Curly arrow mechanism

It makes good sense to draw free-radical mechanisms in the manner shown by these examples. However, shorter versions may be encountered in which not all of the arrows are actually drawn. These versions bear considerable similarity to two-electron curly arrow mechanisms, in that a fishhook arrow is shown attacking an atom, and a second fishhook arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type of representation is quite clear if the complement of electrons around a particular atom is counted each time but, if in any doubt, use all the necessary fishhook arrows. 

Drawing curly arrow mechanisms is a bit like riding a bike. Before you ve mastered the skill, you keep falling off. Once you ve mastered the skill, it seems so straightforward that you wonder how you ever did without it. You still come across busy streets and complex traffic junctions, but the basic skill remains the same. 

The OH group is said to be ortho, para-directing towards electrophiles. No substimtion occurs in either meta position. We can understand this by looking at the curly arrow mechanisms or by looking at the molecular orbitals. In Chapter 21 (p. 000) we looked at the it system of an enolate and saw how the electron density is located mainly on the end atoms (the oxygen and the carbon). In phenol it is the ortho and para positions that are electron-rich (and, of course, the oxygen itselO- We could show this using curly arrows. 

Curly arrow mechanism The movement of electrons, usually in pairs, is indicated by curly arrows to indicate which bonds are being formed and which are being broken within the reacting molecules. 

Benzene is symmetrical and the structure with a circle in the middle best represents this. However, it is impossible to draw curly arrow mechanisms using this representation so we shall usually make use of the Kekule form with three double bonds. This does not mean that we think the double bonds are localized It makes no difference which Kekule structure you draw— any mechanism can be equally well drawn using either. 

Drawing curly arrow mechanisms is discussed in Section 4.11... 

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. 

These simple examples illustrate the basic rules for mechanism and the use of curly arrows. The concepts are no different from those we have elaborated for drawing resonance structures (see Section 2.10) ... 

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. 

In this example, the student remembered that a series of curly arrows was required, and they are generally in the right places, but not coming from electron-rich species, and not flowing in the right direction. This is typical of trying to remember a mechanism, which then fails to obey the general rules. 

Too many steps at once it is tempting to draw a mechanism with a series of curly arrows leading to the product via the minimum number of structures. We can often use several curly arrows in the same structure, but only provided we do not destroy the rationale for the mechanism. 

A shorthand addition-elimination mechanism sometimes encountered is also shown. This employs a double-headed curly arrow to indicate the flow of electrons to and from the carbonyl oxygen we prefer and shall use the longer two-step mechanism to emphasize the addition intermediate. 

Note that if we choose not to put in all the curly arrows, we could write the mechanism in two ways either considering the radical as the attacking species or the double bond as the electron-rich species. The first version is perhaps more commonly used, but it is much more instmctive to compare the second one with an electrophilic addition mechanism (see Section 8.1). The rationalization for the regiochemistry of addition parallels that of carbocation stability (see Section 8.2). 

Currently accepted mechanisms. We have tried to be complete in showing steps, intermediates and the necessary curly arrow notations. 

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... 

Fig. Use of half curly arrows in a mechanism (homolytic cleavage).

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. 

Drawing your own mechanisms with curly arrows... 

---