What's the big deal about mechanisms?

1. Definition

   Everything that involved in the transformation of starting material into product(s). Includes each step in which electron changes take place, as well as information about energy changes. The study of mechanisms can be divided into two areas: thermodynamics and kinetics.

2. Progress of reaction

   

   

   1. How many steps?
      1. one
      2. three for each product
   2. Reactive intermediates?

           a.  none (concerted reaction)

          

   3. Transition states (especially for RDS)

      

      

      There are also transition states for the formation of the two products in the second reaction. The formation of the substitution product has two further steps and the elimination product has one.

       

   4. Rates of reaction (activation energies - kinetics)

      

      

   5. Overall energy change (equilibria - thermodynamics)

      

      

      and an equivalent equilibrium constant for butene.

   6. Curved arrow convention

      

      b.     A separate mechanism is usually written to explain each product in the reaction.

      Exercise: Complete the mechanism for the second reaction, showing how each product is formed from the carbocation intermediate.

3. Energy diagrams
   1. One step

      

   2. Multistep

      

4. Determining reasonable mechanisms
   1. Experimentally
      1. Measuring rates
      2. Orders of reactions

         Sum of the exponents of the concentration terms in the experimentally determined rate law

      3. Molecularity of a reaction

         Number of molecules involved in the rate-determining step

   2. By analogy
      1. Functional group chemistry
   3. Example

      

5. Writing rate laws
   1. Each step in a mechanism has a rate expression = change in concentration with time.
   2. Rate µ disappearance of starting material in a step with time µ appearance of product or intermediate in that step with time.

      

   3. Overall experimental rate is the rate for the rate-determining step.
   4. Concentrations in the rate-determining step need to be related to the concentrations of the actual starting materials.

      

6. Energy diagram