Temperature plays a crucial role in determining reaction outcomes. At lower temps, kinetic control favors products that form fastest, while higher temps allow thermodynamic control, favoring more stable products. This balance shapes the products we see in organic reactions.
Understanding kinetic vs thermodynamic control is key to predicting and manipulating reaction outcomes. By tweaking conditions like temperature, we can steer reactions towards desired products, whether they're the fastest-forming or most stable options.
Kinetic versus Thermodynamic Control of Reactions
Temperature effects on diene addition products
- Temperature influences relative rates of competing reaction pathways
- Lower temperatures favor kinetic product due to lower activation energy barrier and lower transition state energy (e.g. 1,2-addition in conjugated dienes)
- Higher temperatures favor thermodynamic product which is more stable with lower overall Gibbs free energy ($\Delta G$) and system has enough energy to overcome higher activation energy barrier (e.g. 1,4-addition in conjugated dienes)
- Conjugated dienes undergo 1,2-addition or 1,4-addition
- 1,2-addition typically kinetic product forming less stable allyl carbocation intermediate (e.g. 1,2-addition of HBr to 1,3-butadiene)
- 1,4-addition typically thermodynamic product forming more stable alkene product with extended conjugation (e.g. 1,4-addition of HBr to 1,3-butadiene)
Kinetic vs thermodynamic control
- Kinetic control favors product that forms fastest determined by relative activation energies ($E_a$) of competing reaction pathways with lowest $E_a$ favored and kinetic products often less stable than thermodynamic products (e.g. SN1 reaction of tertiary alkyl halides)
- Thermodynamic control favors most stable product determined by relative Gibbs free energies ($\Delta G$) of products with lowest $\Delta G$ favored and thermodynamic products often more stable than kinetic products (e.g. SN2 reaction of primary alkyl halides)
- Interconversion between kinetic and thermodynamic products possible if activation energy barrier for interconversion is low allowing kinetic product to convert to thermodynamic product over time through equilibration (e.g. cis-trans isomerization of alkenes)
Product ratios in diene reactions
- Low temperature conditions favor kinetic control
- 1,2-addition product (kinetic product) formed in higher proportion
- Ratio of 1,2-addition to 1,4-addition products higher (e.g. 80:20 ratio of 1,2 to 1,4 addition of HBr to 1,3-butadiene at 0 ℃)
- High temperature conditions favor thermodynamic control
- 1,4-addition product (thermodynamic product) formed in higher proportion
- Ratio of 1,4-addition to 1,2-addition products higher (e.g. 20:80 ratio of 1,2 to 1,4 addition of HBr to 1,3-butadiene at 40 ℃)
- Curtin-Hammett principle applies when interconversion between kinetic and thermodynamic products is fast relative to formation of products
- Product ratio determined by difference in transition state energies leading to each product
- Relative stability of products does not affect product ratio under Curtin-Hammett conditions (e.g. addition of HCl to 3,3-dimethyl-1-butene)
Reaction Energy Profile Analysis
- Reaction coordinate diagram visually represents energy changes during a reaction
- Shows relative energies of reactants, products, and transition states
- Helps identify the rate-determining step, which is typically the step with the highest activation energy
- Hammond postulate relates transition state structure to nearby stable species
- For exothermic reactions, transition state resembles reactants
- For endothermic reactions, transition state resembles products