Reaction mechanisms curly arrows

Part Z The Mechanism of Substitution and Part 3 Elimination and Addition Pathways and Products are concerned with organic reaction mechanisms. Curly arrows are introduced and the key features of the two common mechanisms of nucleophilic substitution are reviewed. Including kinetic features, stereochemical outcome and reaction coordinate diagrams. This leads to a discussion of the features of El and E2 elimination reactions. The book finishes with a discussion of the factors that affect the competition between substitution and elimination reactions. Much of the teaching of substitution mechanisms Is carried out via interactive CD-ROM activities. 

Predict the products of the following elimination reactions. Use curly arrows to illustrate the reaction mechanisms. What is the stereochemistry of the product... 

Further symbols are used to indieate reaction mechanisms, in particular the use of curly arrows (to represent the movement of pairs of electrons) and fish-hooks (to represent the movement of single eleetrons). Students need to understand the precise meaning of these arrows (whieh electrons move, and where from and where to) to appreeiate how they represent stages in reaetion mechanisms. Students who have been taught the formalism are not neeessarily able to identify the outcome of... 

Here, we have used single-headed curly arrows or fishhook arrows to indicate. the movement of single electrons. The tail of the curly arrow shows the source of the electron and the head shows its destination. Using single-headed curly arrows, the mechanism for the methane/chlorine chain reaction is completed below. 

We have now encountered a number of different types of arrow routinely used in chemistry to convey particular meanings. We have met curly arrows used in mechanisms, double-headed resonance arrows, equilibrium arrows, and the simple single arrows used for reactions. This is a convenient point to bring together the different types and provide a checklist for future reference. We are also showing how additional information about a reaction may be presented with the arrow. 

Experience tells us that whilst many students find mechanisms easy and logical, others despair and are completely bewildered. We cannot guarantee success for all, but we hope that by showing a few of the common mistakes we may help some of the latter group join the former. In order to make the examples chosen as real as possible, these have all been selected from students examination answers. The mechanisms relate to reactions we have yet to meet, but this is not important. At this stage, it is the manipulation of curly arrows that is under consideration. You may wish to return to this section later. 

By the end of the 1950s the main features of ionic and radical reactions were reasonably well understood, but pericyclic reactions were not even recognized as a separate class. Diels-Alder reactions, and a good many others, were known individually. Curly arrows were used to show where the bonds went to in these reactions, but the absence of a sense of direction to the arrows was unsettling. Doering provocatively called them no-mechanism reactions in the early 1960s. 

First, a reaction has to be characterised, i.e. identities and yields of products must be determined, and these aspects are covered generally in Chapter 2. Once these are known, alternative possible paper mechanisms can be devised. Each may take the form of a sequence of linear chemical equations, or a composite reaction scheme replete, perhaps, with curly arrows . The next stage is to devise strategies for distinguishing between the alternatives with a view to identifying the correct one or, more realistically, eliminating the incorrect ones wherever possible, one seeks positive rather than negative evidence. 

To understand what happens to the valence electrons during a reaction mechanism there is a diagrammatic way making use of curly arrows. For example, the above mechanism can be... 

Most molecules are at peace with themselves. Bottles of water, or acetone (propanone, Me2C=0), or methyl iodide (iodomethane CH3I) can be stored for years without any change in the chemical composition of the molecules inside. Yet when we add chemical reagents, say, HC1 to water, sodium cyanide (NaCN) to acetone, or sodium hydroxide to methyl iodide, chemical reactions occur. This chapter is an introduction to the reactivity of organic molecules why they don t and why they do react how we can understand reactivity in terms of charges and orbitals and the movement of electrons how we can represent the detailed movement of electrons—the mechanism of the reaction— by a special device called the curly arrow. 

To understand organic chemistry you must be familiar with two languages. One, which we have concentrated on so far, is the structure and representation of molecules. The second is the description of the reaction mechanism in terms of curly arrows and that is what we are about to start. The first is static and the second dynamic. The creation of new molecules is the special concern of chemistry and an interest in the mechanism of chemical reactions is the special concern of organic chemistry. 

Organic chemists use curly arrows to represent reaction mechanisms... 

You have seen several examples of curly arrows so far and you may already have a general idea of what they mean. The representation of organic reaction mechanisms by this means is so important that we must now make quite sure that you do indeed understand exactly what is meant by a curly arrow, how to use it, and how to interpret mechanistic diagrams as well as structural diagrams. 

We shall not be discussing this reaction anywhere in the book We have included it just to convince you that, once you understand the principle of curly arrows, you can understand even very complicated mechanisms quite easily. At this stage we can summarize the things you have learned about interpreting a mechanism drawn by someone else. 

Another equally important reason for mastering curly arrows now, before you start the systematic study of different types of reactions, is that the vast number of different reactions turn out not to be so different after all. Most organic reactions are ionic they therefore all involve nucleophiles and electrophiles and two-electron arrows. There are relatively few types of organic electrophiles and nucleophiles and they are involved in all the different reactions. If you understand and can draw mechanisms, the similarity between seemingly unrelated reactions will become immediately apparent and thus the number of distinct reaction types is dramatically reduced. 

The curly arrows we used in this representation arc slightly different from the curly arrows we used (Chapter 5) to represent mechanisms by the forming and breaking of bonds. We still arrive at the second structure by supposing that the curly arrows mean the movement of two electrons so that the right-hand structure results from the reaction shown on the left-hand structure, hut these reactions would be the movement of electrons and nothing more. In particular, no atoms have moved and no o bonds have been formed or broken. These two structures are just two different ways of... 

Now, this four-membered ring (like many others) is unstable, and it can collapse in a way that forms two double bonds. Here are the curly arrows the mechanism is cyclic, and gives the alkene, which is the product of the reaction along with a phosphine oxide. 

The mechanism of the reaction that forms diazomethane is shown below. The key step is base-catalysed elimination, though the curly arrows we have to draw to represent this are rather tortuous ... 

The mechanism of this type of reaction depends on whether the carbene is a singlet or a triplet, and the outcome of the reaction can provide our first chemical test of the conclusions we came to in the previous section. Singlet carbenes, like this one here (remember that electron-rich substituents stabilize the singlet spin state), can add to alkenes in an entirely concerted manner the curly arrows for the process can be written to show this. 

---