Question

For two reversible competing one-step reactions, A-Y and A-Z, how does the Y/Z ratio change with a) reaction temperature, b) reaction time if Y is the kinetic product and Z is the thermodynamic product? Please explain with the help of a reaction energy diagram.

Answer 1

When a reaction produces more than one product, the product that is formed most rapidly is called the kinetic product, and the most stable product is called the thermodynamic product.

Thermodynamic Versus Kinetic Control of Reactions

When a conjugated diene undergoes an electrophilic addition reaction, two factors—the temperature at which the reaction is carried out and the structure of the reactant—determine whether the 1,2-addition product or the 1,4-addition product will be the major product of the reaction. When a reaction produces more than one product, the product that is formed most rapidly is called the kinetic product, and the most stable product is called the thermodynamic product. Reactions that produce the kinetic product as the major product are said to be kinetically controlled. Reactions that produce the thermodynamic product as the major product are said to be thermodynamically controlled.
“The thermodynamic product is the most stable product.”

For many organic reactions, the most stable product is the one that is formed most rapidly. In other words, the kinetic product and the thermodynamic product are one and the same. Electrophilic addition to 1,3-butadiene is an example of a reaction in which the kinetic product and the thermodynamic product are not the same: The 1,2-addition product is the kinetic product, and the 1,4-addition product is the thermodynamic product.
“The kinetic product is the product that is formed most rapidly” For a reaction in which the kinetic and thermodynamic products are not the same, the product that predominates depends on the conditions under which the reaction is carried out. If the reaction is carried out under sufficiently mild (low-temperature) conditions to cause the reaction to be irreversible, the major product will be the kinetic product. For example, when addition of HBr to 1,3-butadiene is carried out at -80 °C, the major product is the 1,2-addition product.
“The kinetic product predominates when the reaction is irreversible.” If, on the other hand, the reaction is carried out under sufficiently vigorous (high-temperature) conditions to cause the reaction to be reversible, the major product will be the thermodynamic product. When the same reaction is carried out at 45 °C, the major product is the 1,4-addition product. Thus, the 1,2-addition product is the kinetic product (it is formed more rapidly), and the 1,4-addition product is the thermodynamic product (it is the more stable product).
“The thermodynamic product predominates when the reaction is reversible.” A reaction coordinate diagram helps explain why different products predominate under different reaction conditions (Figure 8.2). The first step of the addition reaction—addition of a proton to C-1—is the same whether the 1,2-addition product or the 1,4-addition product is being formed. It is the second step of the reaction that determines whether the nucleophile (Br^-) attacks C-2 or C-4. Because the 1,2-addition product is formed more rapidly, we know that the transition state for its formation is more stable than the transition state for formation of the 1,4-addition product. This is the first time we have seen a reaction in which the less stable product has the more stable transition state! Figure 8.2 Reaction coordinate diagram for the addition of HBr to 1,3-butadiene.

At low temperatures (-80 °C), there is enough energy for the reactants to overcome the energy barrier for the first step of the reaction and therefore form the intermediate, and there is enough energy for the intermediate to form the two addition products. However, there is not enough energy for the reverse reaction to occur: The products cannot overcome the large energy barriers separating them from the intermediate. Consequently, at -80 °C, the relative amounts of the two products obtained reflect the relative energy barriers to the second step of the reaction. The energy barrier to formation of the 1,2-addition product is lower than the energy barrier to formation of the 1,4-addition product, so the major product is the 1,2-addition product.
In contrast, at 45 °C, there is enough energy for one or more of the products to go back to the intermediate. The intermediate is called a common intermediate because it is an intermediate that both products have in common. The ability to return to a common intermediate allows the products to interconvert. Because the products can interconvert, the relative amounts of the two products at equilibrium depend on their relative stabilities. The thermodynamic product reverses less readily because it has a higher energy barrier to the common intermediate, so it gradually comes to predominate in the product mixture.
Thus, when a reaction is irreversible under the conditions employed in the experiment, it is said to be under kinetic control. When a reaction is under kinetic control, the relative amounts of the products depend on the rates at which they are formed. A reaction is said to be under thermodynamic control when there is sufficient energy to allow it to be reversible. When a reaction is under thermodynamic control, the relative amounts of the products depend on their stabilities. Because a reaction must be reversible to be under thermodynamic control, thermodynamic control is also called equilibrium control. For each reaction that is irreversible under mild conditions and reversible under more vigorous conditions, there is a temperature at which the changeover happens. The temperature at which a reaction changes from being kinetically controlled to being thermodynamically controlled depends on the reactants involved in the reaction. For example, the reaction of 1,3-butadiene with HCl remains under kinetic control at 45 °C even though addition of HBr to 1,3-butadiene is under thermodynamic control at that temperature. Because a C – Cl bond is stronger than a C – Br bond (Table 3.1), a higher temperature is required for the products to undergo the reverse reaction. (Remember, thermodynamic control is achieved only when there is sufficient energy to allow one or both of the reactions to be reversible.)
It is easy to understand why the 1,4-addition product is the thermodynamic product. We saw in Section 4.11 that the relative stability of an alkene is determined by the number of alkyl groups bonded to its sp² carbons: The greater the number of alkyl groups, the more stable is the alkene. The two products formed from the reaction of 1,3-butadiene with one equivalent of HBr have different stabilities since the 1,2-addition product has one alkyl group bonded to its sp² carbons, whereas the 1,4-product has two alkyl groups bonded to its sp² carbons. The 1,4-addition product, therefore, is more stable than the 1,2-addition product. Thus, the 1,4-addition product is the thermodynamic product. Now we need to see why the 1,2-addition product is formed faster. In other words, why is the transition state for formation of the 1,2-addition product more stable than the transition state for formation of the 1,4-addition product? For many years, chemists thought it was because the transition state for formation of the 1,2-addition product resembles the contributing resonance structure in which the positive charge is on a secondary allylic carbon. In contrast, the transition state for formation of the 1,4-addition product resembles the contributing resonance structure in which the positive charge is on a less stable primary allylic carbon. However, when the reaction of 1,3-pentadiene + DCl is carried out under kinetic control, essentially the same relative amounts of 1,2- and 1,4-addition products are obtained as are obtained from the kinetically controlled reaction of 1,3-butadiene + HBr. The transition states for formation of the 1,2- and 1,4-addition products from 1,3-pentadiene should both have the same stability because both resemble a contributing resonance structure in which the positive charge is on a secondary allylic carbon. Why, then, is the 1,2-addition product still formed faster? When the π electrons of the diene abstract D⁺ from a molecule of undissociated DCl, the chloride ion can better stabilize the positive charge at C-2 than at C-4 simply because when the chloride ion is first produced, it is closer to C-2 than to C-4. So it is a proximity effect that causes the 1,2-addition product to be formed faster. A proximity effect is an effect caused by one species being close to another. Because the greater proximity of the nucleophile to C-2 contributes to the faster rate of formation of the 1,2-addition product, the 1,2-addition product is the kinetic product for essentially all conjugated dienes. Do not assume, however, that the 1,4-addition product is always the thermodynamic product. The structure of the conjugated diene is what ultimately determines the thermodynamic product. For example, the 1,2-addition product is both the kinetic product and the thermodynamic product in the reaction of 4-methyl-1,3-pentadiene with HBr, because not only is the 1,2-product formed faster, it is more stable than the 1,4-product. The 1,2- and 1,4-addition products obtained from the reaction of 2,4-hexadiene with HCl have the same stability—both have the same number of alkyl groups bonded to their sp² carbons. Thus, neither product is themodynamically controlled.