Arrows double-barbed

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

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

Scheme 1.9 Application of arrow pushing to heterolytic cleavage using double-barbed arrows.

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. 

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.]... 

Double-barbed arrows show the attack (or movement) of an electron pair. 

According to resonance theory, we consider Kekule structures I and II below as resonance contributors to the real structure of benzene, and we relate them to each other with one double-headed, double-barbed arrow (not two separate arrows, which we reserve for equilibria). 

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. 

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. 

In looking at Figure 3.71, don t forget the curved arrow formalism (p. 23). The double-barbed arrows track the movements of pairs of electrons. The color-coding should help. Notice also the red and green equilibrium arrows. The different lengths reflect the exothermicity or endothermicity of each step. The first step is endothermic, and the second is exothermic. 

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. 

A heterolytic cleavage of an ion. For an odd electron ion, inductive cleavage results from the pair of electrons that forms a bond to the atom at the apparent charge site moving to that atom while the charge site moves to the adjacent atom. The movement of the electron pair is represented by a double-barbed arrow. 

A normal, double-barbed curved arrow Pn shows the movement of a pair of electrons. 

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 ... 

Each of these single-barbed arrows indicates the motion of one electron, while double-barbed arrows indicate the motion of two electrons. All of the mechanisms presented in this chapter utilize single-barbed arrows. Make sure to draw them properly. 

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 ). 

Fig. 2 Earthquake mechanisms in oceanic environments. Earthquakes occur mainly on active faults (double line spreading center single line transform fault barbed line subduction zone, barbs point in direction of subducting plate). Dashed line is an inactive fracture zone. Arrows show relative motion sense...

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