Let's take a look at a real example

Vinyl Allyl Ether4-Pentenal

If we didn't have a computer or quantum mechanics what would we do to design chemical reaction systems?

• As a first option, head to the library to see whether any published data is available for this reaction; however, this is not available for many reaction systems.
• Head to the laboratory to do a bunch of experiments
  • Use a calorimeter to calculate the heat of formation, heat of reaction, and heat capacity
  • Use a bench-scale differential reactor or batch reactor to gain data such that the activation energy and preexponential factor could be calculated
• Spend a lot of time and money on experiments, remember that "six weeks in the lab may save you a hour in the library"-George Quanderer Dow Chemical.

Because we do have computers and quantum mechanics, we will proceed like this:

• Start to think about the reaction on a molecular level
• Try to postulate a logical mechanism for the reaction
• Use a computational chemistry software package such as Cerius2, Spartan, or Gaussian to calculate thermodynamic properties of reactants, transition state, and products
• Use these properties to calculate the activation energy and the preexponential factor

Presently:

Computational chemistry has not yet advanced to the point where large-scale plants can be designed solely on a theoretical basis. However, the calculated quantities can help you:

1. Decide if a reaction system is even feasible for further development, before you spend a lot of money and time doing experiments
2. Determine whether experimental results (which aren't always correct and reproducible!) are consistent with what you expect for the system at hand
3. Help identify an improved process (e.g., a new catalyst)

You will now begin thinking on a molecular level

What do you think the transition state would look like?

If we take a close look at the reaction above we can see that three things happen:

• A carbon-oxygen bond is broken
• A carbon-carbon bond is formed
• A carbon-oxygen double bond is formed

Since we start and end with the same atoms in the molecule the reaction must be intramolecular. So if we just bring the two free ends of the molecule together and "push" some electrons around like this:

Figure 3.

The arrows that you see in figure 3 represent the movement of the electrons as the molecule transitions from the reactant to the transition state.

And thus the transition state would be:

Figure 4.

In figure 4 the dashed lines represent the bonds being formed and broken in the transition state.