The key difference between concerted and stepwise reactions is that concerted reactions are single-step reactions, whereas stepwise reactions are multistep reactions.
The terms concerted and stepwise reactions come under the field of physical chemistry where the rates of reactions are determined using the change in the reactant and product amounts with time. All chemical reactions we know can be categorized into two different groups as concerted reactions and stepwise reactions depending on the mechanism of the reaction.
CONTENTS
1. Overview and Key Difference
2. What are Concerted Reactions
3. What are Stepwise Reactions
4. Side by Side Comparison – Concerted vs Stepwise Reactions in Tabular Form
5. Summary
What are Concerted Reactions?
Concerted reactions are chemical reactions that involve only one step. That means; all single-step reactions fall under the category of concerted reactions. Therefore, bond breaking and bond-forming reactions occur simultaneously to form the products. In other words, there are no reactive intermediates or unstable high energy complexes formed during this type of chemical reactions.
Figure 01: An Example of a Concerted Reaction (contains an unstable intermediate)
Generally, concerted reactions do not depend on the solvent polarity, and the mechanism of the reaction is named as a concerted mechanism. Some common examples for this type of reactions include pericyclic reactions, SN2 reactions and some rearrangement reactions such as Claisen rearrangement.
What are Stepwise Reactions?
Stepwise reactions are multiple-step reactions. Therefore, there are on or more chemical intermediates in these reactions. These intermediates are usually highly reactive due to high energy which makes them unstable. Stepwise reactions typically contain two or more elementary reactions.
Figure 02: An Example of a Stepwise Reaction
In contrast to stepwise reactions, the bond breaking and bond-forming in the stepwise reactions occur in different steps (not in a single step). Therefore, the reactants cannot convert into products directly. The “overall reaction” in a stepwise reaction that gives the conversion of reactants into products. We can get the overall reaction by accurately balancing all the steps in the stepwise reaction. Nevertheless, there is one elementary reaction in a stepwise reaction that determines the rate of the reaction. It is the slowest reaction of the reaction series, and it may or may not have reactants or products involved (it may contain only intermediates of the reaction).
What is the Difference Between Concerted and Stepwise Reactions?
The terms concerted and stepwise reactions come under the field of physical chemistry where the rates of reactions are determined using the change in the reactant and product amounts with time. The key difference between concerted and stepwise reactions is that concerted reactions are single-step reactions, whereas stepwise reactions are multistep reactions. The rate-determining step of a concerted reaction is itself a concerted reaction. But, in stepwise reactions, the slowest elementary reaction is the rate-determining step.
Moreover, there are no reaction intermediates in concerted reactions because the reactants directly convert into the products. However, in stepwise reactions, there are one or more intermediate compounds which are highly unstable due to the high energy level of the compound. The overall reaction of a stepwise reaction gives the conversion of the reactants into products.
Below infographic provides more details on the difference between concerted and stepwise reactions.
Summary – Concerted vs Stepwise Reactions
All the chemical reactions we know can be categorized into two groups as concerted reactions and stepwise reactions. The key difference between concerted and stepwise reactions is that concerted reactions are single-step reactions, whereas stepwise reactions are multistep reactions.
Reference:
1. “Concerted Reaction.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
2. “Concerted Reaction.” Wikipedia, Wikimedia Foundation, 2 June 2017, Available here.
Image Courtesy:
1. “Sn2EtCl+bromide” By Smokefoot – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Oxymercuration Stepwise” By TheJackal at English Wikipedia – Transferred from en.wikipedia to Commons by Ronhjones using CommonsHelper (Public Domain) via Commons Wikimedia