Isomerism in organic molecules

Isomers are compounds, which have the same chemical formula, but a different structure. There are several classes of isomer.

Structural isomers (also called constitutional isomers):

Structural isomers are molecules that have the same formula, but are connected in different ways. This can include branching of a chain, different positions of functional groups or the splitting up of a functional group, (for example, a carboxylic acid may become a ketone and an alcohol).

Conformational isomers (also rotational isomers or “rotamers”):

Conformational isomers occur when there is rotation around a single bond. It is less important for small, chain molecules as they are constantly rotating, but it is very important for bulky or cyclic alkanes.

Stereoisomers, (also geometric or optical isomers):

This broad class of isomers includes any molecule with the same order of atoms, but different geometries, or different arrangement of the atoms in space. There are two different types of stereoisomers:

Enantiomers: non-superimposable mirror images.

Non-superimposable mirror images

Diastereomers: stereoisomers that are not mirror images. These can include cis-trans (or E-Z) isomers, and molecules with more than one stereocentre.

To get stereoisomerism, you need a chiral molecule – one that is not identical to its mirror image. The most common example of chirality is your hands – your left and right hands are mirror images of each other, but you can’t superimpose them.

Chiral molecules can be spotted by looking for a chiral (or stereo) centre. This will be a carbon with four different substituents attached to it.

It is important to be able to distinguish between different enantiomers, so we use the R/S nomenclature. To designate R or S, you use the Cahn-Ingold-Prelog rules:

1. Prioritise the atoms directly attached to the chiral centre by atomic number.

2. If two or more atoms are the same, look at the next atom out, continuing until you find a point of difference.

3. Multiple bonds are counted as the same number of bonds to the same atom, (for example, a double bond to an oxygen counts as two single bonds to oxygen atoms)

Now, with the 4th priority substituent pointing into the page, draw a curved line from priority 1 to 2 to 3. If it is clockwise, the molecule is R, anticlockwise is S.

This molecule is S

Here are some links that might be useful:



Filed under Chem 1, Organic Chemistry

2 responses to “Isomerism in organic molecules

  1. John

    Shouldn’t the last conformation in the figure below “Conformational isomers (also rotational isomers or “rotamers”)” be labelled eclipsed, and not staggered? They are two opposites.

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