors, directing groups, reaction conditions, solvents, and catalysts.

Common examples illustrate the concept. In electrophilic aromatic substitution, substituents direct incoming electrophiles to specific positions on an aromatic ring. For instance, a methyl group generally directs electrophilic substitution to the ortho and para positions, while a nitro group directs to the meta position. In addition reactions to alkenes, HX adds with Markovnikov regioselectivity, placing the hydrogen on the carbon with more hydrogens and the halogen or other group on the more substituted carbon; under hydroboration-oxidation, anti-Markovnikov regioselectivity places the new group on the less substituted carbon. Nucleophilic opening of epoxides typically proceeds with attack at the less substituted carbon under basic conditions, whereas protonated (acidic) epoxides favor attack at the more substituted carbon. In conjugate (Michael) additions to enones, nucleophiles tend to add at the beta position, reflecting regiochemical control.

Regiochemical reasoning guides synthesis planning, enabling selective construction of target isomers in pharmaceuticals, agrochemicals, and polymers. It often intersects with stereochemical considerations, but focuses specifically on the site of bond formation rather than three-dimensional orientation.