Arrows, use

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. 

This open question is symbolized by the unusual type of equilibrium arrows used in Scheme 11-2. 

Stoichiometry, do not need these visualization skills. Rather, they involve the understanding of representational conventions, such as the meaning of arrows used in diagrams, and how the verbal or mathematical mode of representation relates to and is translated into the visual mode. 

Here is where it can be confusing as to what is exactly going on. These arrows do not represent an actual process (such as electrons moving). This is an important point, because you will leam later about curved arrows used in drawing reaction mechanisms. Those arrows look exactly the same, but they actually do refer to the flow of electron density. In contrast, curved arrows here are used only as tools to help... 

The overall result is the addition of H and X across the double bond. We have specifically used a starting alkene that avoids issues of regiochemistry or stereochemistry we will soon see other examples in which we must explore both of those issues. For now, focus on the curved arrows used in both steps. It is absolutely critical to master the art of drawing curved arrows properly. Let s quickly practice ... 

We have now encountered a number of different types of arrow routinely used in chemistry to convey particular meanings. We have met curly arrows used in mechanisms, double-headed resonance arrows, equilibrium arrows, and the simple single arrows used for reactions. This is a convenient point to bring together the different types and provide a checklist for future reference. We are also showing how additional information about a reaction may be presented with the arrow. 

Strychnine was used in its impure powder form centuries before it was isolated. The nuts that yielded the seeds were given names such as poison nut or vomit nuts. Natives prepared poison arrows using the seeds and excretions of Strychnos species, and Europeans... 

Alcohols, which contain a hydroxyl functional group, can undergo dehydration reactions with either carboxylic or phosphoric acid to form esters. The double arrows used in this and subsequent equations indicate that the reaction can go in either direction. 

In this they somewhat resemble the curly arrows used to show resonance. in benzene, where the arrows show where to draw the new bonds, and which ones not to draw in the canonical structure but in this case there is neither a sense of direction nor even an actual movement. The analogy between the resonance of benzene and the electron shift in the Diels-Alder reaction is not far fetched, but it is as well to be clear that one is a reaction, with starting materials and a product, and the other is not. 

Fig. 6.32. Stereoisomeriza-tion-free synthesis of an oligopeptide following the original DCC procedure (black reaction arrow top left) and a modified DCC procedure (red reaction arrows) using a polymer support known as the Wang resin [Fmoc = (fluo-roenylmethoxy) carbonyl].

Retrosynthetic arrow (Section 11.14) An arrow used in the development of a synthetic scheme that points from the target to the reactant from which it can be prepared. 

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. 

The enolate ion is one of those three-atom four-electron systems related to the allyl anion that we met in Chapter 7. The negative charge is mainly on oxygen, the most electronegative atom. We can show this with curly arrows using the simplest enolate possible (from MeCHO). 

The arrows used in the chart have three functions. 

The curly arrows are drawn clockwise, but they could equally well have been drawn anticlockwise. Thus, there is no absolute sense in which the hydrogen atom that moves from one carbon atom to the other in the ene reaction is a hydride shift, as seems to be implied by the clockwise curly arrow, or a proton shift, as it would seem to be if the arrows were to have been drawn in the opposite direction. In other words, neither component can be associated with the supply of electrons to any of the new bonds. The curly arrows therefore have a somewhat different meaning from those used in ionic reactions. They share with all curly arrows the function of showing where to draw the new bonds and which ones not to draw in the resulting structure. They are related to the arrows used to illustrate resonance in benzene, in having no sense of direction, but the Diels-Alder reaction has starting materials and a product, and aromatic resonance in benzene does not. 

Draw a line in the direction of the material or process flow from or to the outside entity with an arrow. Use icons to help clarify in what format the information is moving (e.g., use an envelope to indicate it is mailed, and an envelope with an E in the middle to indicate it s e-mail). 

Table 6.1 summarizes the many kinds of arrows used in describing organic reactions. Curved arrows are especially important because they explicitly show what electrons are involved in a reaction, how these electrons move in forming and breaking bonds, and if a reaction proceeds via a radical or polar pathway. 

A more complete summary of the arrows used in organic chemistry Is given in the table Common Abbreviations, Arrows, and Symbols, located on the inside back cover. 

Fishhook (Section 6.3B) A half-headed curved arrow used in a reaction mechanism to denote the movement of a single electron. 

F igure 12.6. Illustration of the HMBC correlations (arrows)used to assign the positions of two of the methyl quaternary methyl groups in taxol. 

Note that the arrows used to show the flow of unpaired electrons (radicals) have only a half head. Also, the intermediate in the reaction is a diradical. (Radicals are discussed in more detail in Chapter 5.) Rotation about the highlighted single bond takes place fast enough that the stereochemistry of the starting olefin is lost. 

The second compound has more possibilities, one of which is very stable indeed as it has a benzene ring (delocalization shown). We haven t drawn the mechanism for enolization this time but notice the different reaction arrows used for tautomerism (equilibrium arrows) and delocalization (double-headed arrow). 

Arrows used to show electron spin and to indicate location of the electrons in the separated atoms and the molecule... 

Step 2. Approximate daily dosage is shown directly above the arrow. When the line con nects between the arrows, use the crossbars between the arrows to choose high, medium, or low dosages or estimate an intermediate amount. Exercise caution when choosing dosages in the high range. 

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