7.5 Product Distributions
- Page ID
- 17192
Quite frequently, chemical reactions produce more than one product. In such circumstances, we need to consider the product distribution, the ratio of one product to another. How much of one product we get in relation to other products can be determined from energetic considerations, but what energies one must consider depends on the type of reaction: reversible vs. irreversible.
Reversible Reactions
Because reversible reactions are in equilibrium, the ratio of potential products depends entirely on their relative energies (∆G°) using the relationship ∆G° = -RTlnK. Algebraic rearrangement produces K = e-∆G°/RT and the equiibrium constant K is merely the ratio of one product concentration to the other. Thus, the ratio of products in a reversible equilibrium depends only on the relative energies of the competing products. The major product in a reversible reaction will be the more stable one.
Irreversible Reactions
Conversely, in an irreversible reaction, the ratio of products from competitive pathways is not dependent on the stabilities of the various products because the reaction is not operating under equiibrium conditions. Instead, the ratio of products in an irreversible sequence depends on the rates at which each product is produced. Algebraic manipulation of the rate equations gives us an expression for the ratio of products that depends only on the ratio of rate constants. That ratio in turn depends on a difference in the energies of the respective transition states. Thus, the product ratio of an irreversible reaction depends only on the difference in energies between the two competing transition states leading to the different products.