In organic chemistry, many students learn about 1,2 versus 1,4 addition in their second semester. This addition happens with conjugated dienes (double single double bond). When a conjugated diene is reacted with HX (where X is the halogen), under different temperatures, it produces different products. Under low temperature, the reaction is said to be under kinetic control. In the first step, the double bond attacks hydrogen, creating an allylic carbocation (resonance stabilized). Since the temperature is low, there is not enough time for the X to go around the molecule. Therefore, X attaches to the carbocation right away, and we call it 1,2 addition.
Under thermodynamic control, which happens at high temperatures, there is enough energy for extra changes to happen. Thus, while the first step is the same, attack of hydrogen and creation of carbocation, it is then followed by resonance. The resonance leads to another carbocation to which the halogen can attach. Thermodynamic control creates the most stable product. How do we know when the product is most stable? It is the one with the most substituted double bond. Let's note that thermodynamic control does not always result in 1,4 product but is often 1,2. Why? Again we must always check the substitution of the double bond. In some cases the 1,2 mechanism results in the most substituted double bond and is therefore, both kinetic and thermodynamic product. Therefore, the student must be very careful the showing the thermodynamic product.
Furthermore, when two bond are not the same and we are not sure which one to break first, we must try both pathways with resonances. We then can check what kind of carbocations both pathways produce. The one with the most stable carbocation is the one we will choose for both kinetic and thermodynamic product.
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