Chemistry · General Principles Of Organic Chemistry · NEET
Structural isomerism is when two or more compounds have the SAME molecular formula but different structures — meaning the atoms are joined together in a different order. Because the connectivity (which atom bonds to which) is different, the compounds have different properties. This is also called constitutional isomerism. Example: C3H8O can be propan-1-ol (CH3CH2CH2OH) or propan-2-ol (CH3CHOHCH3).
In CHAIN isomerism the carbon skeleton is different — a straight chain versus a branched chain. Example: C5H12 gives pentane, 2-methylbutane, and 2,2-dimethylpropane. In POSITION isomerism the carbon skeleton stays the same but the functional group or substituent sits at a DIFFERENT position on that skeleton. Example: C3H8O gives propan-1-ol (OH on C-1) and propan-2-ol (OH on C-2). Quick check: skeleton changed = chain; only the group moved = position.
Metamerism happens when compounds have the same functional group (usually an ether, thioether, ester, or amine) but different ALKYL groups on either side of that group. Example: C4H10O gives methyl n-propyl ether (CH3-O-CH2CH2CH3) and diethyl ether (CH3CH2-O-CH2CH3). Both are ethers, so the functional group is the SAME — that is why it is NOT functional isomerism. It is the alkyl groups around the oxygen that differ.
The formula C3H6O can be written as an aldehyde, propanal (CH3CH2CHO), or as a ketone, propanone/acetone (CH3COCH3). One has the -CHO group and the other has the C=O in the middle (>C=O ketone). Because they have DIFFERENT functional groups but the SAME molecular formula, they are functional isomers. This is why NCERT uses C3H6O as the standard example of functional isomerism.
Structural (constitutional) isomers differ in the ORDER in which atoms are bonded — the connectivity itself is different. Stereoisomers have the SAME connectivity (same bonds in the same order) but differ only in the 3D arrangement of atoms in space. Geometrical (cis/trans) and optical isomerism are stereoisomerism. Chain, position, functional, and metamerism are all structural isomerism.
No. This is a common trap. In metamerism the functional group is the SAME (both are ethers, or both are amines). Only the alkyl groups on the two sides differ. In functional isomerism the functional group itself is DIFFERENT (aldehyde vs ketone, alcohol vs ether). So metamerism and functional isomerism are two separate categories of structural isomerism.
Look for a formula that can hold a divalent linking group (like -O- in ethers or -S- in thioethers) with enough carbons to split into two different pairs of alkyl groups. C4H10O works because oxygen can sit as ethyl-O-ethyl OR methyl-O-propyl. A formula like C3H8O only gives methyl-O-ethyl (one arrangement), so it does not show metamerism.
The compound which shows metamerism is:
The pair of molecules that are metamers among the following is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Chain isomerism (different carbon skeleton), position isomerism (group at a different position), functional isomerism (different functional group), and metamerism (different alkyl groups around the same functional group). Ring-chain isomerism is sometimes added as a fifth type.
Yes. Structural isomerism and constitutional isomerism mean the same thing — isomers that differ in how their atoms are connected. Both terms are used interchangeably in NEET and in modern IUPAC language.
Ethers (R-O-R'), thioethers, esters, ketones, and secondary/tertiary amines — any group with a central linking atom that can carry two different alkyl groups on its two sides. Ethers are the classic NEET example.
No. Because their atoms are connected differently, structural isomers usually have different boiling points, melting points, solubility, and chemical reactivity. That is why they are counted as distinct compounds.
