Isomer types

Chapter 7 : Stereochemistry

Compounds that have the same molecular formula but different chemical structures are called isomers.
Depending on what types of differences there are between the structures, it is possible to classify isomers into various sub-types.
If you "click" on the named boxes there is a link to a definition and an example.

Types of Isomer tree

Constitutional (or structural) isomers differ in the order in which the atoms are connected so they contain different functional groups and / or bonding patterns:

example: 1-propanol, 2-propanol and ethyl methyl ether (C₃H₈O)

Stereoisomers contain the same functional groups and differ only in the arrangement of atoms in space.

Conformational isomers (or conformers or rotamers) are stereoisomers produced by rotation about sigma bonds, typically rapidly interconverting at room temperature:

example 1: butane : staggered (left) and eclipsed (center).The C2-C3 sigma bond rotation is animated (right). Try rotating the model to look along the C-C to see the two forms.

example 2: cyclohexane : chair (left) and boat (right).These two forms can be interconverted by twisting the ring structure.

Configurational isomers are stereoisomers that do not readily interconvert at room temperature and can (in principle at least) be separated.

Geometric isomers are configurational isomers that differ in the spatial position around a bond with restricted rotation (e.g. a double bond):

(E)-2-butene and (Z)-2-butene

Optical isomers are configurational isomers that differ in the 3D relationship of the substituents about one or more atoms:

Enantiomers are optical isomers that are non-superimposable mirror images.

(R)- or (S)-2-chlorobutane. Try moving the images to show that they are mirror images.

Diastereomers are optical isomers that are not enantiomers.

(S,R)- or (R,R)-2-bromo-3-chlorobutane

Deriving Possible Isomers

There is no simple way to know how many isomers exist for a given molecular formula, and therefore, it is important to have a systematic method to derive them. Important preliminary steps in the process are to determine the IHD to see what types of structures you should be considering, then list the functional groups that are possible based on the molecular formula and IHD. Once you have done that, here are a few suggestions to aid the drawing process.....

start with the longest continuous chain (or ring) first
move substituents systematically along the chain
if you have C=C consider E- and Z- possibilities or cis- and trans- for ring systems
decrease the the chain length one step at a time and introduce new alkyl groups
watch out that you don't repeat structures
if chiral centers are present then need to consider R and S stereoisomers, do this by identifying the chiral centers.