Benzyl anion and cation

Benzene oxide-oxepin equilibrium, 326-27 Benzbydrol, 397 Benzocyclobutene, 3S0, 433 Benzonorbonadienes. substituted. 437 Benzophenone.267-68. 407, 424, 467 Jablonski diagram, 232 oxciane formation, 407, 424 phoioreduction, 397-98,467 as sensitizer, 294, 367, 407 substituted. 32 Benzopinacol, 397 Benzoyloxy chromophore, 134 Benzvaiene, 264-63, 302,448-31 Benzyl anion and cation, 171 Benzyl radical, 102 Biacctyl, 266. 291, 423,469 Jablonski energy diagram, 231 9,9 -Bianthryl, 48... 

Because the 7t-charge densities q of neutral AHs (including odd AH radicals such as benzyl) are unity on all atoms (rule 5), the excess charges q - 1 of the benzyl anion and cation obtained by adding or removing an electron to or from the NBMO are equal t0 -( 2nbmo.m and 4BMOji, respectively. 

It is well known that the tricarbonylchromium-complexed benzylic anions and cations are stabilized due to overlapping between d-orbital of the chromium and p-orbital of the benzylic carbon [1]. Tricarbonylchromium complexes of a-te-tralone and a-indanone having a carbonyl group at the side chain underwent a deprotonation of the exo-benzylic protons by treatment with base to give the stereo-controlled tricyclic compounds (Eqs. 1 and 2) [2]. In these cases, Robinson annulation products were formed in less than 10% yield. Also, base treatment of benzyl ether chromium complex having a chlorine at the side chain 3 gave cyclization product as a diastereomeric mixture (Eq. 3) [3]. 

Consider the benzyl anion and cation, C6H5CH2". The NBMO coefficients in benzyl are easily found [see (7)] by the method of Section 3.3. From these, one arrives at the distributions of formal charge for the anion and cation indicated in (8) and (9), respectively. 

Although the ion pairs of a-substituted benzyl anions and the corresponding cations are chiral species, which, in addition, often bear a pyramidal and hence stereogenic carbon atom, in most cases rapid racemization of the alkali and alkaline earth metal derivatives occurs in solution... 

The relative stability of benzylic radicals and cations makes photoinduced C-X cleavage of benzylic derivatives a widespread occurrence, and in fact according to different modes. These are i) fragmentation of an excited state, formed by direct irradiation or via energy transfer sensitization ii) atom abstraction by a chemical sensitizer iii) fragmentation of a benzylic radical cation or anion generated via electron transfer sensitization or quenching (Sch. 2). 

It was found in this experiment that both anionic and cationic species reacted efficiently with methanol in bulk styrene. The bonded dimer cations and the radical anions were converted to long-lived benzyl radicals, which initiated the radical polymerization. The G value of the propagating benzyl radical was only 0.7 in pure styrene, but it increased up to 5.2 in the presence of methanol. A small amount of methanol converted almost all the charge carriers to propagating free radicals this explains why the mechanism of radiation-induced polymerization is changed drastically from cationic to radical processes on adding methanol. 

Spectra and kinetics were also determined for many other species. The solvated electron was observed and its spectrum was determined in a wide variety of solvents, from ethers and alcohols to hydrocarbons and even supercritical fluids. Other radicals, including the benzyl radical, the first species studied in pulse radiolysis, were observed. Excited states, both singlet and triplet, anions and cations, were determined for aromatic species. The number and variety of species is large. The importance of these studies was that it was now possible to observe the intermediate states in the radiation-chemical reactions and thus confirm or refute reaction mechanisms that had been proposed based on product yield data. 

The microspheres mentioned above are all spherical and no change of the diameter and aggregation of the microspheres takes place during the reaction of surface modification. The surface charge of every microsphere can be determined by electrophoresis. For instance, the zeta potentials of our cellulose triacetate, Cell-OH, crosslinked Cell-OH, Cell-CM, Cell-SE, Cell-NHa, Cell-DEAE, Cell-DEAE(Me), and benzyl cellulose microspheres were —19.9, —2.1, —2.7, —17.1, —20.9, +4.6, +14.2, +15.1, and —65.2 mV, respectively. This result indicates that anionic and cationic microspheres with the same average diameter but different surface charges can be prepared by this method. 

The fact that the benzyl anion and benzyl cation should have opposite MCD signs according to the mirror-image theorem makes it easy to understand why benzene derivatives with mesomeric donor substituents, which are isoelectronic with the benzyl anion, and benzene derivatives with mesomeric acceptor substituents, which are isoelectronic with the benzyl cation, show opposite MCD signs. This fact can be used for a qualitative and even a quantitative characterization of mesomeric substituent effects. (See also Section 2.4 and Example 3.10.)... 

Fig. 3-11 Total rc-electron population (and excess-charge distribution) in the benzyl anion and the benzyl cation...

Radicals are species with at least one unpaired electron, which, in contrast to organic anions and cations, react easily with themselves in bond-forming reactions. In the liquid phase, most of these reactions occur with diffusion-controlled rates. Radical-radical reactions can be slowed only if radicals are stabilized by electronic effects (stable radicals) or shielded by steric effects (persistent radicals). However, these effects are not strong enough to prevent diffusion-controlled recombination of, for example, benzyl radicals or tert-butyl radicals.1 Only in extreme cases are the radical or di-tert-butylmethyl radical recombination rates low.2 While the recombination rates of the triphenyl-methyl radical is reduced due to both steric and radical stabilizing effects, the steric effect alone slows the recombination of the di-/t>/-/-butyl methyl radical. Since neither of the radicals have C-H bonds (I to the radical centre, disproportionation reactions, in which the hydrogen atom is transferred, cannot occur. 

Unfortunately, since a true experimental value for the heat of formation of thiazole is not known, it is not possible to evaluate the accuracy of the calculated values. However a close practical estimation of an experimental AHf° for thiazole can be obtained (see Section 3.06.4). Dewar s AMI semiempirical MO method has also been used by Bean <93JOC7336> to calculate the heats of formation and charges in the 4- and 5-substituted-2-methyl thiazoles and their benzyl-like anions. This study afforded also proton and hydride ion affinities of the benzyl-like anion and cation of 2-methylthiazoles (see Section 3.06.7.3.2). 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

Figure 3.10 Energy costs for the 90° rotation in the (a) allyl radical, cation, and anion (b) the benzyl radical and cation.

Cozens, F. L., Ortiz, W., and Schepp, N. R, Direct observation of the benzyl radical and the benzyl anion within cation-exchanged zeolites a nanosecond laser study, /. Am. Chem. Soc., 120, 13543, 1998. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Because allyltrimethylsilane 82 or benzyltrimethylsilane 83 can be regarded as combinations of the hard trimethylsilyl cation and the soff allyl or benzyl anions, pyridine N-oxide 860 reacts with excess 82 or 83 in the presence of catalytic amounts of tetrabutylammonium fluoride di- or trihydrate in THF to give 2-allyl-or 2-benzylpyridines 948 and 950 [60]. The general reaction of silicon reagents such as 82 and 83 or of trimethylsilyl cyanide 18 with fluoride to generate allyl or... 

The semiempirical AMI MO method has been used to calculate heats of formation of a series of m- and p-substituted benzene and toluene derivatives ArY and ArCHaY, and their phenyl or benzyl cations, anions, and radicals heterolytic and homolytic bond dissociation energies (BDEs) and electron transfer energies for the ions have also been calculated and the relationship A//het = A//et-I-AWhomo has been confirmed (it being noted that A//homo is insensitive to ring substituents). The linear relationship found between and the appropriate HOMO or LUMO... 

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