Carbenium ions phenyl

The longer the chain of unbranched carbenium ions is, the more the calculated values deviate from those found experimentally in the direction of higher stability. However, the expected order of ion stability (primary < secondary < tertiary) remains intact. For cations, which are able to delocalize the positive charge due to conjugation in phenyl rings, the calculated stability is too small. The example of the acetyl cation shows that the reliability of the MINDO/3 method decreases, if charged species, especially those containing hetero atoms with free electron pairs, are calculated. 

We report on the measurement of the propagation rate constants kp of styrene, indene, phenyl vinyl ether (PhViE) and 2-chloroethyl vinyl ether (CEViE) in nitrobenzene at (mostly) 298 K with 4-ClC6H4CO+SbF 6 as initiator. The dependence of the conductivity on the [4-ClC6H4CO+SbF"g] = c0 helped to establish that [Pn+] = c0 and thus to validate the foundation of this work. It is shown that most probably the propagating species are the uncomplexed, unpaired, solvated carbenium ions. Some new enthalpies of polymerisation have been found. 

The regioselectivity in the fluorination of phenyl-substituted alkenes 9 has been reported and in the case of an electron-withdrawing group (C02Me, Ac) and a donating group (4-OMe) a more stable carbenium ion is formed and complete regioselectivity is observed, but it is completely lost in the reaction with the chloro derivative.2... 

We also obtained NMR spectra of the phenylindanyl cation 13 in the large-pore zeolite HY, and a small amount of cation 14 was formed on HZSM-5 by dimerization of a-methylstyrene. The dimethylphenyl carbenium ion 15 was not persistent on any zeolite we examined. This is not surprising if one reads the solution acid literature. 15 cannot be observed in 100% H2S04 stabilizing this cation requires 30% oleum (S03/H2S04) or other superacids (115). HZSM-5 is not a superacid. The observation of the much less stable styryl cation 11 was hailed as a triumph of superacid solution chemistry (116). If the styryl cation, with the phenyl group provid-... 

General support for this mechanism was found in the cyclization of bis(4-phenyl-3-butenyl disulfide) (a Scheme 4) which formed 3-iodo-2-phenyltetrahydrothiophene (e Scheme 4) when treated with iodine (71IJS(A)(1)39>. The intermediate carbenium ion (b) is not stabilized by loss of a proton, since it lacks the driving force of aromaticity, but rather reacts with... 

The rearrangement of light and deuterium-labelled cis- and trans-2-methyl-3-phenyloxiranes (1, 2 and 1, 2 ) was studied on ZnO, Al-O-j and WO, and in the presence of BF,. Both in the gas phase (473-673 K) and in the liquid phase (298-413 K), l-phenyl-2-propanone (3) and 2-phenylpropanal (4) were formed with high selectivities (0-90% and 11-80%, respectively). Ring-opening was found to occur by selective fission of the benzyl C-0 bond. Mechanistic studies revealed the formation of an open carbenium ion or a double-bonded surface intermediate. The acidic (electrophilic) and basic characters of the oxides determine the product distributions by affecting the relative importance of the competing mechanisms. 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

There are a number of other aryl-substituted carbodicationic systems that can be properly described as distonic superelectrophiles. For example, dication 45 has been generated from 2,2 -p-phenylenedi-2-propanol in SbFs at —78°C.4 When compared to the dimethyl(phenyl)carbenium ion (cumyl cation) 46, NMR data indicate that the positive charges are dispersed to a considerable extent into the neighboring methyl groups in the dication 45. 

Zeolite beta has also been used for the racemization of secondary phenylic alcohols in a dynamic kinetic resolution however, in this case water elimination/ addition via a carbenium ion is involved rather than a redox mechanism [75, 76]. 

It has been demonstrated that 1 -phenyl-4- J-butyIcyclohexene is a very appropriate olefin for studying the stereochemistry of fluorination or halofluorination, where isomerization in the primarily formed cyclohexyl fluoro carbenium ion could be avoided by introduction of a J-butyl group67. Acid-catalyzed liquid-phase fluorine addition resulted in equal amount of cis and trans adducts, with the nature of the catalyst little influencing the stereochemistry, while introduction of a methoxy group into the phenyl ring also had no significant effect on the stereochemical course of addition (Scheme 18). 

Of course, a NMR chemical shift value of 225 ppm can be interpreted in different ways. One can assume that 61 represents a free silylium cation in solution that because of considerable -conjugation between 3pn(Si) orbital and the three phenyl rings is internally stabilized and, therefore, does not interact with solvent molecules. The question in this case would be to which extent 11-conjugation has reduced the silylium cation character and how much positive charge is spread over the C framework. It could be that 61 represents more a carbenium ion in the same way as the silaguanidinium ion represents more an ammonium ion (see section 4.1). 

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