sterechemistry

The Two Major Classes of Isomers
Recall that isomers are different compounds with the same molecular formula.
The two major classes of isomers are constitutional isomers and stereoisomers.
Constitutional/structural isomers have different IUPAC names, the same or different functional groups, different physical properties and different chemical properties.
Stereoisomers differ only in the way the atoms are oriented in space. They have identical IUPAC names (except for a prefix like cis or trans). They always have the same functional group(s).
A particular three-dimensional arrangement is called a configuration. Stereoisomers differ in configuration.
Cyclopentane
A planar structure for cyclopentane would have bond angles of 108°, which is very
close to the ideal values for a tetrahedral carbon atom. However, such a structure could
lead to considerable torsional strain resulting from ten H---H eclipsing interactions.
Torsional strain is reduced by moving one or two carbon away from the plane. This
results in an increase in angular strain. Carbon atoms move in and out of the plane
rapidly, resulting in an illusion of rotation of the molecule. This phenomenon is termed
as pseudo-rotation.