The determination of thermodynamic equilibrium constant for

3.6. The determination of thermodynamic equilibrium constant for the hydrolysis reaction of CHO
Although the hydrolysis reaction rate of CHO in the acidic solution is very high, the conversion of CHO is still unsatisfied. Thus the reaction 'NHS-SS-Biotin' tended to be controlled by thermodynamics rather than dynamics. The enthalpy change ΔrHmΘ, entropy change ΔrSmΘ, Gibbs free NHS-SS-Biotin change ΔrGmΘ of the reaction were calculated by Gaussian 09 according to density functional theory (DFT), and equilibrium constant was calculated as follow:equation(1)ΔrGmΘ=-RTlnKΘ
As shown in the Table 2, Gibbs free energy change ΔrGmΘ of the hydrolysis reaction of CHO is a positive number (ΔrGmΘ>0) and the equilibrium constant (KΘ=1.98×10-5KΘ=1.98×10-5) is a extremely small value, which means this reaction cannot happen spontaneously under standard conditions without Brønsted acid. In fact, the hydrolysis reaction of CHO in absence of sulfuric acid had been tested in this work and the obtained results supported the conclusion above-mentioned.