Ballpark figures: Using the Eyring equation, there is a straightforward relationship between Δ"G"‡, first-order rate constants, and reaction half-life at a given temperature. At 298 K, a reaction with Δ"G"‡ = 23 kcal/mol has a rate constant of "k ≈" 8.4 × 10–5 s–1 and a half life of "t"1/2 ≈ 2.3 hours, figures that are often rounded to "k ~" 10–4 s–1 and "t"1/2 ~ 2 h. Thus, a free energy of activation of this magnitude corresponds to a typical reaction that proceeds to completion overnight at room temperature. For comparison, the cyclohexane chair flip has a Δ"G"‡ of about 11 kcal/mol with "k ~" 105 s–1, making it a dynamic process that takes place rapidly (faster than the NMR timescale) at room temperature. At the other end of the scale, the "cis/trans" isomerization of 2-butene has a Δ"G"‡ of about 60 kcal/mol, corresponding to "k ~" 10–31 s–1 at 298 K. This is a negligible rate: the half-life is 12 orders of magnitude longer than the age of the universe.