14.3 Kinetic versus Thermodynamic Control of Reactions

Kinetic versus Thermodynamic Control of Reactions

Many organic reactions can produce more than one product. Which product you actually get depends heavily on reaction conditions, especially temperature. Kinetic control favors the product that forms fastest, while thermodynamic control favors the product that's most stable. Understanding this distinction lets you predict and manipulate which product dominates just by changing conditions.

Temperature Effects on Diene Addition Products

Conjugated dienes like 1,3-butadiene can react with electrophiles (such as HBr) through two competing pathways: 1,2-addition and 1,4-addition. Temperature determines which pathway wins.

1,2-addition places the new groups on adjacent carbons (carbons 1 and 2). This is typically the kinetic product because it forms through the shorter, more direct pathway with a lower activation energy ($E_a$). However, the resulting product retains only an isolated double bond.

1,4-addition places the new groups at the ends of the diene system (carbons 1 and 4). This is typically the thermodynamic product because the resulting internal alkene is more substituted and more stable, giving it a lower Gibbs free energy ($\Delta G$).

• At low temperatures, molecules don't have enough energy to overcome the higher $E_a$ barrier leading to the 1,4-product. The reaction gets funneled through the lower-barrier pathway, so 1,2-addition dominates.
• At high temperatures, molecules have enough thermal energy to cross both barriers. The reaction can reach equilibrium, and the more stable 1,4-product accumulates.

Kinetic vs. Thermodynamic Control

These two modes of control come down to different questions:

• Kinetic control asks: which product forms fastest? The answer depends on the relative activation energies ($E_a$) of competing pathways. The pathway with the lowest $E_a$ wins. Kinetic products are often less stable overall.
• Thermodynamic control asks: which product is most stable? The answer depends on the relative Gibbs free energies ($\Delta G$) of the products. The product with the lowest $\Delta G$ wins.

A reaction is under kinetic control when products form irreversibly, or when there isn't enough energy for the reverse reaction to occur. Thermodynamic control takes over when the reaction is reversible and the system can equilibrate. Given enough time and energy, the kinetic product can convert into the thermodynamic product through this equilibration. A classic example is cis-trans isomerization of alkenes, where the less stable cis isomer (kinetic product) converts to the more stable trans isomer (thermodynamic product) at elevated temperatures.

Product Ratios in Diene Reactions

The ratio of 1,2- to 1,4-addition products shifts dramatically with temperature. Using HBr addition to 1,3-butadiene as the standard example:

• At ~0 °C (kinetic control): roughly 80:20 ratio of 1,2- to 1,4-addition product. The low thermal energy traps molecules in the kinetic product before they can equilibrate.
• At ~40 °C (thermodynamic control): roughly 20:80 ratio of 1,2- to 1,4-addition product. Higher thermal energy allows equilibration, and the more stable 1,4-product accumulates.

Notice that the same reactants give opposite major products just by changing temperature. That's the practical power of kinetic vs. thermodynamic control.

The Curtin-Hammett Principle applies in a specific scenario: when interconversion between intermediates (or products) is fast relative to the rate of product formation. Under these conditions:

• The product ratio is determined by the difference in transition state energies leading to each product, not by the relative stability of the intermediates themselves.
• This means you can't predict the major product just by looking at which intermediate is more stable. You need to compare the barriers going forward from each intermediate.

Reaction Energy Profile Analysis

A reaction coordinate diagram plots free energy on the y-axis against reaction progress on the x-axis. For kinetic vs. thermodynamic control, the key features to identify are:

• The kinetic product sits behind the lower activation energy barrier (smaller energy hill to climb from reactants).
• The thermodynamic product sits at a lower overall energy level (deeper energy well), but the barrier to reach it is higher.
• The rate-determining step is the step with the highest activation energy barrier on the diagram.

The Hammond postulate connects transition state structure to the stability of nearby species on the energy diagram:

• In an exothermic step (energy going downhill), the transition state is early and resembles the reactants in structure.
• In an endothermic step (energy going uphill), the transition state is late and resembles the products in structure.

This postulate is useful because you can't observe transition states directly. If you know whether a step is exothermic or endothermic, you can reason about what the transition state looks like and use that to predict relative rates.