Isomerism — Structural and Stereo, with Worked Examples
Two bottles on a shelf can carry the same label — C4H10 — and hold two different liquids that boil at different temperatures. That is isomerism, and it is the single idea that makes organic chemistry big. Carbon can join the same set of atoms in more than one arrangement, so one molecular formula can describe many real, separable compounds. This guide sorts the whole family in one place, shows how to count isomers without missing any or double-counting, and works through the examples that CBSE, ICSE, JEE, NEET and IIT-JAM keep coming back to.
The definition, stated carefully
Two things follow immediately, and both are marks in an exam. First, isomers always have exactly the same molar mass, because molar mass depends only on the formula. Second, they are different substances — different melting point, boiling point, density, and often very different chemistry. Same formula, different molecule.
The family tree, as a table
Isomerism splits into two branches. In structural (constitutional) isomerism the atoms are joined in a different order. In stereoisomerism the connectivity is identical and only the arrangement in space differs.
| Branch | Type | What differs | Example pair (same formula) |
|---|---|---|---|
| Structural | Chain | Skeleton is straight or branched | n-butane / 2-methylpropane (C4H10) |
| Position | Same skeleton, group sits elsewhere | propan-1-ol / propan-2-ol (C3H8O) | |
| Functional | Different functional group altogether | ethanol / methoxymethane (C2H6O) | |
| Metamerism | Alkyl groups split differently around one function | methoxypropane / ethoxyethane (C4H10O) | |
| Tautomerism | A hydrogen and a double bond migrate; forms interconvert | keto and enol forms of a carbonyl compound | |
| Stereo | Geometrical (cis–trans, E–Z) | Groups fixed on either side of a rigid bond or ring | cis-but-2-ene / trans-but-2-ene (C4H8) |
| Optical | Non-superimposable mirror images | (+)- and (−)-lactic acid (C3H6O3) | |
| Conformational | Rotation about a single bond only | staggered and eclipsed ethane |
Worked example 1 — chain isomers of C4H10
Four carbons can be strung in a line, or three in a line with one branch. Only two skeletons are possible:
• n-butane: CH3–CH2–CH2–CH3
• 2-methylpropane (isobutane): CH3–CH(CH3)–CH3
Check the molar mass is the same for both, using C = 12.011 and H = 1.008 g/mol:
C: 4 × 12.011 = 48.044
H: 10 × 1.008 = 10.080
M = 48.044 + 10.080 = 58.124 ≈ 58.12 g/mol for each isomer.
Yet n-butane boils near −0.5 °C and 2-methylpropane near −11.7 °C. Branching lowers the surface contact between molecules, so the dispersion forces are weaker and the branched isomer boils lower. Identical formula, identical molar mass, different substance.
Worked example 2 — three isomers of C3H8O
M = 3(12.011) + 8(1.008) + 15.999 = 36.033 + 8.064 + 15.999 = 60.096 ≈ 60.10 g/mol for every one of them.
• Propan-1-ol, CH3CH2CH2OH
• Propan-2-ol, CH3CH(OH)CH3 — a position
isomer of the first; the –OH has moved along the same chain
• Methoxyethane, CH3–O–CH2CH3 — a
functional isomer; it is an ether, not an alcohol at all
The chemistry separates them at once. Both alcohols react with sodium to release hydrogen; the ether does not. Propan-2-ol oxidises to a ketone (propanone), propan-1-ol to an aldehyde and then to propanoic acid. This is exactly how identification questions are set.
Worked example 3 — when does geometrical isomerism appear?
A C=C double bond cannot rotate freely, so the groups on either end are locked in place. But that alone is not enough. Both conditions must hold:
But-2-ene, C4H8. Each doubly bonded carbon carries
one CH3 and one H — two different groups, so the test is passed.
• cis-but-2-ene: both methyl groups on the same side
• trans-but-2-ene: methyl groups on opposite sides
But-1-ene, C4H8. The terminal carbon (=CH2) carries two identical hydrogens, so swapping them gives the same molecule. No geometrical isomers. But-1-ene is still a position isomer of but-2-ene — the formula and the molar mass are the same, 4(12.011) + 8(1.008) = 48.044 + 8.064 = 56.108 ≈ 56.11 g/mol.
When the four groups are all different, cis/trans becomes ambiguous and the E/Z system is used instead. Rank the two groups on each carbon by Cahn–Ingold–Prelog priority (higher atomic number wins at the first point of difference). If the two higher-priority groups are on the same side the alkene is Z; on opposite sides it is E. Cis and Z often coincide, but they are not the same rule and one does not define the other.
Worked example 4 — counting optical isomers
A carbon bonded to four different groups is a chiral (asymmetric) centre. The molecule and its mirror image cannot be placed on top of each other, so two distinct compounds exist — a pair of enantiomers that rotate plane-polarised light by equal amounts in opposite directions.
Lactic acid, CH3–CH(OH)–COOH. The middle carbon carries –CH3, –OH, –COOH and –H: four different groups, so n = 1 and 21 = 2 stereoisomers, the (+) and (−) enantiomers. Molar mass of C3H6O3 = 3(12.011) + 6(1.008) + 3(15.999) = 36.033 + 6.048 + 47.997 = 90.078 ≈ 90.08 g/mol for both.
Tartaric acid, HOOC–CH(OH)–CH(OH)–COOH. Two chiral centres, so the formula predicts 22 = 4. In reality there are only three: the (+) form, the (−) form, and one meso form. The meso isomer has an internal mirror plane — its top half cancels its bottom half — so it is optically inactive and is identical to its own mirror image. This is the classic reason the 2n rule is written as a maximum, not an equality.
Conformers are not isomers you can bottle
Rotation about a C–C single bond turns staggered ethane into eclipsed ethane. These are conformations, and at room temperature the molecule flips between them billions of times a second, so you cannot separate them and put them in two bottles. Every other kind of isomer in the table above gives compounds that can, in principle, be isolated. Keep this distinction clear — questions often ask whether two drawings are "different compounds" or "the same compound drawn differently".
Common mistakes that cost marks
- Calling resonance structures isomers. In resonance no atom moves — only electrons are drawn differently, and the real molecule is a single hybrid. Isomers are separate substances with atoms in different places.
- Drawing the same chain twice. Bending a straight chain on paper does not create a new isomer. Before counting, name each structure; two identical names mean one compound.
- Claiming cis–trans isomerism where one carbon has two identical groups. Check both ends of the double bond, not just one.
- Forgetting the meso form and answering 2n for every molecule. Look for internal symmetry first.
- Mixing up cis/trans with Z/E. They agree in simple cases and disagree when priorities do not follow size — always assign priorities properly.
- Assuming isomers have different molar masses. They never do. If your two answers differ in molar mass, one of the formulas is wrong.
Where isomerism appears in exams
| Exam | Typical question |
|---|---|
| CBSE / ICSE Class 11 | Draw and name all structural isomers of a given formula |
| CBSE / ICSE Class 12 | Geometrical isomerism in alkenes and in coordination compounds; distinguishing tests |
| JEE / NEET | Counting total isomers, identifying chiral centres, meso detection |
| IIT-JAM / CUET-PG | R/S and E/Z assignment, optical activity of given structures |
| GATE / CSIR-NET | Conformational analysis, stereochemistry of reaction products |
Confirm two structures really are isomers. Type both formulas into the Molar Mass & Composition calculator — genuine isomers must return exactly the same molar mass and the same percentage composition. If the numbers differ, you have written a different compound, not an isomer.
Open the Molar Mass Calculator →Struggling to see isomers on paper? Organic chemistry rewards guided practice more than any other chapter. ABC Chemistry runs Class 11–12 chemistry coaching at the Gurugram centre and online classes across India — details at abcchemistry.in.