Bonded phase Methylene selectivity

Gilpin, R. K., Jaroniec, M., and Lin, S., Dependence of the methylene selectivity on the composition of hydro-organic eluents for reversed-phase liquid chromatographic systems with alkyl bonded phases, Chromatographia, 30,393, 1990. 

The polarity values of binary acetonitrile/water and methanol/water mobile phases used in RPLC were measured and compared with methylene selectivity (acH2) for both traditional siliceous bonded phases and for a polystyrene-divinylbenzene resin reversed-phase material [82], The variation in methylene selectivity for both was found to correlate best with percent organic solvent in methanol/water mixtures, whereas the polarity value provided the best correlation in acetonitrile/water mixtures. The polymeric resin column was found to provide higher methylene selectivity than the siliceous-bonded phase at all concentrations of organic solvent. 

Phenyl-Type Phases. Phenyl-type phases have been studied for a long time [58,59]. The presence of a phenyl ring on the surface of a bonded phase introduces so-called n-n interactions with some analytes that are capable of these types of interactions. This introduces an additional specificity for HPLC separations on these stationary phases. Compared to common alkyl-type phases, phenyl columns show lower methylene selectivity in other words, the separation of members of homologous series will be less selective on phenyl columns than on alkyl-modified phases. 

An original study was done with a perfluoiinated stationary phase whose properties were compared to a classical CIS bonded phase [27, 28]. An important decrease of the methylene selectivity for a homologue series of alkylphenones was obtained with the fluorinated phase with both an SDS... 

The curvature in the log k vs. n plots, observal with micellar and hybrid mobile phases, was first attributed to the different locations (with different microenvironment polarities) in the micelle, for different members of a homologous series [6, 7], Methylene selectivity decreases as the difference between mobile and stationary phase polarities is reduced. For a given mobile phase composition, the larger and more hydrophobic homologous compounds are located in a less polar environment of micelles, it is then conceivable to assume that these compounds experience a smaller change in their microenvironment polarity upon being transferred from the micellar pseudo-phase to the bonded alkyl stationary phase. The a(CH2) value between n-pentylbenzene and n-butylbenzene is smaller than it is between ethylbenzene and toluene (Table 9.1), because the former pair is located in a more nonpolar environment than the latter. 

Dehydrofluorination by primary and secondary aliphatic amines occurs at room temperature and is the basis of diamine cross linkmg, which occurs by dehydrofluonnation and subsequent nucleophihc substitution of the double bond The locus of dehydrofluonnation is a VDF unit flanked by two perfluoroolefin units This selectively base-sensitive methylene group also undergoes elimination as the first step in phase-transfer-catalyzed cross-hnking with quaternary ammo mum or phosphomum salts, bisphenols, and morganic oxides and hydroxides as HF acceptors [31, 32]... 

C. A rule of thumb in small-molecule GPC is that it is possible to analyze two compounds whose molecular weight differs by 10% or more by the judicious selection of the mobile phase. For instance, the separation of tolnaftate (the active ingredient of an antifungal preparation) (MW = 302) and BHT (which is present as an antioxidant) (MW = 220) can be accomplished using methylene chloride, a nonhydrogen bonding solvent as shown in Figure 11-4. For this assay, sample prep-... 

Early work on the gas-phase interaction of methylene with carbon-hydrogen bonds showed selective reactivity in the order, tertiary C-H > secondary C-H > primary C-H. Methylene from diazomethane was found to be less selective than methylene from ketone . The differences in the relative reactivity of methylene from the two sources have been attributed to excess translational energy of methylene from CH2N2 . Recently, however, it has been shown that the photolysis of ketene produces more triplet CH2 than the photolysis of diazomethane cf. preceeding section). Part of the reported selectivity must be attributed to triplet-methylene effects. 

Whereas a small but significant selectivity of methylene in gas-phase reactions is well established, the evidence for liquid-phase reactions is less definitive. Studies using diazomethane , phenylcyclopropane , or 9,10-dihydro-9,10-methanophenanthrene as the CHg precursor have been interpreted as indicating random attack on primary, secondary and tertiary C H bonds. However, some results for the liquid phase indicate slight selectivity , and recent liquid-phase experiments with isopentane show even greater selectivity at the tertiary position than does singlet-methylene insertion in the gas phase. 

For micelles, the typical selectivities for alkylphenones mid alkylbenzenes are between 1.1 and 1.6 for SDS concentrations in the 0.06-0.5 M range, which is similar to the values observed for organic-rich eluents in aqueous-organic systems. The overall smaller a(CH2) in MLC is an indication of how closely the environment of a methylene group in the micellar mobile phase resembles that of the alkyl-bonded stationary phase. 

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