Arrow, fishhook

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

Half-headed arrows (fishhooks) are used to show the movement of single electrons. 

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... 

Problem 5.3 Using a curved fishhook arrow, propose a mechanism for formation of the cvclopen-tane ring of prostaglandin H2- What kind of reaction is occurring ... 

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. 

Problem 7.19 Oik- of the chain-termination steps that sometimes occurs to interrupt polymerization is the following reaction between two radicals. Propose a mechanism for the reaction, using fishhook arrows to indicate electron flow. 

Fischer projection, 975-978 carbohydrates and, 977-978 D sugars, 980 i., sugars, 980-981 rotation of, 976 R.S configuration of, 977 conventions for, 975-976 Fishhook arrow, radical reactions and, 139, 240... 

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 single-headed curly arrow or fishhook arrow indicates the movement of a single electron. 

Here, we have used single-headed curly arrows or fishhook arrows to indicate. the movement of single electrons. The tail of the curly arrow shows the source of the electron and the head shows its destination. Using single-headed curly arrows, the mechanism for the methane/chlorine chain reaction is completed below. 

In the formation of radicals, a bond is broken and each atom takes one electron from the pair constituting the bond. Bond-making and bond-breaking processes are indicated by single-headed (fishhook) curly arrows representing the movement of one electron. 

It makes good sense to draw free-radical mechanisms in the manner shown by these examples. However, shorter versions may be encountered in which not all of the arrows are actually drawn. These versions bear considerable similarity to two-electron curly arrow mechanisms, in that a fishhook arrow is shown attacking an atom, and a second fishhook arrow is then shown leaving this atom. The other electron movement is not shown, but is implicit. This type of representation is quite clear if the complement of electrons around a particular atom is counted each time but, if in any doubt, use all the necessary fishhook arrows. 

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). 

Fishhook arrows have the same functions of identifying simultaneously where the electrons come from and go to, and which bonds have been broken and which made, However, since each component provides one electron for each new bond, there is strictly no nucleophile and electrophile. 

Pericyclic reactions are the third distinct class. They have cyclic transition structures in which all bond-forming and bond-breaking takes place in concert, without the formation of an intermediate. The Diels-Alder reaction and the Alder ene reaction are venerable examples. The curly arrows can be drawn in either direction—clockwise, as here, but equally well anti- clockwise. They could even be drawn with fishhook arrows, and would still... 

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. 

Following a suggestion of Budzikiewicz, Djerassi, and tyilliams,11 the shift of a lone electron is indicated as a fishhook (N and that of an electron pair as an arrow A, but in contrast to the cited authors we indicate the movement of each electron of a pair. 

Mechanisms of radical reactions are shown by using fishhook arrows. 

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