thermodynamic versus kinetic control

Suppose a reaction can give two different products. Which one you get often depends on a tug-of-war between two questions: which product forms fastest, and which product is most stable? When a reaction can take more than one path, the answer can flip depending on the conditions, and this choice is called thermodynamic versus kinetic control.

Under kinetic control, the product that forms is simply the one made fastest, the one whose path has the lower activation barrier, regardless of whether it is the most stable. This tends to happen at low temperature and short reaction times, where molecules barely have enough energy to react at all and cannot go back. Under thermodynamic control, given enough energy and time the reactions become reversible, products can revert to reactants and try again, and the system eventually settles into the most stable product, the one at lowest energy, even if it formed more slowly. This tends to happen at higher temperature and longer times.

The classic illustration is the addition of HBr to a conjugated diene, where low temperature favors the faster-forming 1,2-product (kinetic) and high temperature favors the more stable 1,4-product (thermodynamic). The deep point is that the fastest product and the most stable product are not always the same molecule; the lower hill does not always lead to the lower valley. Choosing temperature and time lets a chemist deliberately select which one wins.