Types of Mechanism - Mechanisms and Methods of Determining Them - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part I. Introduction

Chapter 6. Mechanisms and Methods of Determining Them

A mechanism is the actual process by which a reaction takes place: which bonds are broken, in what order, how many steps are involved, the relative rate of each step, and so on. In order to state a mechanism completely, the positions of all atoms should be specified, including those in solvent molecules, and the energy of the system, at every point in the process. A proposed mechanism must fit all the facts available. It is always subject to change as new facts are discovered. The usual course is that the gross features of a mechanism are the first to be known and then increasing attention is paid to finer details. The tendency is always to probe more deeply, to get more detailed descriptions.

Although for most reactions gross mechanisms can be written today with a good degree of assurance, no mechanism is known completely.1 There is much about the fine details that is still puzzling, and for some reactions even the gross mechanism is not yet clear. The problems involved are difficult because there are so many variables. Many examples are known where reactions proceed by different mechanisms under different conditions. In some cases, there are several proposed mechanisms, each of which completely explains all the data.

6.A. Types of Mechanism

In most reactions of organic compounds, one or more covalent bonds are broken. Organic mechanisms may be divided into three basic types, depending on how the bonds break.

1. If a bond breaks in such a way that both electrons remain with one fragment, the mechanism is called heterolytic. Such reactions do not necessarily involve ionic intermediates, although they often do. The important thing is that the electrons are never unpaired. For most reactions, it is convenient to call one reactant the attacking reagent and the other the substrate. In this book, the substrate is always designated as that molecule that supplies carbon to the new bond. When carbon–carbon bonds are formed via heterolytic reactions, the reagent generally brings a pair of electrons to the substrate or takes a pair of electrons from it. A reagent that brings an electron pair is called a nucleophile and the reaction is nucleophilic. A reagent that takes an electron pair is called an electrophile and the reaction is electrophilic. For a reaction in which the substrate molecule becomes cleaved, part of it (the part not containing the carbon) is usually called the leaving group. A leaving group that carries away an electron pair is called a nucleofuge. If it comes away without the electron pair, it is called an electrofuge.

2. If a bond breaks in such a way that each fragment gets one electron, free radicals are formed and such reactions are said to take place by homolytic or free radical mechanisms.

3. It would seem that all bonds must break in one of the two ways previously noted. But there is a third type of mechanism in which electrons (usually six, but sometimes some other number) move in a closed ring. There are no intermediates, ions or free radicals, and it is impossible to say whether the electrons are paired or unpaired. Reactions with this type of mechanism are called pericyclic2 (see Reactions 15-58–15-61 and 18-29–18-33.

Examples of all three types of mechanisms are given in Section 6.B.