Cycloalkanes stereoisomers

Stereochemistry refers to chemistry in three dimensions Its foundations were laid by Jacobus van t Hoff and Joseph Achille Le Bel m 1874 Van t Hoff and Le Bel mde pendently proposed that the four bonds to carbon were directed toward the corners of a tetrahedron One consequence of a tetrahedral arrangement of bonds to carbon is that two compounds may be different because the arrangement of their atoms m space IS different Isomers that have the same constitution but differ m the spatial arrangement of their atoms are called stereoisomers We have already had considerable experience with certain types of stereoisomers—those involving cis and trans substitution patterns m alkenes and m cycloalkanes... 

The 1,2-diinethylcyclopropanes are members of a subclass of stereoisomers called cis-trans isomers. The prefixes cis- (Latin "on the same side") and tmns-(Latin "across") are used to distinguish between them. Cis-trans isomerism is a common occurrence in substituted cycloalkanes. 

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

Since bond rotation can t occur, the two 2-butenes can t spontaneously interconveTt they are different, isolable compounds. As with disubstituted cycloalkanes, we call such compounds cis-trans stereoisomers. The compound with substituents on the same side of the double bond is called c/3-2-butene, and the isomer with substituents on opposite sides is fra/iS-2-butene (Figure 6.3). 

R Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order but with a different geometry. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers (both in alkenes and in cycloalkanes). Actually, cis-trans isomers are just another kind of diastereomers because they are non-mirror-image stereoisomers. 

Our present aim is to survey the chemistry and stereochemistry of cycloalkane-fused 1,3-oxazines, 1,3-thiazines, and pyrimidines. Only those derivatives are discussed in which the annelations involve sp carbon atoms, so that the possibility of cis and trans stereoisomers exists. 

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. 

Identify and draw cis and trans stereoisomers of disubstituted cycloalkanes. Problems 3-33,43, and 44... 

Let us now turn our attention to disubstituted cycloalkanes, and draw all possible stereoisomers for 1,3-dibromocycIopentane. Because 1,3-dibromocyclopentane has two stereogenic centers, it has a maximum of four stereoisomers. 

To draw all possible stereoisomers, remember that a disubstituted cycloalkane can have two substituents on the same side of the ring (ds isomer, labeled A) or on opposite sides of the ring (trans isomer, labeled B). These compounds are stereoisomers but not mirror images of each other, making them diastereomers. A and B are two of the four possible stereoisomers. 

Draw all possible stereoisomers for each cycloalkane. Label pairs of enantiomers and diastereomers. Label any meso compound. 

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