Flow, electron

Suppose the level of NADP is low and the level of NADPH is high. Because reduced ferredoxin donates its electron to NADP, the electrons could possibly back up in the two photosystems. In other 

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Now use curved arrows to track electron flow Move electrons from sites of high electron density (unshared pairs negative charge) toward sites of lower electron density... 

Showing electron flow is not just one more as pect of organic chemistry to learn it is a genuinely useful aid to understanding what happens m a par ticular reaction... 

FIGURE 6 6 Electron flow and orbital interactions in the transfer of a proton from a hydrogen halide to an alkene of the type H2C=CHR... 

Step 1 A molecule of borane (BH3) attacks the alkene Electrons flow from the 7C orbital of the alkene to the 2p orbital of boron A 7C complex is formed... 

FIGURE 6 10 Orbital inter actions and electron flow in the hydroboration of 1 methylcyclopentene... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

Table 6 3 shows that the effect of substituents on the rate of addition of bromine to alkenes is substantial and consistent with a rate determining step m which electrons flow from the alkene to the halogen Alkyl groups on the carbon-carbon double bond release electrons stabilize the transition state for bromonium ion formation and increase the reaction rate... 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

If a gas such as argon is held in a glass envelope that has two electrodes set into it (Figure 6.4), application of an electric potential across the electrodes leads to changes in the gas as the electrons flow from the cathode (negative electrode) to the anode (positive electrode). This passage of electrons... 

A different form of miniature electron multiplier. The curved shape is used to reduce backscattering of the electrons. The final output of electrons flows along a wire to an amplifier. 

Traditionally, the electron and proton transport pathways of photosynthetic membranes (33) have been represented as a "Z" rotated 90° to the left with noncycHc electron flow from left to right and PSII on the left-most and PSI on the right-most vertical in that orientation (25,34). Other orientations and more complex graphical representations have been used to depict electron transport (29) or the sequence and redox midpoint potentials of the electron carriers. As elucidation of photosynthetic membrane architecture and electron pathways has progressed, PSI has come to be placed on the left as the "Z" convention is being abandoned. Figure 1 describes the orientation in the thylakoid membrane of the components of PSI and PSII with noncycHc electron flow from right to left. 

Taken as a group, PV cells comprise soHd-state devices in which photons of light coUide with atoms and transfer thek energy to electrons. These electrons flow into wkes that ate connected to the cells, thereby providing current to electrical loads. 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

When two conducting phases come into contact with each other, a redistribution of charge occurs as a result of any electron energy level difference between the phases. If the two phases are metals, electrons flow from one metal to the other until the electron levels equiUbrate. When an electrode, ie, electronic conductor, is immersed in an electrolyte, ie, ionic conductor, an electrical double layer forms at the electrode—solution interface resulting from the unequal tendency for distribution of electrical charges in the two phases. Because overall electrical neutrality must be maintained, this separation of charge between the electrode and solution gives rise to a potential difference between the two phases, equal to that needed to ensure equiUbrium. 

In dry oxidation we quantified the tendency for a material to oxidise in terms of the energy needed, in kj mol of O2, to manufacture the oxide from the material and oxygen. Because wet oxidation involves electron flow in conductors, which is easier to measure, the tendency of a metal to oxidise in solution is described by using a voltage scale rather than an energy one. 

Figure 12.13 Photosynthetic pigments are used hy plants and photosynthetic bacteria to capture photons of light and for electron flow from one side of a membrane to the other side. The diagram shows two such pigments that are present in bacterial reaction centers, bacteriochlorophyll (a) and ubiquinone (b). The light-absorbing parts of the molecules are shown in yellow, attached to hydrocarbon "tails" shown in green.

Reaction centers convert light energy into electrical energy by electron flow through the membrane... 

While this electron flow takes place, the cytochrome on the periplasmic side donates an electron to the special pair and thereby neutralizes it. Then the entire process occurs again another photon strikes the special pair, and another electron travels the same route from the special pair on the periplasmic side of the membrane to the quinone, Qb, on the cytosolic side, which now carries two extra electrons. This quinone is then released from the reaction center to participate in later stages of photosynthesis. The special pair is again neutralized by an electron from the cytochrome. 

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