Reaction Mechanisms and Operational Species A reaction Mechanism describes the series of steps that the reactants take to form the products. Each step in a reaction mechanism is referred to as an elementary process.

The following reaction constitutes three elementary processes. Reactions consist of one or more elementary processes. The overall reaction is the sum of the elementary processes and the collection of these elementary processes describe the reaction mechanism.

Arrow Pushing… Organic chemists use arrows to indicate the flow of electrons in a mechanism. The importance of this formalism cannot be overstated. Things to remember when pushing electrons: Arrows indicate the movement of electrons NEVER the movement of atoms. Electrons (i.e. arrows) always flow from electrons rich species to electron poor species. That is, they flow from Lewis Bases (NUCLEOPHILES) to Lewis Acids (ELECTROPHILES). One in, one out. Don’t draw multiple arrows going to or from one atom. Curly arrows with two ‘barbs’ represent the movement of TWO electrons. The movement of a lone electron is represented with curly arrows that have one ‘barb. After pushing electrons, the species that you end up with must represent a reasonable Lewis structure.

Arrow Pushing Practice…

We can use a reaction profile diagram to follow the change in energy during an elementary process. The highest energy point on the reaction profile diagram occurs at the transition state for the reaction. A transition state is highly unstable and short lived and cannot be isolated or observed.

Each elementary process will have its own transition state but it is NOT shown in the reaction mechanism. In multi step mechanisms (i.e. more than one elementary process) there will be intermediates, the products of elementary processes. Intermediates are semi stable molecules or ions that often can be observed. For a spontaneous reaction, intermediates will never be more stable than the final product. Why? Label the intermediates for the following reaction mechanism:

A catalyst is a species that increases the rate of reaction by providing an alternative mechanism. Catalysts are neither created nor consumed during a reaction. In order to speed up the rate of reaction, a catalyst must speed up the rate determining step of the reaction. The rate determining step is the slowest elementary process. The rate determining step is the only step which affects the overall rate of the reaction – it is ‘bottleneck’ of the reaction mechanism. Which is the RDS?

Operational Species – Nucleophiles In reaction mechanisms, electrons flow from electron rich species to electron poor species. The electron donors are known as nucleophiles (nucleus loving) and the electron rich parts of them are the nucleophilic sites. Nucleophiles: donate electron pairs to form a new bond to any atom except hydrogen. are species with lone pairs or π bonds. Nucleophiles

Operational Species – Nucleophiles Trends in nucleophilicity: charged species vs neutral species basicity periodic trends

Operational Species – Electrophiles The electron acceptors are known as electrophiles (electron loving) and the electron poor parts of them are the electrophilic sites. Electrophiles are species with the ability to accept electrons. Therefore they tend to be species with: an incomplete octet a π bond that is easy to break, or a good leaving group Carbocations make particularly good electrophiles because they are by definition electron poor.

Operational Species – Electrophiles When evaluating whether a site is electrophilic or not, attack with a nucleophile and push electrons. If you can draw a reasonable Lewis structure, then it is most likely an electrophilic site. If not, you will have to look elsewhere: Electrophiles

Operational Species – Leaving Groups A leaving group is an atom or group of atoms which breaks away from a molecule during a reaction taking a pair of electrons with it. When discussing a leaving group, we refer to the molecule/ion after it has left. A good leaving group is stable after leaving (if not, it wouldn’t leave!). We often say that “a good leaving group is a weak base”. This is often measured by referencing the strength of its conjugate acid. Consider the following: R-Cl R-OH 2 + R-OH

Some common leaving groups Operational Species Generic MoleculeLeaving GroupConjugate AcidpK a of Conjugate Acid R-II - HI-11 R-BrBr - HBr-9 R-ClCl - HCl-7 R-OTsTsO - HOTs-6 R-OH 2 + H2OH2OH3O+H3O+