14.3 Isomerism
What are isomers?
- Isomers are two or more compounds with the same molecular formula but a different arrangement of atoms in space. Organic molecules which exhibit this property show isomerism
- This excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular For example, both of the following are the same molecule. They are not isomers, both are butane.
- Isomers must have different names. A good way to identify isomers is by naming the molecules, molecules with the same name are not isomers.
- There are two types of isomerism
- Structural isomerism
- Stereoisomerism
Structural isomerism
- Chain isomerism arises due to the different arrangement of carbon atoms in a chain. The carbon atoms may be arranged in a straight chain or branched chain
- Positional isomerism arises due to different positions of functional group in the carbon
- Functional isomerism arises due to different functional groups.
- More examples
Stereoisomerism – Geometrical (cis–trans or E-Z) isomerism
- Geometrical isomerism arises due to the rotation about a bond is restricted. It is common in compounds containing carbon-carbon double bond(C=C bond) and certain ring systems.
- i. A C=C bond cannot be rotated due to the presence of π bond. A π bond will break if a rotation occurs. Conversely, a carbon-carbon single bond is rotatable.
ii. In ring systems, rotation about a bond is restricted due to the linkage of the ring because the C-C bond is now part of the ring system. - Geometrical isomers occur in pairs, differing from each other in the positioning of the two groups across the double bond.
i. A cis-isomer has the two groups on the same side of the double bond.
ii. A trans-isomer has the two groups on the opposite sides of the double bond
- To identify geometrical(cis–trans) isomers, it is essential to draw the groups around the double bond showing the correct bond angles(120°).
- Geometrical(cis–trans) isomerism cannot exist if either carbon carries identical groups. In short, to have geometrical isomers, it is essential to have two different groups on the left and two different groups on the right
- More examples:
- Geometrical isomers have similar(not identical) chemical properties but different physical properties
- cis-isomer generally has a higher boiling point than trans-isomer. This is because dipoles in a cis-isomer do not cancel out each other, causing the entire molecule to have a net dipole moment and it is polar. Permanent dipole- dipole forces exist and more energy is required to overcome it
- trans-isomer generally has a higher melting point than cis-isomer. This is because in the solid state, trans-isomers pack more efficiently in the crystalline lattice due to its shape
Stereoisomerism – Optical isomerism
- Optical isomerism arises due to the ability of compounds to rotate the plane of polarisation of a plane-polarised light.
- A compound is said to be optically-active(optical isomerism exists) if it rotates the plane of polarised light. If it does not, it is said to be optically-inactive.
- For a compound to be optically active:
- It needs to have an asymmetrical carbon with four different groups attached to it so that there is no plane of The carbon atoms with four different groups attached to it is called the chiral carbon or chiral centre.
- The isomers must be mirror-images of each other and are non-superimposable. That is, no matter how the molecules are rotated, they never fully resemble each other.
- An example:
- To identify optical isomers:
- In chain systems, check which carbon has four different groups attached to it
- In ring systems, check also which carbon has four different groups attached to it. A different group here can be identified by tracking around that ring from a particular carbon atom in either clockwise or anti-clockwise direction. If the pattern along the way is the same, that carbon atom is not chiral, and the converse is also true
- An example:
