Isomerism cis-trans in cycloalkanes

In many respects, the chemistry of cycJoaJkanes is like that of open-chain, acyclic alkanes Both classes of compounds are nonpolar and fairly inert. There are, however, some important differences. 

Rotation occurs around the carbon-carbon single bond in ethane because of r bond cylindrical symmetry. 

Larger cycloalkanes have increasingly more rotational freedom, and the very large rings ( 25 and up) are so floppy that they are nearly indistinguishable from open-chain alkanes. The common ring sizes (C3, C4, C5, C, C7), however, are severely restricted in their molecular motions. 

The structure of cyclopropane. No rotation i possible around the carbon-carbon bonds without breaking open the ring. 

There are two different 1,2-dimethylcyclopropane isomers, one with the methyl groups on the same side of the ring and the other with the methyl groups on opposite sides of the ring. 

Draw structures corresponding to the following lUPAC names  

FIGURE 4.1 (a) Rotation occurs around the carbon-carbon bond in ethane, but (b) no rotation is possible around the carbon-carbon bonds in cyclopropane without breaking open the ring. 

Unlike the constitutional isomers butane and isobutane (Section 3.2], which have their atoms connected in a different order, the two 1,2-dimethyl-cyclopropanes have the same order of connections but differ in the spatial orientation of the atoms. Such compounds, which have their atoms connected in the same order hut differ in three-dimensional orientation, are called stereochemical isomers, or stereoisomers. 

In many respects, the chemistry of cycloalkanes is like that of open-chain alkanes both are nonpolar and fairly inert. There are, however, some important differences. One difference is that cycloalkanes are less flexible than open-chain alkanes. Jn contrast with the relatively free rotation around single bonds in open-chain alkanes (Sections 3.6 and 3.7), there is much less freedom in cycloalkanes. 

Constitutional isomers (different connections between atoms) 

Stereoisomers (same connections but different three-dimensional geometry) 

Cycl( propane, for example, must be a rigid, planar molecule l)ecause three points (the carbon atoms) define a plane. No bond rotation can take place around a cyclopropane carbon -carbon bond vvitliout breaking open the ring (k igure 4 1). 

Constitutional isomers (different connections between atoms) CH3- CH3 1 -CH- CH3 and CH3 

The methyl groups are said to be cis (Latin, on the same side) or trans (Latin, across) to each other. 

Cis-trans isomers differ from one another only in the way that the atoms or groups are positioned in space. Yet this difference is sufficient to give them different physical and chemical properties. (Note, for example, the boiling points for the two 1,2-dimethylcyclopentane structures.) Therefore, cis-trans isomers are unique compounds. Unlike conformers, they are not readily interconverted by rotation around carbon-carbon bonds. In this example, the cyclic structure limits rotation about the ring bonds. To interconvert these dimethylcyclopentanes, one would have to break open, rotate, and re-form the ring, or carry out some other bond-breaking process. 

Cis-trans isomers can be separated from each other and kept separate, usually without interconversion at room temperature. Cis-trans isomerism can be important in determining the biological properties of molecules. For example, a molecule in which two reactive groups are cis will interact differently with an enzyme or biological receptor site than will its isomer with the same two groups trans. 

In three dimensions, the molecules are nearly spherical. Of these compounds, only the first two are known 

Repetitive branching in compounds can iead to moiecuiar shapes that resembie this tree. 

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Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Cis-trans isomerism in cycloalkanes. Like alkenes, cycloalkane rings are restricted from free rotation. Two substituents on a cycloalkane must be either on the same side (cis) or on opposite sides (trans) of the ring. 

Cis-Trans Isomerism in Cycloalkanes A WORD ABOUT... Isomers—Possible and Impossible Summary of Isomerism Reactions of Alkanes A WORD ABOUT... Alternative Energy The Benefits of Hydrogen A WORD ABOUT... Methane, Marsh Gas, and Miller s Experiment... 

Conformations of Alkanes and Cycloalkanes HOW TO Draw Alternative Chair Conformations of Cyclohexane 2- Cis,Trans Isomerism in Cycloalkanes and Bicycloalkanes HOW TO Convert Planar Cyclohexanes to Chair Cyclohexanes 2-7 Physical Properties of Alkanes and Cycloalkanes 2-8 Reactions of Alkanes... 

We begin our study of stereoisomers with the study of cis,trans isomerism in cycloalkanes. 

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