Single-barbed curved arrows

This simple example illustrates the way we use single-barbed curved arrows to show the movement of a sii e electron (not of an electron pair as we have done earlier). In this instance, each gtoup, A and B, comes away with one of the electrons of the covalent bond that joined them. 

Curved arrows (Sections 1.8, 3.2, and 10.1) Curved arrows show the direction of electron flow in a reaction mechanism. They point from the source of an electron pair to the atom receiving the pair Double-barbed curved arrows are used to indicate the movement of a pair of electrons single-barbed curved arrows are used to indicate the movement of a single electron. Curved arrows are never used to show the movement of atoms. 

Fishhook arrow A single-barbed, curved arrow used to show the change in position of a single electron. 

Radicals and their chemistry are discussed in more detail in Chapter 11. For now, we will just point out that mechanistic steps involving radicals utilize single-barbed curved arrows, often called fishhook arrows, rather than double-barbed curved arrows (Figure 10.5). Single-barbed curved arrows indicate the movement of one electron, while double-barbed arrows indicate the movement of two electrons. Notice the use of single-barbed curved arrows in the first step of the mechanism to form the intermediate radical anion. The nature of this intermediate explains the stereochemical preference for formation of a trans alkene. Specifically, the intermediate achieves a lower energy state when the paired and unpaired electrons are positioned as far apart as possible, rninimizing their repulsion. 

Notice in these illustrations that we have used curved arrows to show the movement of electrons. We will have more to say about this convention in Section 3.5, but for the moment notice that we use a double-barbed curved arrow to show the movement of a pair of electrons and a single-barbed curved arrow to show the movement of a single electron. 

This simple example illustrates the way we use single-barbed curved arrows to show... 

A single-barbed curved arrow instance, each group, A and B, comes away with one of the electrons of the covalent bond... 

A double-barbed curved arrow (O) shows the movement of two electrons (an electron pair). [Later, we will see that a single-barbed arrow ( ) can be used to show the movement of a single electron.]... 

A curved arrow shown as a single barbed fishhook signifies the movement of one electron Normal curved arrows track the movement of a pair of electrons... 

There are two kinds of curved arrows. A double-barbed arrow (/ ) shows the movement of a pair of electrons, either a bonded pair or a lone pair. A single-barbed, or fishhook, arrow P) shows the movement of one electron. For now, weTl concern ourselves only with reactions that involve electron pairs and focus on double-barbed arrows. 

HOMOo - HOMOd Note that the curved arrows are single-barbed, denoting the move-... 

Notice the difference in the red curved arrows in the two parts of Figure 1.44. The curved arrow showing electron movement in the heterolytic cleavage has the standard double-barbed arrows, representing the movement of two electrons to the same atom. The homolytic cleavage pathway uses single-barbed or fishhook arrows representing the movement of one electron to each atom. 

Recall from p. 37 that there is another way of breaking a two-electron bond, and that is to allow one electron to go with each atom involved in the breaking bond (Fig. 2.14). This homol)Tic bond cleavage in methane gives a hydrogen atom (H ) and leaves behind the neutral methjiradical ( CH3). Note the single-barbed fishhook curved arrow convention is used to represent movement of one electron. 

The two previous tutorials on drawing curved arrows showed the simultaneous movement of two electrons. The movement of two electrons is indicated by an arrowhead with two barbs. Electron movement in radical systems involves the movement of only one electron. The movement of a single electron is indicated by an arrowhead with one barb. 

Curved single-barb arrows are used to generate resonance structures of an allylic or benzylic radical. 

Use curved single-barb arrows to show the mechanism of the following step in a radical reaction. 

When drawing a mechanism for a radical process, make sure that all curved arrows are single-barbed (called fishhook arrows), rather than double-barbed. For example, look closely at the head of each of the following fishhook arrows ... 

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