Disubstituted Cycloalkanes Stereoisomers

When a cycloalkane bears two substituents on different carbons— methyl groups, for example—these substituents may be on the same or on opposite sides of the ring. When substituents are on the same side, we say they are cis to each other if they are on oppo- 

PROBLEM 3.8 Exclusive of compounds with double bonds, four hydrocarbons are constitutional isomers of cis- and trans-1,2-dimethylcyclopropane. Identify these compounds. 

The cis and trans forms of 1,2-dimethylcyclopropane are stereoisomers. Stereoisomers are isomers that have their atoms bonded in the same order—that is, they have the same constitution, but they differ in the arrangement of atoms in space. Stereoisomers of the cis-trans type are sometimes referred to as geometric isomers. You learned in Section 2.15 that constitutional isomers conld differ in stability. What about stereoisomers  

We can measure the energy difference between cis- and 1,2-dimethylcyclopropane by comparing their heats of combustion. As iUnstrated in Fignre 3.20, the two compounds are isomers, and so the difference in their heats of combnstion is a direct measure of the difference in their energies. Because the heat of combustion of trans-1,2-dimethylcyclopropane is 5 kJ/mol (1.2 kcahmol) less than that of its cis stereoisomer, it follows that frfl -l,2-dimethylcyclopropane is 5 kJ/mol (1.2 kcal/mol) more stable than di-l,2-dimethylcyclopropane. 

The prefix stereo- is derived from the Greek word stereos, meaning solid. Stereochemistry is the term applied to the three-dimensional aspects of molecular structure and reactivity. 

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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). 

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. 

The lack of free rotation around C—C bonds in disubstituted cycloalkanes leads to an extremely important kind of isomerism called stereoisomerism. Two different compounds that have the same molecular formula and the same structural formula but different spatial arrangements of atoms are called stereoisomers. For example, consider a molecule of 1,2-dimethylcyclopentane. The cyclopentane ring is drawn in I Figure 1.17 as a planar pentagon with the heavy lines indicating that two of the carbons are in front as one views... 

Stereoisomers have the same connectivity of atoms but differ in their spatial arrangement. The terms cis and trans are used to differentiate stereoisomeric alkenes as well as disubstituted cycloalkanes. 

Since bond rotation can t occur, the two but-2-enes can t spontaneously interconvert they are different, isolable compounds. As with disubstituted cycloalkanes, we call such compounds cis-trans stereoisomers. 

All but the 1,1-disubstituted cycloalkanes exist as two isomers If both substituents are on the same face of the molecule, they are cis if they are on opposite faces, they are trans. Cis and trans isomers are stereoisomers—compounds that have identical connectivities but differ in the arrangement of their atoms in space. 

Rule 4. Identify any stereoisomers. In a 1,2-disubstituted ethene, the two substituents may be on the same side of the molecule or on opposite sides. The first stereochemical arrangement is called cis and the second trans, in analogy to the cis-trans names of the disubstituted cycloalkanes (Section 4-1). Two alkenes of the same molecular formula differing only in their stereochemistry are called geometric or cis-trans isomers and are examples of dia-stereomers stereoisomers that are not mirror images of each other. 

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