Welcome. This video explains the symbols used to depict reaction mechanisms. For a chemical transformation to occur, many intermediate reactions are usually involved, called mechanistic steps. During these mechanistic steps, chemical bonds are broken and formed, generating intermediate structures or molecules along the way. The energy diagram or reaction coordinate diagram shows that a certain activation energy is required for each step. Reactions can be endothermic or exothermic overall. How many mechanistic steps are involved in this reaction? Six mechanistic steps are involved. You can figure this out by counting the number of transition states in the energy diagram or by counting the number of intermediates. There's one step to generate each of the four intermediates and add one for the step that generates the product. Chemists analyze many possible mechanisms and conduct experiments to eliminate impossible mechanisms and identify reasonable ones. In your courses and in later modules, you will learn how chemists do this and learn to predict plausible mechanisms yourself. In this module, we focus on the symbolism used in these mechanisms, such as the electron pushing arrows. Chemists use electron pushing arrows as a language to depict bond breaking and forming processes. These arrows, which are also called curved arrows, are used to explain, analyze and predict chemical reactions. A double-headed arrow means that two electrons are involved, while a single-headed arrow, also called a fish-hook arrow, means that one electron is involved. There is a set of reactions that represent common mechanistic steps. What do the curved arrows depict? What components of the starting material are conserved or still found in the product? The curved arrows depict electron movement. That's why they're also called electron pushing arrows. They always start from electrons and point to an atom or bond. This is a key principle that we will revisit frequently. For example, this red arrow shows the carbon-oxygen pi bond breaking. The two electrons from that bond go to the oxygen atom. Formal charges can be determined in a couple of ways. We can go back to the original formal charge formula. The formal charge on the oxygen on the right is equal to 6, minus 1, minus 6, or negative 1. The analogous calculation for the carbon atom gives a formal charge of plus 1. We can also look at whether an atom has gained or lost electrons to determine the formal charge. Gaining electrons makes the atom more negative. Losing electrons makes it more positive. In this example, the oxygen started out neutral and gained electrons, so it became negative. The carbon started out neutral and lost electrons, so it became positive. The second key principle is that atoms, electrons, and charges are conserved between starting materials and products. We have three carbon atoms and one oxygen atom in both the starting structure and the final one. There is the same number of electrons on both sides, and there is the same overall charge on both sides. In this example, the arrow describes the carbon-bromine bond breaking and the two electrons going to bromine. Have the principles of the reaction been respected? How many of each atom type is present in the starting material and products? How many electrons? What is the overall charge? The principles of the reaction have been respected. There is the same number of atoms, electrons, and charges in the starting materials and products. The previous two examples involve breaking bonds. Electron pushing arrows are also used to show bond formation. In this third example, an electron pair on oxygen forms a bond between the oxygen and carbon atoms. What is the charge on the oxygen and the indicated carbon atoms in the products? The oxygen and carbon atoms are both neutral now. Notice that the principle of conservation of atoms, electrons, and charges has been respected. In this fourth and final example, one bond forms and another bond breaks simultaneously. This is also called a concerted reaction. The left hand arrow shows a new bond forming between the nitrogen and carbon atoms. The right hand arrow shows the carbon-oxygen bond breaking with the electrons moving to the oxygen atom. There is a new nitrogen-carbon bond. The electrons from the carbon-oxygen bond are now in the oxygen. We also double-check that the principle of conservation has been respected. Yes, they are the same number of atoms, electrons, and charges on both sides. This example illustrates key principle number three. Electrons from a bond stay with one of that bond's atoms. The electrons from the carbon-oxygen bond stayed with the oxygen atom. One of the atoms from that bond. In this video, we saw that electron-pushing arrows are used to depict the movement of electrons in a reaction mechanism. Key principles are respected in every mechanistic step. The arrows start from electrons and point to an atom or bond. Atoms, electrons, and charges are conserved between starting materials and products. Electrons from a bond stay with one of that bond's atoms. Now that you've watched this introductory video, you're ready to look more closely at each of this module's intended learning outcomes. Enjoy!