**Equilibrium Constant and Free Energy Changes** **, ΔG°**

_{eq}, is a mathematical expression that relates the amount of starting material and product at equilibrium.

_{eq}tells about the position of equilibrium; that is, it expresses whether the starting materials or products predominate once equilibrium has been reached.

**(1)**When K

_{eq}> 1, equilibrium favors the products (C and D) and the equilibrium lies to the right as the equation is written.

**(2)**When K

_{eq}< 1, equilibrium favors the starting materials (A and B) and the equilibrium lies to the left as the equation is written.

_{eq}> 1.

**(1)**When K

_{eq}> 1, log K

_{eq}is positive, making ΔG° negative, and energy is released. Thus, equilibrium favors the products when the energy of the products is lower than the

**(2)**When K

_{eq}< 1, log K

_{eq}is negative, making ΔG° positive, and energy is absorbed. Thus, equilibrium favors the reactants when the energy of the products is higher than the energy of the reactants.

_{eq}, a small change in energy corresponds to a large difference in the relative amount of starting material and product at equilibrium. Several values of ΔG° and K

_{eq}are given in the following table:

For example, a difference in energy of only ~6 kJ/mol means that there is 10 times as much of the more stable species at equilibrium. A difference in energy of ~18 kJ/mol means that there is essentially only one compound, either starting material or product, at equilibrium.

**Conclusion**

❒Conditions Favoring Product Formation:

**Energy Changes and Conformational Analysis**

❒For example, the energy difference between the two chair conformations of phenylcyclohexane is –12.1 kJ/mol, as shown in the accompanying equation. Using the values in table above, this corresponds to an equilibrium constant of ~100, meaning that there is approximately 100 times more B (equatorial phenyl group) than A (axial phenyl group) at equilibrium.

**Enthalpy and Entropy**

**This equation tells us that the total energy change in a reaction is due to two factors:**

**(A) The change in the bonding energy**

**(B) The change in disorder (Entropy).**

**(1)**When the number of molecules of starting material differs from the number of molecules of product in the balanced chemical equation.

**(2)**When an acyclic molecule is cyclized to a cyclic one, or a cyclic molecule is converted to an acyclic one.

**(1)**The product is favored in reactions in which DH° is a negative value; that is, the bonds in the product are stronger than the bonds in the starting material.

**(2)**The starting material is favored in a reaction in which DH° is a positive value; that is, the bonds in the starting material are stronger than the bonds in the product.

**Reference:***Organic chemistry / T.W. Graham Solomons , Craig B.Fryhle , Scott A.snyder , / ( eleventh edition) / 2014.*