Chemistry · Ch 9 — Organic Chemistry – Some Basic Principles and Techniques
Structural Isomerism
Structural Isomerism
Compounds that share the same molecular formula but differ in how their atoms are connected are called structural isomers, and the phenomenon is structural isomerism. The connectivity — the order in which atoms are bonded — is what changes, not the number or type of atoms. Because different connectivity leads to different physical and chemical properties, recognising the types of structural isomerism is essential for understanding organic chemistry.
The textbook identifies four distinct types of structural isomerism. Each arises from a different kind of variation in the carbon skeleton or the placement of atoms and groups.
(i) Chain Isomerism
When two or more compounds have the same molecular formula but different carbon skeletons — that is, the carbon chain is arranged differently — they are called chain isomers, and the phenomenon is chain isomerism.
The carbon skeleton can be a straight chain, a branched chain, or a highly branched chain. All that matters is that the total number of carbon and hydrogen atoms (and any other atoms) is identical.
Example: The molecular formula represents three chain isomers:
- — Pentane (straight chain, no branches)
- — Isopentane (2-Methylbutane, one methyl branch)
- — Neopentane (2,2-Dimethylpropane, two methyl branches on the same carbon)
All three have the same number of carbon and hydrogen atoms, but the connectivity of the carbon atoms differs. In pentane, all five carbons form a continuous chain. In isopentane, four carbons form the main chain with one carbon as a branch. In neopentane, the central carbon is bonded to four other carbons, making a highly branched structure.
Chain isomerism is only possible when the carbon chain has at least four carbon atoms. With three or fewer carbons, there is no way to rearrange the skeleton into a different chain without changing the molecular formula.
(ii) Position Isomerism
When two or more compounds have the same molecular formula and the same carbon skeleton, but differ in the position of a substituent atom or a functional group on that skeleton, they are called position isomers, and the phenomenon is position isomerism.
The carbon skeleton remains unchanged; only the location of the substituent (such as a halogen, hydroxyl group, or double bond) moves to a different carbon atom.
Example: The molecular formula represents two alcohols that are position isomers:
- — Propan-1-ol (the hydroxyl group is on the terminal carbon, C-1)
- — Propan-2-ol (the hydroxyl group is on the middle carbon, C-2)
Both have the same three-carbon chain and the same functional group (alcohol), but the position of the group differs. This difference in position leads to different physical properties (boiling points, solubility) and different chemical reactivity.
When naming position isomers, the locant number (1, 2, 3, etc.) indicates which carbon carries the substituent. Always number the chain from the end that gives the substituent the lowest possible number.
(iii) Functional Group Isomerism
When two or more compounds have the same molecular formula but different functional groups, they are called functional isomers, and the phenomenon is functional group isomerism.
Here, the atoms are rearranged to create entirely different functional groups. Because functional groups determine the chemical family and most of the chemical behaviour, functional isomers often belong to different classes of compounds and have very different properties.
Example: The molecular formula represents two functional isomers:
- — Propanal (an aldehyde, with the group at the end of the chain)
- — Propanone (a ketone, with the group in the middle of the chain)
Both have the same number of carbon, hydrogen, and oxygen atoms, but the arrangement of atoms creates a different functional group. Propanal has an aldehyde group (), while propanone has a ketone group (). These two compounds belong to different homologous series and show different chemical reactions — for example, aldehydes are easily oxidised to carboxylic acids, while ketones are resistant to mild oxidation.
Functional group isomerism is one of the most dramatic forms of isomerism because the compounds often have completely different chemical properties. Always check the functional group first when comparing isomers with the same molecular formula.
(iv) Metamerism
Metamerism arises when compounds have the same molecular formula and the same functional group, but differ in the alkyl chains attached on either side of the functional group.
The functional group itself remains the same; what changes is how the carbon atoms are distributed between the two alkyl groups that flank the functional group. This type of isomerism is most commonly seen in compounds containing a divalent functional group such as ether (), ketone (), or secondary amine (). …