Fish hook

Further symbols are used to indieate reaction mechanisms, in particular the use of curly arrows (to represent the movement of pairs of electrons) and fish-hooks (to represent the movement of single eleetrons). Students need to understand the precise meaning of these arrows (whieh electrons move, and where from and where to) to appreeiate how they represent stages in reaetion mechanisms. Students who have been taught the formalism are not neeessarily able to identify the outcome of... 

Notice the use of fish-hook arrows to show the movement of a single electron. Such arrows are used a great deal when writing mechanisms for reactions involving radical species. 

The loss of nitrogen from an azide to form a nitrene can b /isuatlzed as shown, atthougi. vhether the fragmentation involves two electrons (doubte-headedi arrows) or a single electron (fish-hooks) is not important to this discussion ... 

The flask is charged with about 3 1. of liquid ammonia (Note 1), the stirrer is started, and a rapid stream of acetylene gas (about 5 bubbles per second) is passed in for about 5 minutes to saturate the ammonia. The acetylene from a tank is sufficiently purified by passage through a sulfuric acid wash bottle a safety trap also should be inserted in the line. Sodium (92 g., 4 gram atoms) is cut in strips (about by 3 by 3 in.) so that they can be inserted through the side neck of the flask. One of these pieces of sodium is attached to the fish-hook and is gradually lowered into the liquid ammonia while a rapid stream of acetylene is passed in. fl he sodium should be added at such a rate that the entire solution does not turn blue. If it does, the sodium should l)i raised above the level of the ammonia until the color is discharged (Note 2). I lie rest of the sodium is added in a similar... 

Fragmentation by movement of one electron. Bonds are broken by movement of one electron, represented by a fish-hook arrow ( ). 

The weak Br-Br bond breaks to form two bromine radicals. This can be represented by two single-headed curly arrows, fish hooks, to indicate that only one electron is moving. This is virtually all you will see of this special type of curly arrow until we consider the reactions of radicals in more detail (Chapter 39). When you meet a new reaction you should assume that it is an ionic reaction and use two-electron arrows unless you have a good reason to suppose otherwise. 

When we made the allyl cation from allyl bromide, the bromine atom left as bromide ion taking both the electrons from the C-Br bond with it—the C-Br bond broke heterolytically. What if the bond broke homolytically—that is, carbon and bromine each had one electron A bromine atom and an allyl radical (remember a radical has an unpaired electron) would be formed, This reaction can be shown using the singleheaded fish hook curly arrows from Chapter 5 normal double-headed arrows show the movement of two electrons single-headed arrows show the movement of one. 

The peroxy radical RO" abstracts H from the HBr to give ROH, leaving behind a new radical Br. We have described this process using arrows with half-heads (also known as fish-hook arrows ). 

Use of Curved Arrows Curved arrows are used to describe the movement of electrons in a reaction mechanism. The arrow starts with the electron(s) to be moved and ends at the atom or bond where they move. Full arrows are used to denote the movement of electron pairs and fish hook arrows are used to show the movement of single electrons. ... 

Hall-headed arrows (fish-hooks, are used to show the movement of individual electrons in free radical reactions, i-contrast to the curved arrows used in polar and molecular reactions to show the movement of electron pairs. 

Benzene is described as a resonance hybrid of the two extreme forms which correspond, in terms of orbital interactions, to the two possible spin-coupled pairings of adjacent p electrons structures 1 and 2. These are known as resonance contributors , or mesomeric structures , have no existence in their own right, but serve to illustrate two extremes which contribute to the real structure of benzene. Note the standard use of a double-headed arrow to inter-relate resonance contributors. Such arrows must never be confused with the use of opposing straight fish-hook arrows that are used to designate an equilibrium... 

Fish hook barbs are best but the bronze colored ones break. Tly to get No 2/0 silvery hooks. Bend straight and cut to suit. 

Silvery Flattened Fish Hook Then Ground... 

The fish hook barb will hold it in and the poison will be automatically injected into his system before his reaction time will let him yank it out. By that time, 2 ccs of death will have poured in and only the Great Pumpkin couldn t keep him alive for more than a few seconds. 

For the barb, you ll need a fish hook. Bronze colored hooks are tempered too hard and so are almost imp ible to bend without breaking. Thus, the shaft behind the barb will be too long and also curved, or it will be too short to handle. The silvery ones can be straightened and so can be cut so the shaft rests on the hypo s needle end Uid the barb projects slightly beyond its tip as shown at the right of Figure 3. 

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 ). 

---