Chemistry · Hydrocarbons · NEET
Two conditions must BOTH be true. (1) There must be restricted rotation - this comes free with a C=C double bond because the pi bond locks the two carbons. (2) EACH double-bond carbon must be attached to two DIFFERENT groups. Check both carbons one by one. If even one carbon has two identical groups (two H, two CH3, etc.), the molecule cannot show cis-trans isomerism. Both carbons must pass the test.
Its structure is (CH3)2C=CH2. Look at the right carbon: it holds two H atoms - two identical groups. Since one carbon has a matching pair, the rule fails, so there are no cis and trans forms. The left carbon also carries two CH3 groups, another identical pair. Either way, it does NOT show geometrical isomerism. This is a classic NEET trap option.
No. Its structure is (CH3)2C=C(CH3)2. Each double-bond carbon carries two CH3 groups. Both carbons have identical pairs, so swapping does nothing - there is only one form. NEET 2023 used this exact compound as a wrong option. Always look for a carbon with two same groups to reject a compound quickly.
Its structure is CH3CH2(CH3)C=C(CH3)CH2CH3. Each double-bond carbon is attached to one CH3 group and one CH2CH3 (ethyl) group. These two groups are different, and this is true on BOTH carbons. So both carbons pass the test, and the molecule exists as cis and trans forms. That is why it was the correct answer.
Cis means the two similar or higher-priority groups are on the SAME side of the double bond. Trans means they are on OPPOSITE sides. Because the double bond cannot rotate, cis and trans are truly different molecules with different melting points, boiling points and dipole moments. Cis usually has a higher dipole moment (groups add up); trans is often more stable (groups far apart).
A triple-bond carbon in an internal alkyne (like but-2-yne) is linear and holds only ONE other group, not two - so there is nothing to be cis or trans. A single bond rotates freely, so any 'cis' arrangement instantly flips to 'trans'; the two forms are not separable. Only the C=C double bond gives both restricted rotation and two groups per carbon.
Which amongst the following compounds will show geometrical isomerism?
Which one of the following compounds can exist as cis-trans isomers?
The most suitable reagent for the following conversion is CH3-C#C-CH3 -> cis-2-butene
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Trans is usually more stable because its bulky groups sit on opposite sides, so they crowd each other less. Cis has groups on the same side, causing more strain, so it has higher energy. Exception: some ring or hydrogen-bonded cases flip this.
Cis usually has a higher boiling point because its dipole moments do not cancel, giving a net polarity. Trans is often symmetric so its dipoles cancel (near zero), lowering the boiling point. Trans often has a higher melting point because it packs better in a solid.
Yes. Cis and trans isomers have the exact same molecular formula and the same connectivity of atoms. They differ only in the 3D arrangement of groups around the double bond, so they are stereoisomers, not structural isomers.
Yes. In rings like 1,2-dimethylcyclohexane the ring cannot rotate, so two substituents on different carbons can point on the same side (cis) or opposite sides (trans). NEET 2025 tested exactly this idea.
E-Z is a more precise version of cis-trans. Using priority rules, Z (zusammen) means the two higher-priority groups are on the same side (like cis) and E (entgegen) means opposite sides (like trans). E-Z is needed when the two groups are not simple enough for plain cis-trans naming.
