Final answer:
At low temperatures, 1,2-addition is favored in the reaction of conjugated dienes due to kinetic control, which results in products formed from the addition at the 1 and 2 positions. As temperature increases, there's a shift to the 1,4-addition favored by thermodynamic control, resulting in more stable products.
Step-by-step explanation:
When considering 1,2 vs 1,4-addition at low temperatures, we're discussing a type of chemical reaction typically seen in organic chemistry involving conjugated dienes and electrophilic reagents. These reactions can lead to different product outcomes based on the temperature and other reaction conditions. At low temperatures, 1,2-addition is usually favored due to kinetic control which directs the reaction towards the formation of the more stable carbocation intermediate, resulting in products formed via the addition of substituents at the 1 and 2 positions of the diene.
However, as the temperature increases, there's a shift towards thermodynamic control favoring 1,4-addition. This is because the products of 1,4-addition are generally more stable due to a more substituted and thus more stable alkene being formed, despite the activation energy being greater for this pathway. Increases in temperature provide the necessary energy to overcome this barrier. So, at lower temperatures, the reaction proceeds at a slower rate and is more selective for the pathway with the lower activation energy barrier - the 1,2-addition.
It is also notable that the entropy change (ΔS) at lower temperatures for these reactions is minimal in comparison to the enthalpy-driven aspects, which is why reactions are kinetically controlled and favor the 1,2-addition pathway. As the reaction mixtures are exposed to higher temperatures, they reach a point where there is a dynamic equilibrium between reactants and products, and the amount of each can become roughly equal, but this occurs above the low-temperature range we're focusing on here.