Arrow convention

In this equation /g, is th equilibrium exchange current, and the arrow convention adopted is that / g represents the rate of cathodic reduction... 

Show the important resonance structures for these species. Use the curved arrow convention to show how the electrons are moved to create each new resonance structure. 

Complete these equilibrium reactions in the most reasonable manner possible using the curved arrow convention to show the movement of electrons in the reactions. Predict whether the reactants or the products are favored. 

Fig.3 The arrow conventions for inactive and active holes and particles.

The curved-arrow convention is used to show how electrons in one resonance structure can be moved around to generate a new resonance structure. The curved arrows are entirely a formalism electrons do not actually move from one location to another, because the real compound is a weighted average of the different resonance structures, not an equilibrium mixture of different resonance structures. The curved arrows help you not to lose or gain electrons as you draw different resonance structures. 

Because it is not the case that every line represents exactly two electrons in a drawing of an organometallic or inorganic compound, it follows that the curved-arrow convention for showing the movement of electrons cannot be applied unambiguously when reaction mechanisms inolving transition metals are drawn. For this reason, in mechanisms involving transition metals, the name of each individual mechanistic step (insertion, transmetallation, oxidative addition, etc.) is indicated in place of curved arrows. You may use curved arrows to show electron movement in some steps if you wish, but it is more important for you to name every step. 

Figure 12.1. Types of agostic interactions (half-arrow convention). ...

The double-headed arrow and the brackets are used to indicate that the two structures are resonance structures. Remembering the concept of resonance (that is, the delocalization of electrons), we can see why these two structures are equivalent. Using the arrow convention of moving electrons, we have ... 

Fig. 8.5a,b. False negative diagnosis polyps simulating fecal residue in mobile segments. Differential diagnosis of mobile stool or small sessile lesions in a mobile transverse colon a supine scan shows two lesions in the transverse colon, (arrows) b prone scan shows the lesions in the transverse colon in an apparent different position (arrows). Conventional colonoscopy revealed the presence of two small sessile polyps. Lesson Polyps, located in mobile colonic segments such as the transverse colon can cause erroneous diagnosis of mobile residual stool... 

Fig. 8.38a,b. False positive diagnosis intermittently prolapsing rectal mucosa a axial image shows a smooth soft tissue filling defect at the anorectal region (arrow) b endoluminal 3D image shows an apparent low rectal mass (arrows). Conventional colonoscopy showed edematous mucosa due to rectal prolapse. Lesson Rectal mucosa can appear very prominent, particularly in case of mucosal prolapse, simulating low rectal masses... 

FIGURE 1.5 An application of Hund s rule. The electronic configuration with the largest number of parallel (same direction) spins is lowest in energy. Note the use of the arrow convention to show electron spin. 

It is easy to confuse resonance with equilibrium. On a mundane, but nonetheless important level, this confusion appears as a misuse of the arrow convention. Two arrows separate two entirely different molecules (A and B), each of which might be described by several resonance forms. The amount of A and B present at equilibrium depends on the equilibrium constant. The doubleheaded resonance arrow separates two different electronic descriptions (C and D) of the same species, E (Fig. 1.29). 

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. 

The first necessary clarification must be what is a reaction Chemists describe a full reaction mechanism with the familiar arrow convention for electron pair redistribution over a linked chain or cycle of atoms as in ... 

Hendrickson next recouped this analysis for the even-atom (four and six) pericycles and added the set of odd-atom pericycles typified by equations (3) and (4). He also enumerated the possibilities of each with atom types of different valences in the cycle, permutations which run into the thousands. It may be noted that the full-shell representations like B = 6 in Figure 1 are not reactions since no a-bonds are broken. They just show the generation of one resonance form from another, reflecting the use of the arrow convention for generating either reaction products or resonance forms. 

When we show arrows and distinguish lone pairs, we do not wish to imply that there are somehow arrows running around in the solution, or that we can tell nonbonding electrons apart of course, we cannot. This arrow convention is a book-keeping procedure that we will use many times to keep track of the changes that occur in organic reactions. 

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