Single-headed curved arrow

Double- and single-headed curved arrows indicate the movement of electrons. Double-headed curved arrows (shown in Figure 2-4a) show the movement of two electrons, whereas single-headed curved arrows (Figure 2-4b) indicate the movement of one electron. The electrons always move in the direction indicated by the arrow. The head (point) of the arrow is where the electron is going, and the tail is the electron s source. 

Free-radical mechanisms obviously involve free radicals. A free radical is a species with an unpaired electron. In these mechanisms, single-headed curved arrows eire the norm. In Organic Chemistry 1, these free radicals first appear when excimining the chlorination of an alkane such as methane. The process begins with an initiation step as shown in Figure 2-14. (All initiation steps increase the number of free radicals.)... 

The movement of unpaired electrons in free radical reactions is shown with single-headed curved arrows. 

As a result of this transfer of one electron, both sodium and fluorine form ions that have the same electron configuration as neon, the noble gas closest to each in atomic number. In the following equation, we use a single-headed curved arrow to show the transfer of one electron from sodium to fluorine ... 

Robinson s curly arrow is used to show the movements of pairs of electrons in a reaction mechanism. The tail of a curly arrow starts at a mobile electron pair and its head points to the destination of the electron pair. Fishhook arrows indicate cleavage or movement of a single electron shown as a single-headed curved arrow. They are widely used in radical chemistry to represent the homolytic cleavage and reactions of radicals. ... 

Onr two commandments (never break a single bond, and never violate the octet rule ) reflect the two parts of a curved arrow (the head and the tail). A bad tail violates the first commandment, and a bad head violates the second commandment. 

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). 

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

Two half-headed curved arrows are needed for two single electrons. 

When more than one curved arrow is present, they should all point in the same general direction and never toward each other or away from each other. However, curved single-headed arrows do not necessarily follow this rule. 

Figure 25.1 Addition polymerization to form polyethylene from ethylene. Each curved, single-headed arrow represents the movement of one electron.

These difficulties have led to the convention of representing molecules that cannot adequately be written as a single classical structure by a combination of two or more classical structures, the so-called canonical structures, linked by a double-headed arrow. The way in which one of these structures can be related to another often being indicated by curved arrows, the tail of the curved arrow indicating where an electron pair moves from and the head of the arrow where it moves to ... 

We use a single double-headed arrow between resonance forms (and often enclose them in brackets) to indicate that the actual structure is a hybrid of the Lewis structures we have drawn. By contrast, an equilibrium is represented by two arrows in opposite directions. Occasionally we use curved arrows (shown in red above) to help us see how we mentally move the electrons between one resonance form and another. The electrons do not actually resonate back and forth they are delocalized over all the resonance forms at the same time. 

Use curved arrows to show the movement of electrons. Full-headed arrows are used for electron pairs and half-headed arrows are used for single electrons. 

A curved half-headed arrow indicates the movement of a single electron in the direction of the arrowhead... 

In chemical structures, the unpaired electron of a radical is represented by a dot. Radical mechanisms are depicted in one of two ways. Most commonly, each individual step of the mechanism is written without the use of arrows to show electron movement. The resulting series of equations shows the order of events, and it is assumed that one-electron transfers are taking place throughout. A second method uses curved, half-headed arrows ( - ) to show electron movement. The half-headed arrow is used to denote movement of a single... 

From the first glance it is obvious that the same trends syiply to both emulsions, but earlier in the case of emulsion A which may be qualified as h s stable. The way to quantify this concept is to intercept the two curves by a single line. For instance, the (vertical) white-headed arrow locked at a fixed time (here 10 h) intercepts both curves at a point (white dot) whose ordinate indicates the coalesced volume after 10 h. It is larger for emulsion A, which is thus less stable. In the present case there is quite a difference in coalescence fraction VJV between the two emulsions. However, this is not the general case, since the emulsion-coalesced volume fraction does not vary but over a sometimes narrow period of lime. For instance, if the interception is made at 100 h instead of 10 h. then both emulsions are completely coalesced, while after 1 h none of them has started to coalesce. Thus at I h and iOO h the diagnostic is the same for both emulsions, in discrepancy with the actual difference. 

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) ... 

In order to determine if either rule has been broken, we must look carefully at the tail and the head of the curved arrow. The tail is placed on a double bond, and therefore, this curved arrow does not break a single bond. So the first rule is not violated. 

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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