Constructing Transition States

We've shown a few transition states in the previous examples.  Note that you can not isolate (put in a bottle) a transition state structure.

Let's look at how we generate these structures.

In the previous example, we looked at the two-step electrophilic addition of H-X to alkenes.  Each step will have a unique transition state.

Let's look at the first step, i.e. the electrophilic addition of H-Br to propene to form a 2^o carbocation, as illustrated below.

A transition state is a structure intermediate between the reactant and product of an elementary mechanistic step.

To draw the transition state follow these simple steps.

1. Draw all the reactant molecules or atoms involved in the reaction. 
   
   
   
2. Examine the bonds that are formed or broken in these reactant molecules by comparing with the product molecules.
   
   
   • There is one C-H bond formed between the top most C atom and the H atom of the H-Br.
   • There are two bonds broken.  The pi bond of the C=C and the H-Br bond. 
     
     Draw all these as dashed bonds (i.e. a partial bond to indicate it is not completely formed or broken.
     
     
     
     
3. broken between 
   
   bonds and atoms that have not changed in going from the reactant to the product.