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. Evaluate the changes in bonds between the reactants and products.
   
   
   • 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. Examine the changes in formal charges between the reactants and products.
   
   • The bottom most carbon is neutral in the reactant, while its positive in the product.  We don't know what the exact charge is in the transition state, but we know it is somewhere between 0 and +1.  So we assign a partial + (i.e. δ⁺) to it.
   • The Br atom goes from neutral to -1 charge.  Again we don't know what the exact charge is but we know its between 0 and -1, so we assign a partial - charge (i.e. δ^-)