Electrophilic Addition to Alkenes: The Relation between Reactivity and Enthalpy of Hydrogenation: Regioselectivity is Determined by the Stability of the Two Conceivable Products

• Published in: Chemistry : a European journal 2015-07, Vol.21 (29), p.10348-10361
• Main Authors: Schnatter, Wayne F. K., Rogers, Donald W., Zavitsas, Andreas A.
• Format: Article
• Language: English
• Subjects: Alkenes; Carbenes; Chemistry; electrophilic addition; Enthalpy; Halogenation; heats of formation; Hydration; Hydrogenation; NMR spectroscopy; Solvents; Stability
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201500314

Although electrophilic addition to alkenes has been well studied, some secrets still remain. Halogenations, hydrohalogenations, halohydrin formations, hydrations, epoxidations, other oxidations, carbene additions, and ozonolyses are investigated to elucidate the relation of alkene reactivities with their enthalpies of hydrogenation (ΔHhyd). For addition of electrophiles to unconjugated hydrocarbon alkenes, ln(k) is a linear function of ΔHhyd, where k is the rate constant. Linear correlation coefficients are about 0.98 or greater. None of the many previously proposed correlations of ln(k) with the properties of alkenes or with linear free‐energy relationships match the generality and accuracy of the simple linear relationship found herein. A notable exception is acid‐catalyzed hydration in water or in solvents stabilizing relatively stable carbocation intermediates (e.g., tertiary, benzylic, or allylic). 13C NMR chemical shifts of the two alkene carbons also predict regioselectivity. These effects have not been noted previously and are operative in general, including addition to heteroatom‐substituted alkenes. Rate data show that ln(k) is a linear function of the enthalpy of hydrogenation of the alkenes for halogenations, hydrohalogenations, halohydrin formations, epoxidations, other oxidations, carbene addition, and ozonolyses in various solvents. Acid‐catalyzed hydration is the notable exception. The regioselectivity of addition is determined by the relative thermodynamic stability of the two conceivable products.