Kinetic Vs Thermodynamic Control in Competing Reactions

Kinetic vs Thermodynamic Control in Competing Reactions

In most of our reactions, we are simply concerned about the products of the reaction. However, chemists study all aspects of reactions to include reaction rates and energy changes.

Kinetics is the study of reaction rates. A reaction rate tells us how fast a given reaction is occurring and includes time. For example, a reaction rate might be expressed by how much of a specific reactant disappears in a given time. If the concentration of the reactant R is in moles per liter, its concentration is [R], where the square brackets [ ] mean concentration in moles per liter. The reaction rate can be written in terms of the rate of disappearance of R, which is expressed as -d[R]/dt and is read as the change in concentration of R with respect to the time t. The minus sign means that the concentration of R is decreasing with time and reflects the physical reality that R is being used up during the reaction. The rate law of a reaction is an experimentally determined equation that has the form of rate = k[R1]x[R2]y, where R1 is reactant 1 and R2 is reactant 2, k is the specific rate constant and the exponents x and y are experimentally determined variables.

Thermodynamics is the study of energy changes that accompany a chemical reaction or a physical change such as the melting of a solid. For example, the enthalpy change (H) of a reaction tells us the change in heat content. The sign of H tells us whether heat is added (+) to the reaction or evolved (-) by the reaction (i.e., the reaction is endothermic or exothermic). In most cases, a reaction is conducted in such a way that the more stable of two potential products is obtained as the actual product. For example, the major product obtained when we dehydrate 2-butanol is trans-2-butene. trans-2-Butene is the most stable of the three potential alkene products. Stability and internal (potential) energy are inversely proportional. Thus, the lower the potential energy of a substance, the more stable it is. The Gibbs free energy G is related to H by the well-known Gibbs equation G = H -TS, where T is the absolute temperature and S is the entropy change of the reaction. In many cases, S is very small, and the term TS becomes negligible when it is compared to H. Therefore, in many cases G and H are very nearly the same and either one of them can be used in those instances to describe energy changes in a reaction. Therefore, it is largely the author's choice to use H or G to show changes in internal energy. G is more precise, but either term can generally be used to make the same points. Going back to trans-2-butene. It is more stable than cis-2-butene or 1-butene, the two minor products of the dehydration of 2-butanol.

Let us consider a hypothetical reaction in which two reactants A and B produce two products C and D. These products (C and D) form by two

significantly different reaction pathways and differ in their internal energy. Each pathway proceeds through a single transition state, which is shown as a maximum on a reaction profile or coordinate. The reaction is special, because the faster forming product C is less stable than D and the slower forming product D is more stable than C. This special case is shown in the reaction profile of Figure 1. Each product is formed from A and B. Product C forms via the blue pathway and product D via the red pathway.

Figure 1. A reaction profile of a reaction that can go under either thermodynamic or kinetic control.

In CHM 202, we learn that conjugated dienes can undergo either 1,4- or 1,2- addition, depending on the temperature. The more stable 1,4-adduct forms at a higher temperature than does the 1,2-adduct. Likewise, if two enolates can be produced from a ketone, the more stable enolate forms at a higher temperature, the less stable enolate forms at a lower temperature.

In this experiment, we are actually conducting two different reactions. An aldehyde and a ketone compete with each other to form a product with a limited amount of reagent. The aldehyde is 2-furfural, which is also called furfuraldehyde. 2-Furfural is a derivative of furan, a cyclic ether. The ketone is cyclohexanone, and the reagent is semicarbazide in the form of its hydrochloride salt. Semicarbazide is a derivative of hydrazine, H2NNH2. The structures below show several derivatives of hydrazine that undergo condensation reactions with aldehydes and ketones. Cyclohexanone and 2-furfural react similarly with all of these reagents.

The Reaction

The reaction between an aldehyde or ketone and hydrazine or its derivatives involves an addition followed by a dehydration so that the overall reaction is a condensation reaction (i.e., one in which water is lost). Figure 1 shows the generalized two-step reaction.

Figure 1. General Reaction.

Mechanism

Figure 2 shows the mechanism of the reaction. In Step 1, the

Figure 2. Mechanism of Reaction.

hydrazine derivative adds to the carbonyl group. The non-bonded pair of electrons on nitrogen forms a new sigma bond with the carbonyl carbon, and the  bond breaks to leave a negative charge on oxygen. In the product of the first step, nitrogen has four bonds and a positive formal charge. In Step 2, a proton rapidly exchanges in the aqueous medium to form an aminoalcohol. In Step 3, the OH group is protonated (i.e., the oxygen of the OH group abstracts a proton from H3O+ to make a good leaving group. In Step 4, the double bond between carbon and nitrogen forms as the aqueous solvent abstracts a proton from the nitrogen and the good leaving group (H2O) departs. Hydrazine and all of its derivatives follow this general mechanism. The R group determines which compound is the reactant. For example, if R = phenyl, the reactant is phenylhydrazine.

Simple Product Formation

Once we know the reaction involves two steps and we know the mechanism of the reaction, we can look at the reactants and see that the double bond from carbon to oxygen (i.e., the carbonyl double bond) becomes a double bond between the carbonyl carbon and a hydrazine (or derivative) nitrogen atom.

Figure