Structural and Geometric Isomers
Name: Henry S.
How can you tell the difference between a structural and
These terms are often used ambiguously, so one has to be careful. Within
the context of organic chemistry consider the two compounds: dimethyl ether
and ethanol. Both have the formula: C2H6O. However, the atoms are put
together differently: CH3--O--CH3 in the case of dimethyl ether, and
CH3--CH2--OH in the case of ethanol. They are entirely different chemical
substances. There are other groups of compounds that are also different
chemical substances. For example: CH3--CH2--CH2--CH3 and CH3--CH(CH3)--CH3
n-butane and 2-methyl propane both have the same formula C4H10 but are the
same bonds just rearranged. These are frequently termed geometric isomers,
in contrast to the previous that are typically termed structural isomers.
But, like I said, these terms are often carelessly used interchangeably.
There is another kind of isomer referred to as "optical isomers" that are
the same arrangement of chemical bonds that are non-superimposable mirror
images of one another. They differ mainly in the direction each rotates
polarized light. This also makes macroscopic crystals of the pair of optical
isomers mirror images of one another.
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Update: June 2012