Trans isomers of cycloalkanes

In Section 11.3, we observed that the rotation around the carbon-carbon bonds of cycloalkanes was restricted. The consequence of the absence of free rotation was the formation of geometric or cis-trans isomers. The cis isomers of cycloalkanes had substituent groups on the same side of the ring (L., cis, "on the same side"). The trans isomers of cycloalkanes had substituent groups located on opposite sides of the ring (L., trans, "across from"). 

Cis and trans isomers of cycloalkanes furnish us with another example of stereoisomers that are diastereomers. Consider the following two compounds ... 

Cis-trans isomers of cycloalkanes are a type of stereoisomer, also called geometric stereoisomers, in which substituents are on the same side ids) or on the opposite sides trans) of the ring. 

These stereoisomers are not readily interconverted by rotation around the double bond at room temperature. Like cis-trans isomers of cycloalkanes, they are configurational stereoisomers and can be separated from one another by distillation, taking advantage of the difference in their boiling points. 

Stereoisomers (Section 4.2) are compounds whose atoms are connected in the same order hut with a different arrangement in space. Among the kinds of stereoisomers we ve seen are enantiomers, diastereomers, and cis-trans isomers of cycloalkanes. Actually, cis-trans isomers are just one class of diastereomers because they are non-mirror-image stereoisomers ... 

Recognize, draw, and name cis-trans isomers of substituted cycloalkanes. 

Geometric isomers are not restricted to compounds containing the C=C bond. Eor example, certain di-substituted cycloalkanes can exist in the cis and the trans forms. Label the following molecules as the cis and trans isomer of the same compound ... 

The stereochemistry of cycloalkanes is discussed extensively in the chapter by Anderson, and it is important for an understanding of the analyses offered for these compounds to distinguish between isomers and conformers. Because of the low energetic barrier for equilibration between axial and equatorial conformers, only low-temperature HNMR or CNMR as well as IR will freeze the equilibration and allow the spectral analysis of the conformers. However, disubstituted cycloalkanes will exhibit, in addition to structural isomerism, also cis, trans isomerism. Both cis and trans isomers of 1,4-dimethylcyclohexane undergo, at room temperature, a conformer equilibration. In the case of the trans isomer the conformer with two equatorial CH3 groups is more stable, while for the cis isomer both conformers are equally stable and cannot be separated. 

Stereoisomers have the same molecular formula and the same connectivity of atoms in their molecules, but different three-dimensional orientations of their atoms in space. The one example of stereoisomers we have seen thus far is that of cis-trans isomers in cycloalkanes (Section 3.7) and alkenes (Section 4.1C) ... 

Stereoisomers Compounds that have the same molecular formula, the same connectivity of their atoms, but a different orientation of their atoms in space. The term "stereoisomer"includes cis,trans isomers in cycloalkanes and alkenes as well as enantiomers, diastereomers, meso compounds, and atropisomers. Conformational isomers are also stereoisomers, whether they are isolable or not. 

One consequence of three-dimensionality is the existence of stereoisomers, namely cis and trans isomers of substituted cycloalkanes (Section 4-1). We shall see in Chapter 5 that the phenomenon of stereoisomerism is more general and occurs in acyclic molecules as well. These concepts influence such diverse areas as relative reactivities and biological effectiveness. Because of its fundamental importance and its powerful utility in biological applications, stereochemistry constitutes a recurring theme through the remainder of this book. 

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... 

Review Chapter 5. The inability of atoms in rings to rotate completely about their [Pg.167]

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. 

Which of the following cycloalkanes are capable of geometric (cis-trans) isomerism Draw the cis and trans isomers. 

A -Cycloalkene-lA ioiits. a,couple intramolecularly to cyclic ene-l,4-diones in the presence of Cu(acac)2 in benzene at 60°. Other copper catalysts are unsatisfactory. The procedure fails when n is S or 6, but is satisfactory V ien n is 4 and >6. Both cis- and trans-isomers are formed, with the latter predominating exeept when n is 4. The products are reduced by sodium dithionite to cycloalkane-1,4-diones, which can be cyclized by NaOH to fus d-ring cyclo-pentenones when n is 7, 9,10, and 16. 

Cis and trans isomers are named by adding the prefixes cis and trans to the name of the cycloalkane. Thus, A Is cis-1,2-dimethylcyclopentane, and B is trans-, 2-dimethylcyclopentane. 

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