Curly arrows electrons

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... 

The S+ hydrogen atom seeks out the electron density in the double bond. The curly arrow El represents the movement of a pair of electrons. 

In 1925, Robinson first published his schematic "curly arrows," indicating electron drift or displacement, 124, defining nitrosobenzene as a crotenoid reactant. 

A single-headed curly arrow or fishhook arrow indicates the movement of a single electron. 

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. 

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

Curly arrows must start from an electron-rich species. This can be a negative charge, a lone pair, or a bond. 

Resonance stractures can be interconverted by the movement of electrons indicated by curly arrows. 

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. 

The proton thus contains no electrons. This seems a rather unnecessary statement, but it means a proton can only be an acceptor of electrons, and can never donate any. Curly arrows may be directed towards protons, but can never start from them This would be a serious mechanistic error. Nevertheless, most students seem to make this error at some time or other. 

In the formation of radicals, a bond is broken and each atom takes one electron from the pair constituting the bond. Bond-making and bond-breaking processes are indicated by single-headed (fishhook) curly arrows representing the movement of one electron. 

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. 

Mistakes with valencies As electrons are moved around via curly arrows, it is imperative to remember how many electrons are associated with a particular atom, and not to exceed the number of bonds permitted. The usual clanger is five-valent carbon, typically the result of making a new bond to a fully substituted carbon (four bonds, eight electrons) without breaking one of the old bonds. This is the case in the example shown. 

Arrows from protons Ask yourself how many electrons are there in a proton We trust the answer is none, and you will thus realize that arrows representing movement of electrons can never ever start from a proton. It seems that this mistake is usually made because, if one thinks of protonation as addition of a proton, it is tempting to show the proton being put on via an arrow. With curly arrows, we must always think in terms of electrons. 

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. 

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). 

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... 

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. 

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. 

---