Isomers which have their atoms connected in the same sequence but differ in the way the atoms are oriented in space - i.e. the difference between two stereoisomers lies only in the three dimensional arrangement of atoms. Stereoisomers can be classed as cis-trans isomers or optical isomers .
Discussion
Cis-trans isomers - sometimes referred to as geometrical isomers - are stereoisomers. They occur when double bonds prevent rotation of atoms around a bond. For example cis-butenedioic acid and trans-butenedioic acid are stereoisomers. In the cis-isomer, both of the acid groups lie on the same side of the molecule. In the trans-isomer, the acid groups are on opposite sides of the molecule.
cis and trans isomers of butenedioic acid - the carbon-carbon double bond is highlighted in red
Optical isomers - also called enantiomers - are mirror images of one another.
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
Mirror Image configurations of Bromo-chloro-fluoro-methane Cannot be Superimposed
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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