Elimination electron flow paths

Section 4.5.1 Addition-Elimination Electron Flow Paths... 

Line structure can be deceptive in eliminations because the important C-H bond is not explicitly drawn out, in this example, when there is a lone pair stabilized carbocation intermediate in the El. The loss of a formal charge may be difficult to notice in reactions that drop off a proton because the proton is commonly not drawn as one of the products. Since the proton lost was not drawn in the line structure on the left, it was simply forgotten. The arrow made a double bond in the right place but flowed the electrons the wrong way away from the positive center and not toward it. Besides the loss of the formal charge, the lone pair on nitrogen was lost it was assumed in the product, but the incorrect arrow did not form it. The right side shows the correct electron flow path Dg, deprotonation of a carbocation to form a pi bond. 

The AdE2 is the conceptual reverse of the El elimination. The first step, Ag, (Association, Electrophilic) is a new electron flow path and has the following general form. The arrow starts from the pi bond, breaking it and forming a new sigma bond to the electrophile, creating a carbocation next to the carbon-electrophile bond. 

The Ad]sf2 is the conceptual reverse of the ElcB elimination and has a new electron flow path, AdN, as its first step. The first arrow of the Ad starts from the nucleophile lone pair and makes a bond to the partial positive carbon of the pi bond. Next, an arrow starts from the pi bond, breaking it, and creates a carbanion next to the carbon-nucleophile bond. For the energetics of the AdN path to be reasonable, the carbanion needs to be stabilized by an adjacent electron-withdrawing group. The best crosscheck for the A4n2 is the rule (the carbanion formed should not be any more than 10 pAa units more basic than the incoming nucleophile). 

This time we are just applying known addition and elimination routes to a new reactant. The first task is always to map changes on a balanced reaction. Is the medium acidic or basic The pH of the medium is very important in deciding which addition-elimination route to take, basic or acidic. Again visualize the reactant street comer on the addition surface to decide which way to go, then make sure you draw the arrows correctly, keep track of charge balance, and use the known electron flow paths. 

Chapter 3 covered the proton transfer electron flow path and reviewed the factors that contribute to acidity. Chapter 4 introduced all the rest of the major electron flow paths along with the four reaction archetypes, substitution, elimination, addition, and rearrangement. This chapter gathers together all the major electron flow paths, introduces a few minor paths, and reviews common path combinations. Section 7.4, Variations on a Theme, shows how the 18 electron flow paths might be reasonably extended and modified. 

Our approach is to separate the conduction paths for H+ ions and electrons through the incorporation of a ceramic second phase. This approach essentially eliminates the combined dependence of hydrogen flux on electronic and proton conductivities. The approach is to short-circuit the electron flow-paths so that the overall flux is limited only by the proton conductivity. A similar mixed conducting requirement exists for electrodes in high-temperature proton conducting fuel cells, and some work has been carried out to develop mixed conductors as electrodes [24]. 

Next, using the electron sink as a guide, find the few electron flow pathways that fit those sources and sinks (Chapter 7). Look at restrictions to limit the choice of pathways, such as acidic vs. basic media. Narrow down the choice of pathways further by using steric, electronic, and solvent limitations if available. Combine the best source with the best sink through an appropriate pathway to yield a preliminary product (Chapter 8). Only about half of all the possible sources and sinks are common, and those common combinations were outlined in the correlation matrix of Table 8.1 and discussed in the sections that followed. Be aware of alternatives like competing paths (Chapter 4) and decisions like substitution vs. elimination (Chapter 9). 

The Earth acts as an infinite store from which elecfrons (current flow) can be drawn or to which they can return. Providing a path for that flow can eliminate any undesirable excess or deficiency. Gaining electrons can neutralize positive ions in a system, and electrons can be conducted to Earth (called earthing in some countries). In the United States, the term grounding is preferred, and the path to Earth or the Earth itself is a ground. In some instances (such as in electronic equipment), a massive metallic body acts as the reservoir of electrons and ions (the ground) in place of the Earth. 

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