Cycloalkanes configuration

Table 2.12. chemical shifts (relative configurations of cycloalkanes, pyranoses and alkenes (application of y-effects) The shifts which are printed in boldface reflect y-effects on C atoms in the corresponding... 

As for the stereochemistry, for the case of complete cyclization, besides the usual tacticity (possibly, isotactic or syndiotactic, referred to relative configurations of equivalent stereogenic carbons of subsequent monomeric units), the cis or trans configuration of the 1,3-cycloalkane rings which are present in the polymer main chain also has to be considered.70,74... 

The cycloalkanes also are known as naphthenes, cycloparaffins, or alicyclic hydrocarbons. In the petroleum industry, this class of hydrocarbons is known as naphthenes. Naphthenes have saturated rings. The general formula for the ring without substituents is CnH2n. This is the same as the general formula for the alkene series however, the structural configurations differ completely and, thus, the physical and chemical properties are not at all similar. 

Stereoisomers have the same order of atom attachments but different arrangements of the atoms in space. Cis-trans isomerism is one kind of stereoisomerism. For example, two substituents on a cycloalkane can be on either the same (c/s) or opposite (trans) sides of the mean ring plane. Stereoisomers can be divided into two groups, conformational isomers (interconvertible by bond rotation) and configurational isomers (not interconvertible by bond rotation). Cis-trans isomers belong to the latter class. 

Fig. (a) Butane (C4H]0) (b) 2-methlypropane (C4H10). Configurational Isomers — Alkenes and Cycloalkanes Definition... 

Table 2.12.13C chemical shifts (5C) and relative configurations of cycloalkanes, pyranoses and alkenes (application of y-effects)4 8. The shifts which are printed in boldface reflect y-effects on C atoms in the corresponding --------------------------------------------------isomer pairs---------------------------------------------------... 

A cis-trans isomer is a t)rpe of stereoisomer. Stereoisomers are molecules that have the same structural formula and bonding pattern but different arrangements of atoms in space. A cycloalkane is in the cis configuration if two substituents are on the same side of the ring (either both above or both below). A cycloalkane is in the trans configuration when one substituent is above the ring and the other is below the ring. The cis-trans isomers are not interconvertible. 

The next example is of the Isomers for substituted cycloalkanes [77], assumed to undergo such rapid configurational changes as to average the environment of equatorial/awal substituents, so that only the avers e planar ring conformation need be considered [78]. We shall illustrate the enumeration with substituted cyclopropanes whose cycle indices can be obtained from the symmetries of the graphs S-7,... 

The addition of substituted pyridines to [VO(acac)2] has been studied, and cis- and trans-isomers identified by i.r. spectroscopy. This proposed structural isomerism has been confirmed by an Z-ray diffraction study of [VO(acac)2(p-PhC6H4N)] which has the cis-configuration (29). A number of oxo-vanadium(iv) p-diketonato-complexes have been shown to be active catalysts for the epoxidation of cycloalkanes. 4-Benzoyl-3-methyl-l-phenylpyrazolin-... 

Base-induced eliminations of y-haloalkyl-9-BBN derivatives give cycloalkanes (C3 to Cs) with inversion of configuration at both carbon centers.1,1-Diboryl adducts from the dihydrob-oration of 1-alkynes with 9-BBN-H serve as useful precursors to B-cyclopropyl-9-BBN derivatives by a similar process (eq 28). 

The use of MS in addition to GC/MS includes methods such as capillary zone electrophoresis (CZE/MS), thin layer chromatography (TLC/MS) and others. The major problem in employing these separation-MSD methods is the interface. The very high vacuum required for MS operation requires complete removal of the carrier gas or liquid . Since the present review treats specifically the use of the MSD for the analysis of alkanes and cycloalkanes, we concentrate on the GC/MSD/C (gas chromatography/ mass spectrometry/computerized) method. The MS used can be quadrupole or magnetic and its configuration will, of course, control the power of resolution (PR), the mass range, etc. The ionization mode is also important, e.g. whether El or Cl (see Sections, III.A.l and III.A.2). 

Configuration of cycloalkanes is based on the combinations of tetrahedrons, similarly to the case of alkanes. However, in some cases the ring structure requires that the angles between C-C bonds deviate from the normal tetrahedral values (109°28 ). Almost ideal tetrahedral angles are present in the molecule of cyclohexane. For the pictorial representation of three dimensional molecular structures of cyclic molecules special descriptive projection is used. Let us analyse the structure of cyclohexane molecule in more detail. 

---