Arrows, curly

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... 

FIGURE 20.6 Phase images of ethylene-propylene-diene terpol3mier (EPDM) samples at different scales. Images of the unvulcanized sample are shown in (a, d) and images of samples, which were cross-linked with different amount of sulfur curative—1 phr—in (b, e) and 2 phr—in (c, f). White arrows in (f) most likely indicate locations with small sulfur crystals. 

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. 

They distinguished the molecule "in repose" from the molecule in action. They also suggested use of circled + and - symbols to indicate "tendency" to charge rather than actual charge. As we will see later, the "curly arrow" and the d + /d-notation, not their symbols, were to accomplish this notation. Prevost and Kirrmann, "Essai," 201, 208210. 

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. 

You will notice that the head of the curly arrow points midway between the two ions, indicating where the covalent bond forms. 

You will gain more experience of using double-headed curly arrows later. 

Using curly arrow notation, write an equation for (a) the homolytic fission of bromine and (b) the heterolytic fission of hydrogen chloride. 

For a short video on bond fission and curly arrow notation visit www. brightredbooks.net... 

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. 

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) ... 

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. 

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