# What is the difference between structural isomer, geometric isomers, and enantiomers?

Mar 10, 2018

Well, they all got the same chemical formula....

#### Explanation:

Structural isomers have the same chemical formula (and typically they are organic compounds)...but different connectivity. For example...butane, ${C}_{4} {H}_{10}$ generates TWO structural isomers...each with different $C - C$ connectivity...

${H}_{3} C - C {H}_{2} C {H}_{2} C {H}_{3}$, $\text{n-butane}$...

${H}_{3} C - C H \left(C {H}_{3}\right) C {H}_{3}$, $\text{isobutane}$...

$\text{Pentane}$, ${C}_{5} {H}_{12}$, generates 3 structural isomers...

$\text{Hexane}$, ${C}_{6} {H}_{14}$, generates 5 structural isomers...

As the formula becomes bigger, with more carbons, the number of isomers is magnified...$\text{heptane}$ generates 9 structural isomers. In each case ... it is the $C - C$ connectivity that alters, and each could be given a distinct and descriptive name.

On the other hand, geometric isomers each have the SAME $C - C$ connectivity...but different geometries. And this is best illustrated by taking a simple example, $\text{2-butylene}$, the which demonstrates TWO geometric isomers depending on the geometry of the methyl groups across the olefinic bond...

Just to persuade you that this does make a difference, the boiling point of $\text{cis-butylene}$ is $3.7$ ""^@C, and that of $\text{trans-butylene}$ is $0.9$ ""^@C.

And the third type of isomerism, occurs when the connectivity is the same..and yet the spatial properties of the molecule gives the result that the structure generates two or more stereoisomers...and typically this occurs when a carbon atom in the molecule has 4 different substituents...such a carbon can generate left-handed, and right-handed enantiomers that can only be equated by a mirror plane.