Six-electron

We term the in-phase combination an aromatic transition state (ATS) and the out-of-phase combination an antiaromatic transition state (AATS). An ATS is obtained when an odd number of electron pairs are re-paired in the reaction, and an AATS, when an even number is re-paired. In the context of reactions, a system in which an odd number of electrons (3, 5,...) are exchanged is treated in the same way—one of the electron pairs may contain a single electron. Thus, a three-electron system reacts as a four-electron one, a five-electron system as a six-electron one, and so on. 

The next simplest loop would contain at least one reaction in which three electron pairs are re-paired. Inspection of the possible combinations of two four-electron reactions and one six-electron reaction starting with CHDN reveals that they all lead to phase preseiwing i p loops that do not contain a conical intersection. It is therefore necessary to examine loops in which one leg results in a two electron-pair exchange, and the other two legs involve three elechon-pair exchanges fip loops). As will be discussed in Section VI, all reported products (except the helicopter-type elimination of H2) can be understood on the basis of four-electron loops. We therefore proceed to discuss the unique helicopter... 

In elements of Periods 2 and 3 the four orbitals are of two kinds the first two electrons go into a spherically symmetrical orbital—an s orbital with a shape like that shown in Figure 2.7—and the next six electrons into three p orbitals each of which has a roughly doublepear shape, like those shown unshaded in each half of Figure 2.10. 

The elements in this group have six electrons in their outer quantum level, and can thus achieve a noble gas configuration by acquiring two electrons. 

In the case of 1,3-butadiene, RAMSES combines the two double bonds to form a single, delocalized r-electron system containing four electrons over all four atoms (Figure 2-50a). The same concept is applied to benzene. As shown in Figure 2-50b, the three double bonds of the Kekule representation form one electron system with six atoms and six electrons. 

Lewis s concept of shared electron parr bonds allows for four electron double bonds and SIX electron triple bonds Carbon dioxide (CO2) has two carbon-oxygen double bonds and the octet rule is satisfied for both carbon and oxygen Similarly the most stable Lewis structure for hydrogen cyanide (HCN) has a carbon-nitrogen triple bond... 

The green oxygen m Figure 1 5 owns three unshared pairs (six electrons) and shares two electrons with nitrogen to give it an electron count of seven This is one more than the number of electrons m the valence shell of an oxygen atom and so its formal charge is —1... 

Boron trifluoride is a trigonal planar molecule There are six electrons two for each B—F bond associated with the valence shell of boron These three bonded pairs are farthest apart when they are coplanar with F—B—F bond angles of 120°... 

There is ample evidence from a variety of sources that carbocations are mterme diates m some chemical reactions but they are almost always too unstable to isolate The simplest reason for the instability of carbocations is that the positively charged car bon has only six electrons m its valence shell—the octet rule is not satisfied for the pos itively charged carbon... 

The chief reason why ethylenebromonium ion m spite of its strained three membered ring IS more stable than 2 bromoethyl cation is that both carbons and bromine have octets of electrons whereas one carbon has only six electrons m the carbocation... 

Of the two resonance forms A and B A has only six electrons around its positively charged carbon B satisfies the octet rule for both carbon and oxygen It is more stable than A and more stable than a carbocation formed by protonation of a typical alkene... 

The picture of benzene as a planar framework of ct bonds with six electrons m a delo cahzed rr orbital is a useful but superficial one Six elecfrons cannof simulfaneously occupy any one orbifal be if an afomic orbifal or a molecular orbifal We can fix fhis wifh the more accurate molecular orbital picture shown m Figure 114 We learned m Section 2 4 that when atomic orbitals (AOs) combine to give molecular orbitals (MOs) the final number of MOs musf equal fhe original number of AOs Thus fhe six 2p AOs of SIX sp hybridized carbons combine fo give six tt MOs of benzene... 

Oxygen lone pair cannot be used to stabilize positive charge in any of these stmctures all have six electrons around positively charged carbon... 

The transition state for the first step of the Claisen rearrangement bears much m common with the transition state for the Diels-Alder cycloaddition Both involve a con certed six electron reorganization... 

Nine electron pairs p to Six electron pairs p to Three electron pairs p to... 

Triple bond (Section 1 4) Bond formed by the shanng of six electrons between two atoms... 

In this titration the analyte is oxidized from Fe + to Fe +, and the titrant is reduced from CryOy to Cr +. Oxidation of Fe + requires only a single electron. Reducing CryOy, in which chromium is in the +6 oxidation state, requires a total of six electrons. Conservation of electrons for the redox reaction, therefore, requires that... 

Controlled-potential coulometry also can be applied to the quantitative analysis of organic compounds, although the number of applications is significantly less than that for inorganic analytes. One example is the six-electron reduction of a nitro group, -NO2, to a primary amine, -NH2, at a mercury electrode. Solutions of picric acid, for instance, can be analyzed by reducing to triaminophenol. 

The ground configuration of benzene is obtained by feeding the six electrons, which were originally in 2p AOs on the carbon atoms (z axis perpendicular to the ring), into the lower energy MOs giving... 

The low current efficiency of this process results from the evolution of hydrogen at the cathode. This occurs because the hydrogen deposition overvoltage on chromium is significantly more positive than that at which chromous ion deposition would be expected to commence. Hydrogen evolution at the cathode surface also increases the pH of the catholyte beyond 4, which may result in the precipitation of Cr(OH)2 and Cr(OH)2, causing a partial passivation of the cathode and a reduction in current efficiency. The latter is also inherently low, as six electrons are required to reduce hexavalent ions to chromium metal. 

Here, with six electrons involved, it is the disrotatory mode (Hiickel system) in which the transition state is stabilized. There are numerous examples of interconversion of 1,3,5-... 

Notice that the orbital array is of the Mobius topology with a phase change depicted between the C-1 and C-2 positions. This corresponds to an allowed photochemical process since there are six electrons involved in bonding changes. 

Carbon has six electrons around the atomic core as shown in Fig. 2. Among them two electrons are in the K-shell being the closest position from the centre of atom, and the residual four electrons in the L-shell. TTie former is the Is state and the latter are divided into two states, 2s and 2p. The chemical bonding between neighbouring carbon atoms is undertaken by the L-shell electrons. Three types of chemical bonds in carbon are single bond contributed from one 2s electron and three 2p electrons to be cited as sp bonding, double bond as sp and triple bond as sp from the hybridised atomic-orbital model. 

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

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