What is the major product of the reaction of 2-methylbuta-1,3-diene with HBr?
The regioselectivity of the overall reaction depends on the temperature.  The carbocation intermediate can pass through the two different transition states  that lead to the 1,2- and 1,4- products, respectively. If we choose a temperature low enough, then the product distribution will reflect the difference in energy between the two activation energies Ea (1,2) and Ea (1,4). So long as the reaction is not reversible, the product with the lower energy transition state will dominate which is decided by the stability of the intermediate, so in that case 3-bromo-3-methylbut-1-ene (product-1) will be more stable product.
At higher temperatures, the reaction has the potential to be reversible, this means that both the 1,2- and 1,4- products can ionize This sets up an equilibrium. The product ratio will now reflect the relative stabilities of the 1,2- and 1,4- products, not the transition states leading to their formation. In the case of 2-methylbuta-1,3-diene, the product 1-bromo-3-methylbut-2-ene (product-2) will be more stable and dominant at higher temperatures.