Trans-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. 

Ring structures confer) restricted rotation around the bonds and thereby give rise to geomemcs4sdmer. In trans-l,3-dichlorocyclopentane one chlorine is above the plane of the ring and on is below In cis-1,3 dichlorocyclopentane both chlorines are above the plane of the ring,... 

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. 

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. 

How does the proposed mechanism invoking a bridged halonium ion intermediate explain the observed trans products of halogenation For example, chlorination of cyclopentene affords both enantiomers of rram-l,2-dichlorocyclopentane, with no cis products. 

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. 

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

Dichlorocyclopentane (b) its trans isomer (c) cw-l,3-dichlorocyclopentane (d) its trans isomer ... 

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