Carbonium ions , spectroscopic

Carbonium ions (alkyl), spectroscopic observation in strong acid solutions, 4, 305... 

A detailed spectroscopic examination should settle the question of whether the ion has the open or the cyclic structure. In general halo-chromic salts lose their color when a covalent bond is established to the central carbon atom, but the bromonium ion might resemble the carbonium ion. Compounds of similar color but which are certainly not cyclic halonium ions are also known ... 

A. Gandini, P. H. Plesch, Spectroscopic Studies on Carbonium Ions Derived from Aromatic Olefins, Pt. I. Styrene and related Olefins, J. Chem. Soc., 1965, 4765. 

Since the determination of absolute rate constants is one of the most urgent problems in cationic polymerization, and the styrene-perchloric acid system seemed to be so clean and simple, Gandini and Plesch set out first to check Pepper and Reilly s results by determining spectroscopically the concentration of carbonium ions during polymerization, and they intended then to extend the method to other monomers. However, their findings were not as expected. A comparison of spectroscopic and conductivity measurements with rate measurements in an adiabatic calorimeter showed [4] that in methylene dichloride solution ... 

We have shown [1, 2] that, in the polymerisation of styrene by perchloric acid under the conditions reported here, the initiation reaction does not produce carbonium ions and that the monomer is polymerised by non-ionic chain carriers. Since the most likely nonionic reaction product formed from perchloric acid and styrene is the ester 1-phenylethyl perchlorate we attempted its preparation in order to try it as catalyst for the polymerisation of styrene. However, we found this ester to be unstable in methylene dichloride solution. It forms styrene oligomers, polystyryl ions, and perchloric acid, and the preparative technique and the mechanism of the reactions involved will be discussed in a paper dealing with the spectroscopic behaviour of polymerising and polymerised systems. 

Pepper and Reilly s views on the mechanism of this polymerisation implied that with the concentrations of perchloric acid used by them, it should be possible to estimate the concentration of polystyryl ions spectrophotometrically and so to test whether initiation did indeed give carbonium ions in concentration equal to that of the acid. When Gandini and Plesch [5, 29-31] carried out the appropriate measurements, they found from spectroscopic, conductimetric and kinetic studies that no ions were present during the polymerisations, but that they were formed once the styrene concentration had been reduced by polymerisation to less than four times the concentration of acid. Addition of more styrene instantly removed the ions, which reappeared once again when polymerisation had reduced the styrene concentration sufficiently. This formation of ions after polymerisation had misled some workers into concluding that they were also present during that reaction. 

Our evidence concerning the effect of water on carbonium ions shows convincingly that, since the rate of polymerisation is not affected by relatively large quantities of water [22], it is most improbable that carbonium ions present during the polymerisation, at concentrations too low to be detected by our spectroscopic technique, could be responsible for the propagation. If that had been so, no polymerisation would have been obtained with... 

Spectroscopic Studies on Carbonium Ions derived from Aromatic Olefins. Part II. Acenaphthylene, N-Vinylcarbazole, and Tetraphenylethylene, A. Gandini and P.H. Plesch, Journal of the Chemical Society, (B), 1966, 7-10. 

The last years have seen a rapid development of the direct spectroscopic observation of stable carbonium ion intermediates in strongly acidic solutions. Thus many alkylcarbonium ions, previously suggested... 

A further statement in this same article explains the important addition made by modem analytical equipment "... the rapid development of spectroscopical methods of analysis, of which nuclear magnetic resonance spectroscopy was the most important, has enabled Deno (25) and many others to show that carbonium ions can now be directly observed in solution. This has made it possible to determine the structure of such ions. Deno states that the HSO4 salts of substituted cyclopentenylca-tions form the sludge in commercial alkylation acid. He did not encounter any cations possessing less than 10 carbon atoms. "... 

In an attempt to identify the chromophoric systems resulting from reactions of steroids with strong acids ( colour reactions Kober, Allen, Oertel, Talbot, Salkowski, and Liebermann-Burchard reactions) a detailed study has been made, with use of all the usual spectroscopic techniques. Visible colours are attributed generally to charge-transfer between the steroid, as donor, and a delocalized carbonium ion, generated from the steroid in acidic media, acting as acceptor. The structures of the steroidal carbonium ions are discussed. 

Catalytic reactions of hydrocarbons over zeolites are reviewed. The historical development of various mechanistic proposals, particularly of the carbonium ion type, is traced. In spite of numerous catalytic, spectroscopic, and structural studies which have been reported concerning the possible roles of Bronsted acid, Lewis acid, and cationic sites, it still is not possible to formulate a comprehensive mechanistic picture. New activity and product data for cumene cracking and isotope redistribution in deuterated benzenes over Ca-and La-exchanged Y zeolites is presented. Cracking of the isomeric hexanes over alkali metal-exchanged Y and L zeolites has been studied. This cracking is clearly radical rather than carbonium-ion in nature but certain distinct differences from thermal cracking are described. 

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