Alcohol modifiers

Formaldehyde. Pure formaldehyde, CH2O, is a colorless, pungent smelling reactive gas (see Formaldehyde). The commercial product is handled either as soHd polymer, paraformaldehyde (13), or in aqueous or alcohoHc solutions. Marketed under the trade name Formcel, solutions in methanol, / -butanol, and isobutyl alcohol, made by Hoechst-Celanese, are widely used for making alcohol-modified urea and melamine resins for surface coatings and treating textiles. 

In one study, various distinct types of polar modifiers to n-hexane were tested for 3-chloro-l-phenylpropanol (3CPP) and 1-phenylpropanol (IPP) enantiomer separation [53]. Thereby, alcohol modifiers turned out to be more effective displacers of the solutes from the adsorption places on the sorbent surface, yet aprotic polar modifiers provided higher separation factors (with ethyl acetate in n-hexane affording the best separations for these chiral alcohols). It is evident, though, that the optimal choice of polar modifier is strongly solute dependent and can therefore not be generalized. 

FIGURE 4.8 Chiral selectivity as a function of amount and type of alcohol modifiers in the mobile phase in SFC. Conditions Al-(2-heptyl)-p-tolylamide enantiomers as the analyte 250 mm X 4.6 mm ID column, 10 xm silica particles coated with celhtlose trihenxoate carhon dioxide and various types and amounts of modifiers = methanol, o = ethanol, A = 1-butanol, x = 2-propanol, and = 2-butanol 25°C flow rate 4.5 mL/min at 0°C UV detection at 229 nm average column pressure 140 bar. (Reprinted from Macaudiere, R et al. 1989. J. Chromatogr. Sci. 27 383-394. With permission.)... 

Because of the problems encountered with the water system, the use of aliphatic alcohols, ie.g., methanol, ethanol, and isopropanol, as modifiers of the adsorption strength has been recommended (44. 45. 50. 51). Usually, between 0.01 and 0.5% (v/v) alcohol is added to the eluent. As an example, the k values for the benzyl alcohols on a silica column are in the same range when eluted with dichloromethane containing either 0.1% water (50% water-saturated) or 0.15% methanol or 0.3% isopropanol (45). The preparation and preservation of these alcohol-eluent mixtures is accompanied by problems similar to those discussed with water-modified eluents. Also, column equilibration is slow (44). The efficiency of columns operated with alcohol-modified eluents is generally lower than that of water-modulated eluent system. At some alcohol concentrations, distorted peaks with tailing or frontal asymmetry have been observed 44), but olhei workers using another silica could not verify this observa tion (61). 

The alcohols may have inhibited the full recovery of the herbicides under these conditions by providing a liquid phase which could solublize the herbicides. The herbicides could then partition themselves between the liquid alcohol phase and the C02/alcohol phase. This would lead to the herbicides being retained on the celite until the all of the alcohol modifier was removed by the CO2 extraction fluid. 

Temperature and pressure are rarely optimized in HPLC, but these parameters are very important in SFC, hence can alter retention, selectivity, and resolution. Toribio et al. [149] presented the chiral separation of ketoconazole and its precursors on Chiralpak AD and Chiralcel OD CSPs. The authors also reported that alcohol modifiers provided better separation than acetonitrile. Further, Wilson [143] studied the effects of composition, pressure, temperature, and flow rate of the mobile phase on the chiral resolution of ibuprofen on a Chiralpak AD CSP. It was observed that temperature affords the greatest change in resolution, followed by pressure and composition. An increase in methanol concentration, pressure, and temperature has resulted in poor chiral resolution. At first chiral resolution increased with an increase of flow rate (up to 1.5 mL/min) but then started to decrease. Contrary to this, Biermann et al. [135] described the... 

The cesium distribution ratio afforded by alcohol modifiers, such as 1, dissolved in hydrocarbon diluents, such as Isopar L or dodecane, increased dramatically as the concentration of the modifier increased from below 0.10 M to 1 M, then leveled off. 

This general phenomenon was observed when extracting cesium from neutral sodium nitrate as well as from alkaline solutions. In 1,2-DCE, however, the increase in DCs is less pronounced (possibly due to 1,2-DCE s higher polarity, which affords moderate DCs at low modifier concentrations). Nevertheless, both DCs and lij increase as a function of the concentration of the alcohol modifier 1, as shown in Table 4.18. 

Delmau, L. H., Lefranc, T. J., Bonnesen, P. V., Bryan, J. C., Presley, D. J., and Moyer, B. A. Fundamental Studies Regarding Synergism Between Calix[4]arene-bis(ferf-octylbenzo-crown-6) and Alcohol Modifiers in the Solvent Extraction of Cesium Nitrate, Solvent Extr. Ion Exch., in press. 

Alkene esters on the other hand do not require protection as they appear to react primarily via the alkene end, allowing standard chemical transformations to be carried out as summarized in Fig. 5. For example, the terminal esters can be hydrolyzed to form a carboxylic acid terminated surface, or reduced with LiAlfD to form an alcohol modified surface [28]. A concern in carrying out these transformations is that the rather harsh conditions required have the potential to degrade the silicon substrate. Boukherroub and Wayner demonstrated further chemical manipulation of... 

Figure 4. Difference spectra for heat-denatured and rennin/alcohol-modified soy 7S and 11S protein fraction. (-----) Heat denatured, (----) enzyme modified.

The disruption of the hydrophobic interior and exposure of the tyrosine residues in the enzyme and alcohol-modified protein is revealed by the appearance of the positive peaks in the region of 276-278 nm and 284 nm in the UV difference spectrum (Figure 3). The subsequent refolding to a helical or other ordered structure is shown by the strong positive peak at 233 nm. 

With increasing dodecyl amine, the surface area of the catalyst and, hence, the conversion decreased, whereas the selectivities for formation of methacrylic acid and methacrolein increased. The increase in selectivity was ascribed to a reduction in lattice oxygen mobility in the dodecyl alcohol-modified materials, which decreased the amount of COx produced. 

Pirkle-type phases are amino acid derivatives possessing an aromatic entity which can undergo n-n interactions with the solute. The aromatic entity can be either a n donor or n acceptor. The CSP and the solute form a n donor/acceptor pair. This complex is then stabilized by additional interactions such as hydrogen bonding, dipole interactions, or steric repulsion [8]. The Pirkle-type phases are most commonly used in normal-phase mode in order to enhance the n-n and hydrogen bond interactions. Hexane with an alcoholic modifier, such as isopropanol, is the mobile phase of choice. These phases have... 

Figure 1 Representative chiral alcohol modifying agents...

Chiral Amino Alcohol Modifying Agents. A number of chiral amino alcohols have been examined as ligands for the preparation of chiral LAH reducing agents (Figure 2). The complex of (—)-iV-methylephedrine (11) with LAH has been widely studied and has shown promise for the asymmetric reduction of prochiral ketones. It has been found that addition of an achiral component such as 3,5-dimethylphenol (DMP), iV-ethylaniline (NEA), or 2-ethylaminopyridine (EAP) to the complex of LAH with (11) can enhance the enantioselectivity observed in these reductions. Both enantiomers of (11) are commercially available and the ligand can be recovered subsequent to the reaction and reused. 

Primary or secondary amine and amino or monoalkylamino alcohol modifiers 168... 

Most reactions have been conducted using readily available, naturally occurring, chiral modifiers. The auxiliaries employed for the modification of LAH are classified into three types (i) alcohol modifiers (ii) dialkylamino alcohol modifiers and (iii) primary or secondary amines and amino or monoalkylamino alcohol modifiers. Most asymmetric reductions have been investigated with acetophenone (1) as the substrate. Structures (3) to (24) summarize the chiral modifiers, enantiomeric excesses ee) and absolute configurations of the 1-phenyl-1-ethanol (2) produced. 

The effects of alcoholic modifiers and pH on the chromatographic properties of the OVM CSP have been investigated using acidic, basic, and neutral solutes (103). A series of primary, secondary, and tertiary alcohols and pH s ranging from 3.5-6.0 were used in this study. The results indicate that both the shape and hydrophobicity of the alcoholic modifier affect retention (k ) and enantioselectivity (a). In general, an increase in the hydrophobicity of the modifier results in a decrease in k s and a s. However, this is not the case when (-butanol is the modifier, suggesting that the size of the alkyl moiety attached to the carbinol carbon also contributes to the chromatographic results. The pH studies indicated that... 

---