The terms “stepwise reaction” and “concerted reaction” are used in the field of organic chemistry to describe different mechanisms by which chemical reactions proceed. They are used to explain the pathways and processes that molecules undergo when they transform into new substances.
Stepwise reactions occur in multiple sequential steps, forming intermediates, while concerted reactions happen in a single coordinated step without intermediates.
This fundamental difference in their mechanisms underlines how stepwise reactions progress through distinct stages, while concerted reactions execute a rapid, unified rearrangement of molecules.
Stepwise vs. Concerted Reactions
| Stepwise Reactions | Concerted Reactions |
| Stepwise reactions involve a sequence of individual, distinct intermediate steps where molecules undergo changes in a step-by-step manner. | Concerted reactions occur in a single coordinated step where all the necessary bond-breaking and bond-forming processes happen simultaneously. |
| The intermediates are formed between each of the individual steps, contributing to the overall reaction pathway. | They do not involve the formation of intermediates; the reactants transform directly into products without any distinct intermediate stages. |
| Stepwise reactions involve the breaking and forming of specific bonds in each step of the sequence, leading to the final product. | Concerted reactions entail simultaneous bond changes across the reacting molecules, resulting in a rapid rearrangement without distinct bond-breaking or forming steps. |
| They generally have a slower rate due to the cumulative effect of multiple individual steps, each with its own energy barrier. | They often exhibit faster rates since the reaction occurs in a single step, requiring only one energy barrier to overcome. |
| Stepwise reactions have different transition states for each individual step, reflecting the energy profiles of the various intermediates. | Concerted reactions possess a single transition state corresponding to the coordinated rearrangement of all reacting molecules. |
| They involve multiple energy barriers between the successive steps, resulting in a higher overall activation energy for the reaction. | They have a single energy barrier associated with the coordinated transition, leading to a lower overall activation energy. |
| Stepwise reactions can be more complex due to the presence of distinct intermediates and varying mechanisms in each step. | Concerted reactions are generally less complex as they involve a single step and do not produce intermediates. |
| Sn1/Sn2 reactions in organic chemistry are typical examples of stepwise reactions, with multiple steps involving intermediate carbocations or transition states. | The Diels-Alder reaction in organic chemistry is a classic example of a concerted reaction, where a cyclic compound is formed through a single, concerted rearrangement. |
What are Stepwise Reactions?
Stepwise reactions are chemical reactions that occur through a sequence of individual, distinct intermediate steps, where molecules undergo changes in a step-by-step manner. Each step involves the breaking and forming of specific bonds, leading to the eventual transformation of reactants into products.
Examples:
1. SN1 Reaction (Substitution Nucleophilic Unimolecular)
In organic chemistry, an SN1 reaction involves a stepwise process where a leaving group departs, creating a carbocation intermediate. A nucleophile then attacks the carbocation to form the final product.
2. E1 Reaction (Elimination Unimolecular)
In this reaction, a stepwise mechanism leads to the formation of a carbocation intermediate by the removal of a leaving group. A base then removes a proton from a neighboring carbon atom, resulting in the formation of a double bond and the elimination of a molecule.
3. SN2 Reaction (Substitution Nucleophilic Bimolecular)
In this reaction, a nucleophile attacks a substrate while simultaneously displacing a leaving group. The reaction occurs in a single step, resulting in inversion of configuration at the chiral center.
4. Nucleophilic Addition to Carbonyl Compounds
When a nucleophile adds to a carbonyl group, such as in the reaction between an aldehyde/ketone and a nucleophile, the formation of a tetrahedral intermediate leads to the final product after proton transfer.
What are Concerted Reactions?
Concerted reactions, also known as pericyclic reactions, occur in a single, coordinated step where all necessary bond-breaking and bond-forming processes happen simultaneously. These reactions are characterized by their rapid nature and lack of intermediates.
Examples:
1. Diels-Alder Reaction
This is a classic example of a concerted reaction where a conjugated diene and a dienophile react to form a cyclic compound known as a cycloadduct. The reaction involves the simultaneous formation of two new sigma bonds in a concerted manner.
2. Cyclopropane Formation via Electrocyclic Reaction
In this example of an electrocyclic reaction, a conjugated molecule rearranges to form a cyclopropane ring. The reaction occurs through a single, concerted process where pi bonds are broken and sigma bonds are formed simultaneously.
3. E2 Reaction (Elimination Bimolecular)
This reaction involves the simultaneous removal of a leaving group and a proton from adjacent carbon atoms, forming a double bond. It is a concerted process that often occurs in one step.
4. Pericyclic Reactions – Sigmatropic Rearrangements
In sigmatropic rearrangements, atoms shift within a molecule in a concerted manner. An example is the [3,3]-sigmatropic rearrangement of allyl vinyl ethers, which leads to the formation of a new bond and the migration of substituents.
5. Bimolecular Nucleophilic Substitution (SN2-like)
Some reactions, while appearing concerted, can still involve a transition state with some degree of bond cleavage and formation. An example is a bimolecular nucleophilic substitution involving nucleophilic attack and simultaneous departure of a leaving group.
Key Differences Between Stepwise and Concerted Reactions
Similarities Between Stepwise and Concerted Reactions
- Both types of reactions result in the transformation of reactants into products, leading to chemical changes in the involved molecules.
- In both mechanisms, the atoms present in the reactants are conserved in the products, adhering to the law of conservation of mass.
- Both mechanisms involve transition states, which represent the highest energy points along the reaction pathway, regardless of whether the reaction is stepwise or concerted.
- Both types of reactions require activation energy to proceed, regardless of whether the reaction occurs in multiple steps or as a single concerted event.
- The rate at which both stepwise and concerted reactions occur depends on factors such as temperature, concentration, and activation energy.
- Both mechanisms can exhibit stereochemistry effects, resulting in the formation of stereoisomers or changes in molecular geometry.
- Both types of mechanisms can be found across various fields of chemistry and encompass a wide range of chemical transformations.
- Both types of reactions can be investigated using techniques like kinetic analysis, computational chemistry, and isotopic labeling to elucidate their mechanisms.
