Lesson 5 of 13 · 7 min
Isomerism
NCERT § "Isomerism"
Acetic acid is C₂H₄O₂. So is methyl methanoate, a sweet-smelling liquid that boils at 305 K. Same atoms, same count, but one is a sour acid in vinegar and the other a fruity ester. How?
The lesson in notes
In short
Isomers have the same molecular formula but different properties because their structures differ.
Structural isomers differ in the order in which atoms are bonded.
Chain isomers differ in the carbon skeleton: pentane, 2-methylbutane and 2,2-dimethylpropane are all C₅H₁₂.
Position isomers differ in the position of a substituent or functional group on the same skeleton, such as propan-1-ol and propan-2-ol.
Functional group isomers have different functional groups, such as ethanol and methoxymethane (both C₂H₆O).
Metamerism comes from sharing the alkyl groups differently between the two sides of the same functional group: methoxypropane and ethoxyethane are both C₄H₁₀O ethers.
Stereoisomers have the same bonding sequence but differ in the spatial arrangement of atoms; they are classed as geometrical and optical isomers.
When counting isomers of a formula, check each carbon skeleton and then each distinct position; C₃H₈O, for example, has two alcohols (propan-1-ol, propan-2-ol) and one ether (methoxyethane).