Concerted response
A concerted reaction is an elementary chemical reaction in the course of which chemical bonds are both broken and formed. The fact that this is an elementary reaction means that no intermediate products are formed in the course of the reaction and, accordingly, only one transition state occurs. So only one energy barrier has to be overcome to form the product.
Theoretical description
In theoretical chemistry, every reaction can be described by a trajectory on the potential energy surface. The potential hypersurface describes the energy of all conceivable bond states of the totality of all atoms involved in the reaction. The trajectory describes how the bonding conditions change in the course of the reaction. Since the trajectory represents a one-dimensional curve, different binding states on the trajectory can be described by a single coordinate - the reaction coordinate . The plot of the binding energy against the reaction coordinate provides an energy diagram. Compounds that can be theoretically isolated are distinguished by the fact that their bond states are assigned a local energy minimum. The conversion of two compounds into one another takes place via a local energy maximum, the bond state of which is called a transition state. The difference between the energy of the transition state and the starting compound is called the activation energy .
An energy diagram typical for a concerted reaction can be seen in the right part of the adjacent figure. The energy diagram of the concerted reaction shows exactly one transition state (local energy maximum). It follows from this that no energy minima and thus no isolable intermediate products occur in the course of the reaction.
In the left part of the figure, a two-step reaction is shown for comparison. There a transition state (local maximum) is first passed through, whereby an intermediate product (local minimum) is formed. This reacts in a second step via a second transition state to the product.
Examples
Basic reactions in organic chemistry that take place in concert are nucleophilic substitution according to the S N 2 mechanism and elimination according to the E2 mechanism.
A common form of concerted reactions are pericyclic reactions , the course of which can be predicted by the Woodward-Hoffmann rules . Probably the best known example is the Diels-Alder reaction :
The pericyclic reactions also include sigmatropic rearrangement reactions, such as the Cope and Claisen rearrangements .
literature
- Ian Fleming: Frontier Orbitals and Reactions of Organic Compounds . Wiley-VCH, Weinheim 1988, ISBN 3-527-25792-6 .