Curly arrows reaction

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. 

As the name implies, ionic reactions involve the participation of charged entities, i.e. ions. Bondmaking and bond-breaking processes in ionic reactions are indicated by curly arrows that represent the movement of two electrons. The tail of the arrow indicates where the electrons are coming from, the arrowhead where they are going to. 

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 the example shown, the two curly arrows suggest a concerted interaction of three entities. This is improbable, and does not tell us why the reaction should actually take place. Using the longer sequence, we see that the acid catalyst activates the carbonyl group towards nucleophilic attack, and is later regenerated. 

Scheme 11.5 Reaction of 16 with singlet oxygen, and structure motif of the open ring within the cluster framework of 22 and 25. Curly arrows within the formulas 20 and 23 indicate the cluster opening reaction leading to the valence isomers 22 and 25.

Alternate double and single bonds are often used in drawing aromatic structures, although it is fully understood these form a closed loop (tc-system) of electrons. The reason is that these classical structures are used in the valence bond approach to molecular structure (as canonical forms), and they also permit the use of curly arrows to illustrate the course of reactions. 

Pross and Shaik, 1983). The conventional view which describes an SN2 reaction as a two-electron process in contrast to the electron-transfer SRN1 pathway (Bunnett, 1978 Komblum, 1975) is, at best, misleading. The traditional curly arrow picture for an SN2 reaction (78) implies that the nucleophile attacks with two electrons and that the leaving group leaves with two... 

Write down the reaction (with curly arrows) between acetone and hydroxide ion. 

In ionic reactions, curly arrows have two functions they identify where the electrons come from and where they are going, and hence identify which component is the nucleophile and which the electrophile simultaneously they identify which bonds are broken and which new bonds are made. If the tail of an arrow is from a bond, then that bond is breaking, and if the head of the arrow falls between two atoms it shows where the new bond is being made. 

Pericyclic reactions are the third distinct class. They have cyclic transition structures in which all bond-forming and bond-breaking takes place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. The curly arrows can be drawn in either direction—clockwise, as here, but equally well anti- clockwise. They could even be drawn with fishhook arrows, and would still... 

The curly arrows in a pericyclic reaction share the capacity that they have in ionic reactions to show which bonds are breaking and where new bonds are forming, but they do not show the direction of electron flow. 

In this they somewhat resemble the curly arrows used to show resonance. in benzene, where the arrows show where to draw the new bonds, and which ones not to draw in the canonical structure but in this case there is neither a sense of direction nor even an actual movement. The analogy between the resonance of benzene and the electron shift in the Diels-Alder reaction is not far fetched, but it is as well to be clear that one is a reaction, with starting materials and a product, and the other is not. 

Crudely, but adequately for now, we may state rule governing which cycloadditions can take place and which not. A thermal pericyclic cycloaddition is allowed if the total number of electrons involved can be expressed in the form (4n+2), where n is an integer. If the total number of electrons can be expressed in the form 4n it is forbidden. Another way of saying the same thing is that reactions with an odd number of curly arrows are allowed and those with an even number are forbidden. This rule needs to be qualified, as we shall see shortly, and in due course in Chapter 3 made more precise, along with the rules for all the other kinds of pericyclic reaction, in one all-encompassing rule. For now, we need to introduce the rule for photochemical pericyclic cycloadditions. 

Step 1. Draw the bare bones of the reaction 3.10, and put in the curly arrows for the forward and backward reactions. Remember that any substituents, even if they make the diene or dienophile unsymmetrical, do not fundamentally disturb the symmetry of the orbitals directly involved. 

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 envision how chemical reaction arises from differences in polarity it is argued [96] that, since unlike charges attract, electron-rich sites in the functional groups of one molecule react with the electron-poor sites in the functional groups of another molecule. Bonds are made when the electron-rich reagent donates a pair of electrons to the electron-poor reagent. The movement of bonding electron pairs is followed by the use of curly arrows. The formalism is illustrated by some chemical reactions ... 

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... 

Sometimes reactions take place that involve the movement of single electrons rather than pairs of electrons. Such reactions are called radical reactions. For example, a chlorine molecule can be split into two chlorine radicals on treatment with light. One of the original bonding electrons ends up on one chlorine radical and the second bonding electrons ends up on the other chlorine radical. The movement of these single electrons can be illustrated by using half curry arrows rather than full curly arrows ... 

Occasionally, we shall use other colours such as green, or even orange, yellow, or brown, to highlight points of secondary importance. This example is part of a reaction taken from Chapter 19 we want to show that a molecule of water (H20) is formed. The green atoms show where the water comes from. Notice black curly arrows and a new black bond. 

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. 

Since we are describing a dynamic process of electron movement from one molecule to another in this last reaction, it is natural to use some sort of arrow to represent the process. Organic chemists use a curved arrow (called a curly arrow ) to show what is going on. It is a simple and eloquent symbol for chemical reactions. 

The curly arrow shows the movement of a pair of electrons from nitrogen into the gap between nitrogen and boron to form a new G bond between those two atoms. This representation, what it means, and how it can be developed into a language of chemical reactions is our main concern in this chapter. 

Now we shall discuss a generalized example of a neutral nucleophile, Nu, with a lone pair donating its electrons to a cationic electrophile, E, with an empty orbital. Notice the difference between the curly arrow for electron movement and the straight reaction arrow. Notice also that the nucleophile has given away electrons so it has become positively charged and that the electrophile has accepted electrons so it has become neutral. 

When there are no lone pair electrons to supply high-energy nonbonding orbitals, the next best is the lower-energy filled n orbitals rather than the even lower-energy O bonds. Simple alkenes are weakly nucleophilic and react with strong electrophiles such as bromine. In Chapter 20 we shall see that the reaction starts by donation of the 7t electrons from the alkene into the o orbital of the bromine molecule (which breaks the Br-Br bond) shown here with a curly arrow. After more steps the dibromoalkane is formed but the molecules are attracted by overlap between the full 7t orbital and the empty o orbital. 

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

---