SolutionsKinetics vs. Thermodynamics
1. Interpret the diagram
In an energy diagram, reactant A can give product D through a low barrier (B) or product E through a high barrier (C). E is more stable than D. Which product forms faster? Which is the more stable?
D forms faster because its activation energy is lower (kinetic control). E is the more stable (thermodynamic control).
2. What controls each case?
State whether these statements describe kinetic or thermodynamic control:
- The major product is the one that forms fastest.
- The major product is the one of lowest energy.
- Low temperature and short reaction time.
- High temperature, reversible reaction and long time.
- Kinetic.
- Thermodynamic.
- Favours kinetic control.
- Favours thermodynamic control.
3. Halogenation of methylbutane
In the halogenation of 2-methylbutane, why are the primary hydrogens like the 'shoplifting' and the tertiary one like the 'big robbery'?
The primary hydrogens are many (9) and accessible: statistically they are attacked easily, although they give a poorly stable radical. The tertiary one is unique and more hindered, but it gives the most stable radical. With an unselective reagent such as chlorine, the number of hydrogens matters a lot; with the more selective bromine, the stability of the radical dominates.