Thermodynamic control is obtained, however the reaction remains incomplete unless the product enolate is trapped, as in the example below. Since H transfers are very fast, the trapping reaction being slower, the ratio of trapped products largely mirrors the deprotonation equilibrium. In electrophilic additions The electrophilic addition reaction of hydrogen bromide to 1,3-butadiene above room temperature leads predominantly to the thermodynamically more stable 1,4 adduct, 1-bromo-2-butene, but decreasing the reaction temperature to below room temperature favours the kinetic 1,2 adduct, 3-bromo-1-butene. The rationale for the differing selectivities is as follows: Both products result from Markovnikov protonation at position 1, resulting in a resonance-stabilized allylic cation. The 1,4 adduct places the larger Br atom at a less congested site and includes a more highly substituted alkene moiety, while the 1,2 adduct is the result of the attack by the nucleophile (Br−) at the carbon of the allylic cation bearing the greatest positive charge (the more highly substituted carbon is the most likely place for the positive charge). Characteristics In principle, every reaction is on the continuum between pure kinetic control and pure thermodynamic control. These terms are with respect to a given temperature and time scale. A process approaches pure kinetic control at low temperature and short reaction time. For a sufficiently long time scale, every reaction approaches pure thermodynamic control, at least in principle. This time scale becomes shorter as the temperature is raised. In every reaction, the first product formed is that which is most easily formed. Thus, every reaction a priori starts under kinetic control. A necessary condition for thermodynamic control is reversibility or a mechanism permitting the equilibration between products. Reactions are considered to take place under thermodynamic reaction control when the reverse reaction is sufficiently rapid that the equilibrium establishes itself within the allotted reaction time. In this way, the thermodynamically more stable product is always favoured. Under kinetic reaction control, one or both forward reactions leading to the possible products is significantly faster than the equilibration between the products. After reaction time t, the product ratio is the ratio of rate constants k and thus a function of the difference in activation energies Ea or ΔG‡: (equation 1) Unless equilibration is prevented (e.g., by removal of the product from the reaction mixture as soon as it forms), "pure" kinetic control is strictly speaking impossible, because some amount of equilibration will take place before the reactants