Curved arrows fishhook

Double-headed arrow Full-headed curved arrow Half-headed curved arrow (fishhook)... 

Cumulated diene. S ee Allenes Dienes Cuprates. See Lithium diorganocuprates Curl, Robert E, 410 Curved arrows fishhook, 150... 

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... 

Historically, ethylene potymerization was carried out at high pressure (1000-3000 atm) and high temperature (100-250 °C) in the presence of a catalyst such as benzoyl peroxide, although other catalysts and reaction conditions are now more often used. The key step is the addition of a radical to the ethylene double bond, a reaction similar in many respects to what takes place in the addition of an electrophile. In writing the mechanism, recall that a curved half-arrow, or "fishhook" A, is used to show the movement of a single electron, as opposed to the full curved arrow used to show the movement of an electron pair in a polar reaction. 

These arrows (called fishhook arrows) are the hallmark of radical reactions. We use fishhook arrows in radical mechanisms, because they indicate the movement of only one electron, rather than two electrons (by contrast, two-headed curved arrows are used in ionic mechanisms to show the movement of two electrons). 

A covalent bond consists of a shared pair of electrons. Nonbonded electrons important to the reaction mechanism are designated by dots (— OH). Curved arrows (<- ) represent the movement of electron pairs. For movement of a single electron (as in a free radical reaction), a single-headed (fishhook-type) arrow is used ( ). Most reaction steps involve an unshared electron pair (as in the chymotrypsin mechanism). 

Notice the fishhook-shaped half-arrows used to show the movement of single unpaired electrons. Just as we use curved arrows to represent the movement of electron pairs, we use these curved half-arrows to represent the movement of single electrons. These halfarrows show that the two electrons in the Cl—Cl bond separate, and one leaves with each chlorine atom. 

To illustrate the movement of a single electron, use a half-headed curved arrow, sometimes called a fishhook. 

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. 

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

A curved arrow with half a head is called a fishhook. This kind of arrow is used to indicate the movement of a single electron. In eq. 1.6, two fishhooks are used to show the movement of each of the two electrons in the C—C bond of ethane to a carbon atom, forming two methyl radicals (see eq. 1.3) ... 

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 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. 

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