Oxirane ring characterization

The conformations of the six-membered ring systems are better characterized than those of the less stable five-membered analogues. For example, the cyclohexane molecule can occur in two strainless forms, namely in the rigid chair form or in the flexible form (Fig. 2-9). The latter can exist in a variety of shapes of which only the boat and the skew boat (or twist) are regular and easily depictable on paper. The chair form is preferred energetically because it is usually free from steric interactions whereas the flexible forms are not. The half-chair conformation is possible, when a six-membered ring contains either a double bond or an oxiran ring. In the half-chair conformation four adjacent atoms are in the same plane. 

Ethylene oxide and other hazardous epoxides characterized by the presence of three-membered strained oxirane ring structures are discussed individually in the following sections. Miscellaneous epoxy compounds with low toxicity and flammability and low commercial applications are discussed in Table 16.1. 

Characterization of epoxies involves much more than the location of the oxirane ring bands. There are many epoxy resins with different structures, different polimerization degrees...etc. IR spectroscopy can be used to characterize the nature of the epoxy. Figure 4 shows the mIR and nIR spectra of two similar epoxy resins Diglycidylether of bisphenol A (DGEBA) and its hydrogenated derivative (HDGEBA). 

The classification of epoxide adhesives refers to those that contain a predominance of epoxy resins in their formulation, the epoxy resin being characterized by a thiee-membered ring (epoxy or oxirane ring) in which two carbon atoms are singly bonded to an oxygen atom. 

Epoxides undergo a repertoire of fairly well-characterized reactions, perhaps the most typical being the nucleophilic ring opening process. Since the epoxide in question is usually found in a compound with multiple functionality, many new methodologies seek to optimize chemo- and regioselectivity. Fortunately, the oxirane lone pair provides a convenient handle to effect activation of the heterocycle toward C-0 bond cleavage thus, efficient catalysts are of particular interest. 

The term epoxy, epoxy resin, or epoxide (Europe) refers to a broad group of reactive compounds that are characterized by the presence of an oxirane or epoxy ring, shown in Fig. 2.1. This is represented by a three-member ring containing an oxygen atom that is bonded with two carbon atoms already united in some other way. 

Epoxy resins are an important class of polymeric materials, characterized by the presence of more than one three-membered ring known as the epoxy, epoxide, oxirane, or ethoxyline group. 

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