Arrows single-barbed

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... 

Including the ionization process, the free electron pairs, and single-barbed arrows for each moving electron in the scheme is not necessary but presents a valuable aid. Alternatively, the a-cleavage may be indicated in a simplified manner ... 

The mass spectrometric convention, is that a double-barbed arrow ( - ) implies transfer of two electrons, whereas a single barb ( — ) indicates a one-electron transfer. 

Figure 5.3. Connections among r- and p-space densities, density matrices, and form factors. Two-headed arrows signify reversible transformations single-barbed arrows signify irreversible transformations. A Fourier transform is denoted by JF.

For each fragmentation/rearrangement pathway from exercise 1.4, show a detailed mechanism using either single barbed or double barbed arrows as appropriate. 

Scheme 1.8 Application of arrow pushing to homolytic cleavage using single-barbed arrows.

This is an example of the homolytic cleavage of a bromine molecule to form two bromide radicals. Note the use of single-barbed arrows to describe radical-based mechanisms resulting in the movement of single electrons. For clarity, the bond is elongated. Arrow pushing is illustrated below ... 

This is an example of a step in the free-radical-mediated polymerization of ethylene, forming polyethylene. As in the previous example, note that a single-barbed arrow is used for each electron that is moving. Arrow pushing is illustrated below ... 

Radicals can also have delocalized electrons if the unpaired electron is on a carbon that is adjacent to an sp hybridized atom. The arrows in Figure 7.5 are single barbed because they denote the movement of only one electron (Section 3.6). 

A typical radical cyclization involves the attack of a radical center on an sp carbon of a double bond (or other unsaturated group) in the chain. If the chain includes one or more heteroatoms, then a heterocycle will be formed. In Scheme 4.44, we examine the cyclization of a radical to form a 5-membered ring (a pyrrolidine) by this process. Note the practice of showing single-electron processes with single-barbed arrows ( fish hooks ). 

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-... 

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.]... 

We use single-barbed arrows to depict mechanisms involving single electron movements (see Section 3.1A). 

Recall that we use single-barbed arrows to show the movement of single electrons, as In the case of these homolytic bond oleavages and other processes Involving radicals (see Section 3.1A). 

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. 

Single-barbed arrows show the attack (or movement) of an unpaired electron. 

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. 

Arrows used in ionic mechanisms (double barbed) and radical mechanisms (single barbed). 

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. 

Arrows used in ionic mechanisms are double-barbed, while arrows used in radical mechanisms are single-barbed. 

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. 

FIGURE 2.8 Two ways of showing homolytic cleavage of one carbon—hydrogen bond in methane. Each single-barbed arrow represents movement of one electron. 

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. 

We met the methyl radical in Chapter 2 (p. 62). Radicals, sometimes called free radicals, are neutral species that have a single nonbonding electron and can undergo many reactions, as we will see in Chapter 11. Typical and important reactions of radicals include combining with other radicals to form bonds. The combination of two carbon-based radicals is especially important because it results in the formation of a carbon-carbon bond. The arrow formalism for radical reactions uses single-barbed arrows. 

Don t forget—throughout our discussion of these radical reactions, the motion of single electrons is shown with single-barbed, fishhook arrows (see p. 38). 

Uses a single-barbed arrow (fish-hook) to show the movement of a single electron ... 

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