Chemistry · General Principles Of Organic Chemistry · NEET
Position isomerism happens when two compounds have the same molecular formula and the same functional group (or substituent), but that group is attached to a different carbon of the same carbon chain. Classic NEET example: C3H8O gives propan-1-ol (OH on carbon 1) and propan-2-ol (OH on carbon 2). Memory hook: same chain, same group, only the group's SEAT NUMBER changes.
C3H8O forms two position isomers, propan-1-ol and propan-2-ol. The 3-carbon chain and the -OH group stay the same; only the carbon the -OH sits on changes from C1 to C2.
Your doubts, answered
Is position isomerism the same as chain isomerism?
No. In position isomerism the carbon chain stays the SAME length; only the position of a group or double bond moves along it (propan-1-ol vs propan-2-ol). In chain isomerism the carbon SKELETON itself changes shape, like straight vs branched (butane vs 2-methylpropane). One rule: if the main chain changes, it is chain isomerism; if only the group's location changes, it is position isomerism.
Why are propan-1-ol and propan-2-ol position isomers?
Both are C3H8O, both are alcohols (same -OH functional group), and both have a 3-carbon straight chain. The only difference is where the -OH sits: on carbon 1 in propan-1-ol and on the middle carbon 2 in propan-2-ol. Same formula, same group, same chain, different position, so they are position isomers. This exact pair appears in NCERT.
How do I quickly identify position isomers in NEET?
Check three things in order: (1) same molecular formula, (2) same functional group or substituent, (3) same length of main carbon chain. If all three match and only the number in the IUPAC name changes (like but-1-ene vs but-2-ene, or 1-chloropropane vs 2-chloropropane), it is position isomerism.
Can a double bond show position isomerism?
Yes. The position of a C=C or C(triple)C bond counts too. But-1-ene (CH2=CH-CH2-CH3) and but-2-ene (CH3-CH=CH-CH3) are position isomers of C4H8 because the double bond moves from carbon 1 to carbon 2 while the chain stays the same.
Do position isomers have the same physical properties?
No. Even though the group and chain are the same, moving its position changes boiling point, melting point and sometimes reactivity. For example propan-1-ol and propan-2-ol boil at different temperatures. NEET often tests that isomers are DIFFERENT compounds, not the same substance.
⚠️ The NEET trap ✗ Propan-1-ol and propan-2-ol are functional isomers because they are different alcohols. ✓ They are POSITION isomers. Both belong to the same functional class (alcohol); only the -OH position differs. Functional isomerism needs a DIFFERENT functional group, like an alcohol vs an ether (C3H8O could also give CH3-O-CH2CH3, which is the functional isomer). 🧠 Students see two OH compounds of C3H8O and rush to call them functional isomers.
Real NEET questions
2026
The pair of molecules that are metamers among the following is:
A · CH3CH2CH2OH and CH3-CH(OH)-CH3
B · CH3CH2CH2CH2CH3 and (CH3)2CHCH2CH3
C · CH3CH2COOCH3 and CH3COOCH2CH3
D · CH3OCH2CH2CH3 and CH3CH2OCH2CH3 ✓
Solution: The answer is (D), the ethers, which are metamers. This question is useful for position isomerism because option (A), CH3CH2CH2OH (propan-1-ol) and CH3-CH(OH)-CH3 (propan-2-ol), is the classic POSITION isomer pair: same formula C3H8O, same -OH group, same 3-carbon chain, only the -OH position differs. Recognising it is position isomerism (not metamerism) is exactly why (A) is rejected here.
Solved General Principles Of Organic Chemistry NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is when compounds have the same molecular formula and same functional group but the group is attached to a different carbon of the same chain, like propan-1-ol and propan-2-ol.
Which molecular formula is the standard NCERT example?
C3H8O, which gives propan-1-ol (OH on C1) and propan-2-ol (OH on C2) as position isomers.
Is position isomerism a structural (constitutional) isomerism?
Yes. Position, chain, functional and metamerism are all types of structural isomerism, where atoms connect in a different order. It is not stereoisomerism.
What comes next after learning position isomerism?
Learn chain isomerism next, where the carbon skeleton itself changes from straight to branched, so you can clearly separate the two in NEET questions.
