Cis-1,2-Dichlorocyclopentane

Internal mirror plane, cis-1,2-Dichlorocyclopentane has an internal mirror plane of symmetry. 

Internal mirror plane, cis-1,2-Dichlorocyclopentane has an internal mirror plane of symmetry. Any compound with an internal mirror plane of symmetry cannot be chiral. 

Stereoisomers of 1,2-dichlorocyclopentane. The cis isomer has no enantiomers it is achiral. The trans isomer is chiral it can exist in either of two nonsuperimposable enantiomeric forms. 

We have already seen other meso compounds, although we have not yet called them that. For example, the cis isomer of 1,2-dichlorocyclopentane has two asymmetric carbon atoms, yet it is achiral. Thus it is a meso compound, cis-1,2-Dibromocyciohcxanc is not symmetric in its chair conformation, but it consists of equal amounts of two enantiomeric chair conformations in a rapid equilibrium. We are justified in looking at the molecule in its symmetric flat conformation to show that it is achiral and meso. For acyclic compounds, the Fischer projection helps to show the symmetry of meso compounds. 

PRACTICE PROBLEM 4.15 Write structures for the cis and trans isomers of (a) 1,2-dichlorocyclopentane and... 

SOLUTION Seven dichlorocyclopentanes would be obtained as products. Only one isomer is possible for the 1,1-dichloro compound. The 1,2- and 1,3-dichloro compounds have two asymmetric carbons. Each has three stereoisomers because the cis isomer is a meso compound and the trans isomer is a pair of enantiomers. 

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