Cations with isomers compared

The behavior of the isomeric dihydronaphthalenes emphasizes the importance of the relative stabilities of carbocation intermediates in ionic hydrogenations. Treatment of 1,2-dihydronaphthalene with Et3SiH/TFA at 50-60° gives a 90% yield of tetralin after one hour. Under the same conditions, the 1,4-dihydronaphthalene isomer gives less than 5% of tetralin after 70 hours.224 This difference in reactivity is clearly related to the relatively accessible benzylic cation formed upon protonation of the 1,2-isomer compared to the less stable secondary cation formed from the 1,4-isomer.224... 

Distinct evidence for the equilibration of bicyclobutonium with a minor isomer, bisected cyclopropylcarbinyl cation, comes from the ultra-low temperature CPMAS studies of Myhre, Webb and Yannoni25. They have observed a major isomer, the bicyclobutonium ion, with a l3C chemical shift of 15 ppm for the pentacoordinated carbon, and a minor bisected cyclopropylcarbinyl cation, whose cationic center s chemical shift was found to be at 235 ppm. The NMR chemical shifts of the cation are also comparable with those calculated by the IGLO method at that temperature26 27. The energies of these cations were shown to be nearly the same (AAH° = 0.05 kcalmol1). 

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

The plausible mechanism is based on the proposal by Jana and coworkers (Scheme 14). In this case, the sp-hybridized vinyl cation can be attacked by halide, instead of water, to give the ElZ isomer of the alkenyl halide. Compared with the systems using stoichiometric Lewis acid and strong base to prepare substituted alkenyl halides, the present method would provide an excellent alternative due to the environmentally benign system and atom efficiency. 

Detailed structural calculations have been carried out for this system. This is because the neutral isomer, C2HsO, which is implicated in the thermochemistry of ethanol, is of interest in pollution control, atmospheric chemistry, and combustion. Also, there is new information available from photoionization experiments with which to compare theoretical calculations. For details of these comparisons, see Curtiss et al.73 In the earlier theoretical studies of Nobes et al.,74 calculations were performed at the MP2 and MP3 levels with basis sets of double plus polarization (6-13G ) with electron correlation. These studies revealed four stable minima for the system protonated acetaldehyde, CHj-C H-OH <-> CH3-CH=0+H the methoxymethyl cation, CH3OCH2 protonated oxirane, (CH2)2OH+ and vinylox-... 

L-Cysteine is a high value a-amino acid used world-wide in a scale of 1200-15001 year-1 as additive in foodstuffs, cosmetics or as intermediate or active agent (as antidote to several snake venoms) in the pharmaceutical industry. Chemical routes generally lack the efficiency of electrochemical techniques, or they produce mixtures of l- and d- forms rather than the L-isomer. The most common electrochemical route is the cathodic reduction of L-Cystine in acid (usually HC1) solution to produce the stable hydrochloride. In Table 10, the charateristic data for a laboratory bench, laboratory pilot and a product pilot reaction using a DEM filter press are compared [13]. A production scale study was carried out in a filterpress reactor divided by a cation exchange membrane with a total area of 10.5 m2. The typical product inventory was 450 kg/24-hour batch time. For more details see Ref. [13]. 

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