Two molecules are described as stereoisomers of each other if they are made of the same atoms, connected in the same sequence, but the atoms are positioned differently in space. The difference between two stereoisomers can only be seen when the three dimensional arrangement of the molecules is considered. Stereoisomers are a type of isomer.
Stereoisomers can be subdivided into geometric isomers and optical isomers.
Usually, when people refer to geometric isomers, they are referring to cis-trans isomers. Cis-trans isomers can be found when rotation around a chemical bond is impossible, as we find in double and triple bonds.
For example cis-butenedioic acid and trans-butenedioic acid are stereoisomers - specifically geometric isomers.
As you can see in the image below, in the cis-isomer, both of the acid groups lie on the same side of the double bond. In the trans-isomer, the acid groups are on opposite sides of the double bond.
cis and trans isomers of butenedioic acid - the carbon-carbon double bond is highlighted in red
Optical isomers are molecules which are mirror images of one another. Often these mirror image molecules are referred to as enantiomers.
Just as a right-handed glove cannot be superimposed on a left-handed glove, optical isomers cannot be superimposed on one another. For example bromo-chloro-fluoro-methane exists in mirror image forms.
Mirror Image configurations of bromo-chloro-fluoro-methane
Notice in the image below how these mirror image configurations cannot be superimposed
i.e. no matter how we turn these molecules, we cannot arrange them so that the atoms attached to the central atom are identically oriented in space. In the image below, although the red and green beads are oriented identically in the two molecules, there is a mismatch between the blue and yellow beads.
In contrast to the stereoisomers discussed above, structural isomers are not stereoisomers. They arise when compounds have the same molecular formula but the atoms are bonded together in different sequences. For example chloropropane can exist in two forms:
1-chloropropane: CH3CH2CH2Cl and 2-chloropropane: CH3CHClCH3
In 1-chloropropane the chlorine is bonded to an end carbon while in 2-chloropropane the chlorine is bonded to the middle carbon.
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