M2 Lesson 3 - Isomerism: Fischer Structure and Haworth Structure Structural or Constitutional IsomerismStructural or constitutional isomers share the same chemical formulas, but their atoms are arranged differently.The three types of structural isomers are skeletal isomers, positional isomers, and functional group isomers.Structural isomers differ from stereoisomers, which share the same chemical formulas and the same order of atoms, but have different three-dimensional configurations.Types of Structural IsomersThere are three categories of structural isomers:Skeletal isomerism (also called chain isomerism) - structural isomers in which components of the skeleton are arranged in a different order. This is most commonly seen when the skeleton or backbone consists of a carbon chain.Position isomerism (also called regioisomerism) - constitutional isomers in which a functional group or substituent changes position on a parent structure.Functional group isomerism - structural isomers with the same molecular formula, but with atoms connected differently so dissimilar functional groups are formed.Structural Isomer ExamplesButane and isobutane (C4H10) are structural isomers of each other.Pentan-1-ol, pentan-2-ol, and pentan-3-ol are structural isomers that exhibit position isomerism.Cyclohexane and hex-1-ene are examples of functional group structural isomers.Molecules that have multiple stereocenters become difficult to draw quickly.  The Fischer projection is a convention for drawing molecules and quickly designating stereochemistry without laboriously trying to use wedges and hashes to specify the 3-D relationships.A historical note:  this was developed at a time before it was possible to experimentally prove absolute configuration for any molecule.  Emil Fischer, an early carbohydrate chemist, developed it so that it could continue to be used correctly and consistently even after experimental proof of configuration was developed.The definition is that every carbon is specified completely by a cross designating the carbon (at the center) and the four bonds to that carbon.  The stereochemistry of the bonds is defined (now) as the horizontal bonds are in front of the plane (coming toward you, the viewer); the vertical bonds are behind the plane (going away from you).  Sometimes (often, even), bonds to hydrogen are omitted.