In this lesson, we will learn:
- To recall the kinetic factors that influence chemical reactions.
- To understand the influence of orbital and charge interactions in driving chemical processes.
- In kinetics, we introduced the idea of a reaction mechanism, talked about what is required for a chemical reaction and why they happen:
- Two conditions must be met when molecules collide:
Most collisions don’t lead to a reaction because meeting both of these conditions at the same time is unlikely.
- They must have sufficient energy - the activation energy.
- They must have the correct arrangement related to each other.
- Reactions generally have a ‘driving force’ that ‘pushes’ reactants into being products. For example, a weak bond(s) in the reactants breaking so that a strong bond in the products can form is a driving force.
A driving force can be a charge interaction, like ions (Na+ with Cl-), partial charges (+ and - caused by different atoms’ electronegativity) or both.
- Many reactions in organic chemistry don’t involve any ions with formal + or – charges, and some don’t even involve partial charges (like + on carbon and a lone pair). A lot of organic reactions involve orbital interactions:
An example is addition to ethene (C2H4) reacting with bromine (Br2). Both carbon atoms in ethene are neutral and there are no partial charges, but there is overlap between the ethene pi MOs and antibonding sigma MO of bromine for the pi electrons to move into.
- Because electron pairs repel each other, filled molecular orbitals (MOs) of different molecules repel each other. This contributes to activation energy; molecules have to have enough energy to overcome this electron repulsion between filled molecular orbitals.
- Interactions between occupied MOs in one molecule and an empty MO in another are attractive in nature. This is especially true of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These are called frontier molecular orbital (FMO) interactions which will be looked at in more detail later.
See the diagram below:
- Most organic reactions have a mix of charge and orbital interaction. A reaction involves bond(s) breaking and bond(s) forming and covalent bonds are just electron pairs. Reactions, then, involve electrons moving from one place to another. Electrons are more likely to do this if there is an empty orbital they can interact with and a full or partial positive charge to interact with.