1,2-Dichlorocyclopentane

The mirror image of trans- 1,2-dichlorocyclopentane is different from (nonsuper-imposable with) the original molecule. These are two different compounds, and we should expect to discover two mirror-image isomers of trans- 1,2-dichlorocyclopentane. Make models of these isomers to convince yourself that they are different no matter how you twist and turn them. Nonsuperimposable mirror-image molecules are called enantiomers. A chiral compound always has an enantiomer (a nonsuperimposable mirror image). An achiral compound always has a mirror image that is the same as the original molecule. Let s review the definitions of these words. 

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. 

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

Notice in the following figure that the chiral trans isomer of 1,2-dichlorocyclopentane does not have a mirror plane of symmetry. The chlorine atoms do not reflect into each other across our hypothetical mirror plane. One of them is directed up, the other down. 

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. 

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. 

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

OutUne a synthesis of cyclopentene from each of the following (a) Bromocyclopentane (b) 1,2-Dichlorocyclopentane... 

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