Mechanism Basics
Mechanism basics
Each organic reaction has a story. Bonds are broken and bonds are formed. Reactants become products. Organic chemists call this story the mechanism of a reaction. It is how a reaction proceeds. The mechanism helps organic chemists understand why and how the reaction happened. Mechanisms are very important in organic chemistry.
A mechanism is a step-by-step story of how a reaction proceeds. Sometimes, you will be given both the reactants and the products of a reaction and be asked to give the mechanism of how the reaction went. At other times, you may be asked to predict what will happen in a reaction given two or more reactants. This is usually the more difficult task.
To be able to tell the mechanism story, you need to know the conventions organic chemists have agreed upon. The agreement is that we will show where electrons move in a reaction. To show this movement of electrons, we use arrows. A regular two-headed arrow shows the movement of two negative electrons. Did you get that? That last sentence may be one of the most important statements I will teach you. Arrows show the movement of negative electrons. Arrows DO NOT show the movement of positive charges, atoms, or anything else you can think of. Arrows in organic reaction mechanisms show the movement of negative electrons. Students who fail to learn this are almost never successful in organic chemistry. If you don’t learn to focus on the movement of these electrons, organic chemistry will remain a mystery. It is my belief that successful organic chemistry students learn to think in terms of arrows.
There is a small subset of organic reactions called free radical reactions where only one electron is moving at a time. In these cases, we use an arrow with only one barb on its head, sometimes called a fishhook arrow.
For this chapter, we’ll focus on regular, two-headed arrows. Arrows in mechanisms always start at negative electrons and show you where they go. Every time I look at a mechanism, I first ask myself where the electrons are moving. And since electrons have a negative charge, they almost always move towards the most positive atom on another reactant.
2. Draw one arrow in each of the following boxes. Your arrow should ALWAYS begin at the negative electrons and travel towards a positive spot. You can curve your arrows a little bit if you want, but it doesn’t matter.
Of course, for real molecules, there are several atoms. In the following exercise, I’ve made several molecules. On each molecule, I’ve identified one most negative atom and one most positive atom.
3. Draw one arrow in each box showing the movement of the electrons from the negative charge on the molecule on the left to the positive charge on the molecule on the right.
Answers
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