Electron pushing

It has been said that God created an organism especially adapted to help the biologist find an answer to every question about the physiology of living systems if this is so it must be concluded that pyridoxal phosphate was created to provide satisfaction and enlightenment to those enzymologists and chemists who enjoy pushing electrons, for no other coenzyme is involved in such a wide variety of reactions, in both enzyme and model systems, which can be reasonably interpreted in terms of the chemical properties of the coenzyme. Most of... 

Pushing Electrons A Guide for Students of Organic Chemistry, third edition, by Daniel P. Weeks. A workbook designed to help students learn techniques of electron pushing, its programmed approach emphasizes repetition and active participation. (0-03-020693-6)... 

The battery acts as an electron pump, pushing electrons into the cathode, C, and removing diem from the anode, A. To maintain electrical neutrality, some process within the cell must consume electrons at C and liberate them at A. This process is an oxidation-reduction reaction when carried out in an electrolytic cell, it is called electrolysis. At the cathode, an ion or molecule undergoes reduction by accepting electrons. At the anode, electrons are produced by the oxidation of an ion or molecule. 

First we need to locate the part of the molecule where resonance is an issue. Remember that we can push electrons only from lone pairs or bonds. We don t need to worry about all bonds, because we can t push an arrow from a single bond (that would violate the first commandment). So we only care about double or triple bonds. Double and triple bonds are called pi bonds. So we need to look for lone pairs and pi bonds. Usually, only a small region of the molecule will possess either of these features. 

The last possibility does not work, becanse we cannot push electrons from one lone pair to another (at least not in one step). So we only have to consider the first three... 

Many enzymic reactions are a consequence of either nucleophilic or electrophilic attack on the substrate, the former being the more common. If such a species approaches another group it will tend to push electrons away from the positive centre . If the reaction to be catalysed is... 

The three fullerene-centred reductions display features of chemical reversibility and are localized at potential values more negative by about 0.2 V than those of free C6o, Table 4. This proves that also in this case the metal fragment pushes electron density towards the fullerene. 

Since 1990, more and more structures have been solved from HREM images and electron diffraction and more and more scientists have become interested in structure analysis by electron crystallography (Hovmoller 1992 Zou, 1995). This pushed electron crystallography a big step forward. The term electron crystallography was first introduced and soon been accepted. 

Alkyl groups are said to have a positive inductive effect. This means they are electron-donating and can push electrons onto the positively charged carbon atom, thus stabilising the carbocation. It follows that tertiary carbocatlons, with their three alkyl groups, are the most stable species and that primary carbocatlons, with just one alkyl group, are the least stable species. This suggests that tertiary haloalkanes are most likely to react with a nucleophile via an Sj. 1 mechanism. 

The carbocations that are formed to generate these two products are shown on the left. You will recall that alkyl groups can exert a positive inductive effect (see p. 59), i.e. they can push electrons towards the positively charged carbon atom in the carbocation and so stabilise it. Therefore carbocation A will be more stable than carbocation B because it has two alkyl groups directly attached to the positively charged carbon atom, whereas there is only one alkyl group in carbocation B. 

Hint if you draw the alternative Kekule form, you can push electrons around the ring... 

We begin by bringing you up to speed on mechanisms and reminding you how to push electrons around with those curved arrows. We jog your memory with a discussion of substitution and elimination reactions and their mechanisms, in addition to free radical reactions. Next you review the structure, nomenclature, synthesis, and reactions of alcohols and ethers, and then you get to tackle conjugated unsaturated systems. Finally, we remind you of spectroscopic techniques, from the IR fingerprints to NMR shifts. The review in this part moves at a pretty fast pace, but we re sure you can keep up. 

Fluorine in organic compounds is associated with a set of electronic effects inductive and mesomeric, stabilizing and destabilizing, pulling or pushing electrons, which are convincingly... 

An electrolytic cell has two electrodes that dip into an electrolyte and are connected to a battery or some other source of direct electric current. A cell for electrolysis of molten sodium chloride, for example, is illustrated in Figure 18.15. The battery serves as an electron pump, pushing electrons into one electrode and pulling them out of the other. The negative electrode attracts Na+ cations, which combine with the electrons supplied by the battery and are thereby reduced to liquid sodium metal. Similarly, the positive electrode attracts Cl- anions, which replenish the electrons removed by the battery and are thereby oxidized to chlorine gas. The electrode reactions and overall cell reaction are... 

Electrical conductors allow electrons to pass through their conduction band with very little effort. Some effort, however, is nonetheless required to get electrons to flow. Thus, under ordinary conditions, all materials offer some resistance to the flow of electrons. If this were not the case, a current would continue to flow in a loop of copper wire with no need to expend additional energy once the current was started. The energy required to push electrons through a material is a measure of the resistance of the material. 

The lowest energy MLCT transition of Ru polypyridyl complexes of the type tris-[Ru(4,4/-dicarboxy-2,2/-bipyridine)3] (1), can be lowered so that it absorbs more in the red region of the visible spectrum by replacing one 4,4/-dicarboxy-2,2/-bipyridine (dcbpy) with two thiocyanate donor ligands [Ru(dcbpy)2(NCS)2] (2). In complex 2, the two 4,4/-dicarboxylic acid 2,2 -bipyridine ligands pull while the two thiocyanate donor ligands push electrons. The oxidation potential of the complex 2 is 0.85 V vs. SCE, which is cathodically shifted significantly (0.65 V vs. SCE) compared to the homoleptic type of complex 1, which shows Ru(III/II) couple at 1.5 V vs. SCE. Thus, the... 

As the mechanistic steps discussed for the pinacol rearrangement have been illustrated using arrow pushing, it is important to recognize that in all cases, the arrows have been drawn pushing electrons toward positive charges. This point has been previously discussed and will continue to be emphasized. 

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