regiospecificity

Regiospecificity is a property of a chemical reaction in which bond formation or bond cleavage occurs at a particular position within a molecule, producing a definite regioisomer. In a regiospecific reaction essentially only one regioisomer is formed, or the other possible regioisomers appear only in negligible amounts. This concept is closely related to regioselectivity, but regiospecificity implies a stronger, effectively exclusive outcome.

Regiospecificity versus regioselectivity: regioselectivity describes the preference for one regioisomer over others and is usually quantified by product ratios. Regiospecificity denotes an extraordinary or near-complete preference for the formation of a single regioisomer, often due to mechanistic or directing effects.

Examples and mechanisms: additions to alkenes commonly show regiospecific or highly regiospecific outcomes. Hydrohalogenation of alkenes under typical conditions follows Markovnikov regiochemistry, placing the electrophile on the more substituted carbon and the proton on the less substituted carbon, often yielding the major product predominantly. Hydroboration-oxidation of alkenes gives anti-Markovnikov products—primary alcohols predominantly—representing a regiospecific outcome in many substrates. In electrophilic aromatic substitution, directing groups can enforce substitution at a particular position, yielding one major regioisomer with varying degrees of exclusivity.

Factors influencing regiospecificity include electronic distribution, steric effects, directing groups, and the choice of catalysts, reagents, solvents, and temperature. Recognizing regiospecificity helps in planning syntheses and predicting product structures in organic and medicinal chemistry.