Abstract <p>The kinetic and thermodynamic characteristics of the formation of dicyclohexyl disulfide from chlorocyclohexane and sodium disulfide both in the presence and absence of iodide ions were calculated by quantum-chemical methods. When estimating the free energy of dissociation of sodium sulfide for mechanism calculations, a combined approach B2PLYP-D3/6-311+G(2df,2p)//B3LYP/6-31+G(d) with geometry optimization within the IEFPCM continuum model was chosen. It is shown that the reaction does not proceed via the S_N1 mechanism because of the greatly increased free energy during the formation of the cyclohexyl cation from both chloro- and iodocyclohexane. It is demonstrated that the rate-limiting step of the reaction is the S_N2 substitution of the chloride ion in chlorocyclohexane. The free activation energy of substitution of chlorine by the iodide ion is 1.0 kcal/mol lower than that by the sodium disulfide anion. The interaction of the sodium disulfide anion with iodocyclohexane, in turn, occurs with a barrier 1.9 kcal/mol lower than with chlorocyclohexane. The lower activation barriers involving iodine explain the increase in the reaction rate of dicyclohexyl disulfide formation from chlorocyclohexane and sodium disulfide after the addition of potassium iodide to the reaction mixture.</p>