Chemistry · General Principles Of Organic Chemistry · NEET
Cis-trans isomerism (also called geometrical isomerism) happens when rotation is blocked, so groups get locked on the same side (cis) or opposite sides (trans). It appears in two places: across a C=C double bond and across a ring. The one condition to remember: each of the two locked carbons must carry two different groups. Memory hook: "Same side = cis, across = trans; but only if each carbon has two different partners."
Cis and trans but-2-ene: the pi bond blocks rotation, so the two CH3 groups stay locked on the same side (cis) or opposite sides (trans). Isomerism exists only because each double-bond carbon carries two different groups (CH3 and H).
Your doubts, answered
Why does a C=C double bond show cis-trans isomerism but a C-C single bond does not?
A single bond lets the two carbons spin freely (free rotation), so any 'sides' you draw instantly rotate into each other and become the same molecule. A double bond has a pi bond that locks the two carbons in one plane and blocks rotation. Because rotation is frozen, groups on the same side stay on the same side (cis) and groups on opposite sides stay opposite (trans) as two real, separate compounds.
How do I quickly check if a compound shows geometrical isomerism?
Look at each carbon of the C=C (or each ring carbon in question). Ask: does this carbon carry two DIFFERENT groups? If BOTH doubly bonded carbons have two different groups, cis-trans isomerism exists. If even one carbon carries two identical groups (like two H, or two CH3), there is no cis-trans isomerism, because swapping identical groups gives the same molecule.
Why does but-2-ene show cis-trans but but-1-ene does not?
But-2-ene is CH3-CH=CH-CH3. Each double-bond carbon carries one H and one CH3 (two different groups), so cis and trans forms exist. But-1-ene is CH2=CH-CH2CH3. The first carbon (=CH2) has two H atoms, which are identical. When one carbon has two identical groups, no geometrical isomerism is possible.
Can rings show cis-trans isomerism even though there is no double bond?
Yes. A ring also blocks free rotation, just like a double bond. In 1,2-dimethylcyclohexane the two CH3 groups can lie on the same face of the ring (cis) or on opposite faces (trans). The same condition still applies: each ring carbon in question must carry two different groups (here CH3 and H). This is exactly why NEET 2025 asked about 1,2-dimethylcyclohexane.
Is cis-trans isomerism the same as optical isomerism?
No. Both are types of stereoisomerism, but they arise differently. Cis-trans comes from restricted rotation (double bond or ring) and needs different groups on each locked carbon. Optical isomerism comes from a chiral centre (a carbon with four different groups) and produces non-superimposable mirror images. Cis-trans isomers usually differ in melting point, boiling point and dipole moment; optical isomers differ in how they rotate plane-polarised light.
⚠️ The NEET trap ✗ Every compound with a C=C double bond shows cis-trans isomerism. ✓ Only alkenes where EACH doubly bonded carbon carries two different groups show cis-trans isomerism. If one carbon has two identical groups (=CH2, or C bearing two CH3), there is no geometrical isomerism. 🧠 A double bond is necessary but NOT enough. Check each carbon for two different groups before you answer.
Real NEET questions
NEET 2023 Phase 2
Which amongst the following compounds will show geometrical isomerism?
A · 2-Methylprop-1-ene
B · 3,4-Dimethylhex-3-ene ✓
C · Pent-1-ene
D · 2,3-Dimethylbut-2-ene
Solution: Geometrical isomerism needs each doubly bonded carbon to carry two different groups. In 3,4-dimethylhex-3-ene, CH3CH2-C(CH3)=C(CH3)-CH2CH3, each sp2 carbon bears a methyl and an ethyl (two different groups), so cis and trans forms exist. In 2-methylprop-1-ene and 2,3-dimethylbut-2-ene a double-bond carbon carries two identical groups, and pent-1-ene has a terminal =CH2 (two H), so those three do not show it.
NEET 2025
Which one of the following compounds can exist as cis-trans isomers?
A · 1,1-Dimethylcyclopropane
B · 1,2-Dimethylcyclohexane ✓
C · Pent-1-ene
D · 2-Methylhex-2-ene
Solution: Cis-trans isomerism needs two different groups on each of two carbons whose relative arrangement is fixed by a ring or a C=C. In 1,2-dimethylcyclohexane the two methyls sit on adjacent ring carbons and can lie on the same face (cis) or opposite faces (trans). 1,1-Dimethylcyclopropane has both methyls on one carbon; pent-1-ene has a terminal =CH2; and 2-methylhex-2-ene has a carbon bearing two identical methyls, so none of those three qualify.
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.
What is the main condition for cis-trans isomerism?
Each of the two carbons locked by the double bond (or by a ring) must carry two different groups. If any one of them has two identical groups, no cis-trans isomerism is possible.
What is the difference between cis and trans?
In the cis isomer the two reference groups lie on the same side of the double bond or ring; in the trans isomer they lie on opposite sides. This changes properties like melting point and dipole moment.
Why is cis-trans isomerism also called geometrical isomerism?
Because the two isomers have the same connectivity but different geometry, meaning a different arrangement of atoms in space. NCERT lists geometrical and optical isomerism as the two kinds of stereoisomerism.
Do alkynes show cis-trans isomerism?
No. A carbon of a C-C triple bond can hold only one more group, so it can never carry two different groups on each triple-bond carbon. The straight-line geometry also leaves no 'sides' to be cis or trans.
Which isomer usually has the higher melting point, cis or trans?
For solids, the trans isomer usually has the higher melting point because its more symmetric shape packs better in the crystal, as noted in NCERT.
