Q7-20P - Organic Chemistry (Mcmurry)

Question

What about the second step in the electrophilic addition of HCl to an alkene—the reaction of chloride ion with the carbocation intermediate? Is this step exergonic or endergonic? Does the transition state for this second step resemble the reactant (carbocation) or product (alkyl chloride)? Make a rough drawing of what the transition-state structure might look like.

Step-by-Step Solution

Answer

Step-by-Step solution Step 1: Endergonic and Exergonic reaction

An exergonic reaction’s transition state structurally resembles its reactant. An endergonic reaction’s transition state structurally resembles the product.

Step 2: Identification of whether the second step in electrophilic addition is endergonic and exergonic

In the second step of the reaction of the electrophilic addition of HCl to an alkene, a chloride ion reacts with the carbocation intermediate to generate an alkyl halide. The blend of two positively charged species will be fast. The stable neutral species is the product and the energy is released in this reaction. So the second step is exergonic. 

According to the Hammond postulate, the transition state resembles the reactant, the carbocation, in this step.

The transition state can be given as:

Transition state of a particular reaction