Activation Parameters for the Additions of Arylhalocarbenes to Alkenes

By use of nanosecond laser flash photolytic methods, absolute rate constants have been obtained for the additions of p-(trifluoromethyl)phenylbromocarbene (CF₃C₆H₄CBr), phenylbromocarbene (PhCBr), phenylchlorocarbene (PhCCl), phenylfluorocarbene (PhCF), and p-methoxyphenylfluorocarbene (CH₃OC₆H₄CF) to tetramethylethylene (TME) and 1-hexene. The rate constants ranged from 1.7 × 10⁹ (CF₃C₆H₄CBr/TME) to 5.0 × 10⁴ L/(M•s) (CH₃OC₆H₄CF/1-hexene); classical structure/reactivity relations were obeyed. Determination of the rate constants at temperatures between 315 and 200 K gave E_a, log A, ΔH*, ΔS*, and ΔG* for the 10 reactions. All of the reactions were dominated by ΔS*. ΔG* ranged from 5.0 to 11 kcal/mol for the fastest and slowest reactions. Negative values of E_a were observed for some of the faster reactions at higher temperatures. These results are analyzed in terms of single-step addition reactions where ΔG* is dominated by ΔS* and, alternatively, in terms of a two-step mechanism proceeding through a carbene-alkene complex.