Molecular orbitals carbanions

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

Reduction of benzenoid hydrocarbons with solvated electrons generated by the solution of an alkali metal in liquid ammonia, the Birch reaction [34], involves homogeneous electron addition to the lowest unoccupied 7t-molecular orbital. Protonation of the radical-anion leads to a radical intermediate, which accepts a further electron. Protonation of the delocalised carbanion then occurs at the point of highest charge density and a non-conjugated cyclohexadiene 6 is formed by reduction of the benzene ring. An alcohol is usually added to the reaction mixture and acts as a proton source. The non-conjugated cyclohexadiene is stable in the presence of... 

Problem 8.28 (a) Apply the MO theory to the allyl system (cf. Problem 8.26). Indicate the relative energies of the molecular orbitals and state if they are bonding, nonbonding, or antibonding, (b) Insert the electrons for the carbocation C,H, the free radical C,H, and the carbanion CjH, and compare the relative energies of these three species. 

Many such examples are known. In most cases where the stereochemistry has been investigated, retention of configuration is observed,225 but stereoconvergence (the same product mixture from an E or Z substrate) has also been observed,226 especially where the carbanionic carbon bears two electron-withdrawing groups. It is not immediately apparent why the tetrahedral mechanism should lead to retention, but this behavior has been ascribed, on the basis of molecular orbital calculations, to hyperconjugation involving the carbanionic electron pair and the substituents on the adjacent carbon.227... 

Ab initio and semiempirical molecular orbital calculations have been used, together with charge-transfer theories, to investigate the structures of organodioxide anions and related charge-transfer complexes between carbanions and molecular oxygen.219... 

Photoelectron studies of (CH3)3P=CH2 have provided data on the energy of the highest occupied molecular orbital and some of the lower-lying states (4-9). The frontier orbital energy of 6.81 eV is very low and seems to illustrate the carbanionic nature of the carbon, where this orbital is largely localized. This is borne out by semiquantitative CNDO/2 calculations on this molecule. The orbital sequence obtained is satisfactory according to ab initio results. 

The photoelectron spectrum of (CH3) 3ASCH2 shows an energy of only 6.72 eV for the highest occupied molecular orbital, which is predominantly a lone pair localized at the carbanion (69). This energy is lower than in the phosphorus compound (6.81 eV). This result is at least qualitatively in agreement with the picture drawn above. 

What are the geometries of carbon radicals, and how do they differ from those of carbenium ions or carbanions And what types of bonding are found at the carbon atoms of these three species First we will discuss geometry (Section 1.1.1). and then use molecular orbital (MO) theory to provide a description of the bonding (Section 1.1.2). 

A rationale for the regiochemistries observed for the polychloroarene radical anions may be developed by considering the transition states for the two competing processes (Scheme 8). The loss of chloride ion in route (a) generates phenyl radical. The transition state for this process would, therefore, be expected to exhibit some radical localization at C-l. The shape of the transition state might be expected to be bent rather than planar, since heterolytic fission of the carbon-chlorine bond in a coplanar transition state would lead to an excited state (a phenyl cation with an extra electron in the n molecular orbital), while heterolytic fission of a bent system (such as C) could lead directly to a phenyl radical. Thus, the transition state for route (a) might very well possess some of the character of a delocalized anion with a bent localized radical center (C), while the transition state for chlorine atom loss, by a similar argument, would resemble a delocalized radical with a bent localized carbanionic center (D). 

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