Isomerism
Isomerism are molecules that have the same molecular formula, but have a different arrangement of the atoms in space. That excludes any different arrangements which are simply due to the molecule rotating as a whole, or rotating about particular bonds.
There is always 1 question in AS/A-level Chemistry about isomers, you must remember all types of isomers especially (Chain, positional, functional, geometric and optical) 👨🏫
Structural isomers
The atoms are arranged in a completely different order.
Chain isomerism
- Different arrangement of the carbon skeleton
- Similar chemical properties
- Slightly different physical properties
- More branching = lower boiling point
- In the top one, the carbons lie in a horizontal line (linear), without branching. However, the others have branching.
In AS/A-level Chemistry exam, they will always ask for the reasoning of the difference of boiling point of chain isomers! 👩🏫
- As branching increases, boiling points increase, because as a molecule gets more branched, it becomes more compact. As a result, there are weakened intermolecular forces (smaller area) which make it easier to overcome the dipole-dipole Van Der Waals forces within the molecule, thus lowering the melting and boiling points.
- Similar chemical properties because the same functional group is present.
Positional Isomerism
- Same carbon skeleton
- Same functional group
- Functional group is in different position
- Similar chemical properties
- Slightly different physical properties
- These isomers have the same molecular formula and carbon skeleton but differ in the position of a functional or substituted group, or the position of a double or triple bond. Positional isomerism can also occur in benzene rings.
- In halogenoalkanes and alcohols, primary structures are less compact than secondary structures, because the functional group is more exposed. Hence, a lower boiling and melting point for more compact structures (primary<secondary<tertiary).
- Chemically, both undergo nucleophilic substitution reactions, with similar chemical properties.
- In alkenes, both physical and chemical properties are similar. However in unsymmetrical alkenes, during addition reactions two different products can be formed; for symmetrical alkenes, two similar products are formed.
Functional Isomerism
- Molecules have same molecular formula
- Molecules have different functional groups
- Molecules have different chemical properties
- Molecules have different physical properties
- These isomers have the same molecular formula but different functional groups. This usually means they have very different sets of chemical reactions based on the functional group and there can be significant differences in melting/boiling points and solubility.
Aldehydes and Ketones:
- For example, a molecular formula C3H6O could be either propanal (an aldehyde) or propanone (a ketone).
- Both have polar C=O bond which gives dipole-dipole interactions. However, aldehydes are easily oxidised to acids of same number of carbons, whereas ketones only undergo oxidisation under extreme conditions only.
- Aldehydes are reduced to primary alcohols, whereas ketones are reduced to secondary alcohols.
Carboxylic Acids and Esters
- Another common example is illustrated by the molecular formula C3H6O2. Amongst the several structural isomers of this are propanoic acid (a carboxylic acid) and methyl ethanoate (an ester).
- In carboxylic acids, O-H bond gives rise to hydrogen bonding, causing a higher boiling point and solubility in water. Whereas in esters there is no hydrogen bonding, and is insoluble in water.
- Carboxylic acids are acidic and react with alcohols. Whereas Esters are fairly unreactive and are hydrolysed to acids.
This is the end of the topic!!
Drafted by Cherry (Chemistry)