Chemical Thermodynamics and the Equilibrium State
Reactions of Alkenes
Reactions of Conjugated Species
|The same electrophiles that undergo electrophilic addition to alkenes, also will add to conjugated dienes, though forming a resonance stabilized allylic cation intermediate. The delocalization of positive charge in the cation intermediate leads to the possibility of two reaction pathways, direct addition (1,2 addition), which produces the Markovnikov adduct, or conjugate addition (1,4 addition).|
1,2 addition occurs faster than 1,4 addition (the transition state for anion capture is lower for 1,2 addition), but the 1,4 adduct is more stable (with internal double bond). At lower temperature (kinetic conditions) the 1,2 adduct is the favored product. However, higher temperature (thermodynamic conditions) allows equilibration between the possible products, so the more stable 1,4 adduct is favored.
Kinetic versus thermodynamic control in conjugate addition has been on the MCAT a number of times.