Benzene methyl derivatives, retention

Figure 3. Influence of the solute molecule size and structure on the retentions of benzene methyl derivatives. Eluent water/methanol 30/70, Samples benzene, toluene and p-xylene. Stationary phases as given in TABLE I.

Fu and Mak [238,239] dealt with acyl alkyl esters of amino acids with various combinations of acyl and alkyl moieties in comprehensive studies. They evaluated and determined optimal conditions for their preparation they selected acylation with the aid of the anhydride or chloride of the appropriate acid as the first step. They performed the reaction at 20-30°C with vigorous stirring for 10 min, with subsequent esterification with the appropriate alcohol in dry benzene in the presence of Amberlite IR-120 (H+). They further evaluated their retention properties on both Carbowax 20M and GE-XE-60, and the relationships between these properties and their structure. Kolb and Hoser [240] described a microdevice in which the derivatives were prepared with the aid of opened aluminium or gold capsules (20-/J volume) which acted as a microreactor in an automatic capsule—dosage system (MS 41). As an example they demonstrated the application of their device to the preparation of HFB-methyl esters. 

It was found that just contact with neutral alumina at room temperature in benzene or chloroform suffices to promote quantitative isomerization of compound I to the frani-dihydrofuran derivative II only, with retention of the configuration of the aryl and methyl substituents of cyclopropane I. This highly unusual result was underscored by the formation of both II and III during the parallel thermolysis experiment, a result that follows well established lines. Thus, dihydrofurans II and III are very probably the result of a 1,3-sigmatropic rearrangement formally similar to that described in Problem 14. 

Figure 4.26 presents the influence of functional groups on the retention time. Benzene and some aromatic derivates are separated by RP chromatography. Compared with benzene the retention time is decreased for derivates with polar aldehyde or hydroxyl groups while it is increased for the more bulky apolar esters, e.g. benzoic or terephthalic methyl esters. 

Muller (13) studied the retention characteristics of the different silicas for different analytes with different functional groups. For benzene derivatives with polar functional groups, a significant influence of the surface pH of the silica was found only for acidic and basic analytes, such as phenol and anilines. Compared to methyl phenyl ketone, phenol was less retained on the acidic Zorbax silica and more retained on the basic Spherisorb and Hypersil silicas. It also exhibited strong tailing on the basic silicas. Conversely, aniline. 

---