Chemistry · General Principles Of Organic Chemistry · NEET
It is when two molecules have exactly the same atoms and bonds, but the groups sit in different fixed places around a C=C double bond. Because a double bond cannot spin, the two arrangements cannot turn into each other. Example: but-2-ene has a cis form (two methyls on the same side) and a trans form (two methyls on opposite sides).
In the cis isomer, the two similar (or reference) groups are on the SAME side of the double bond. In the trans isomer, they are on OPPOSITE sides. Trans isomers are usually more stable because the bulky groups are farther apart and repel less.
A double bond is made of one sigma bond and one pi bond. The pi bond is formed by sideways overlap of p-orbitals. To rotate, you would have to break this pi bond, which needs a lot of energy. So the groups stay locked in place, and that locking is what creates geometrical isomers. Single bonds have no pi bond, so they rotate freely and show no geometrical isomerism.
Only when EACH carbon of the C=C double bond carries two DIFFERENT groups. If any one double-bond carbon has two identical groups (like two H or two CH3), there is no geometrical isomerism. Rings can also show cis/trans if two ring carbons each carry two different groups.
E/Z is a more exact system used when cis/trans is unclear. On each double-bond carbon, you rank the two groups by atomic number (CIP priority). If the two higher-priority groups are on the same side, it is Z (from German 'zusammen', together). If they are on opposite sides, it is E ('entgegen', opposite). Cis/trans compares 'like' groups; E/Z compares 'priority' groups, so they do not always match.
2-Methylprop-1-ene is (CH3)2C=CH2. One double-bond carbon carries two identical methyl groups and the other carries two H atoms. Since a carbon has two identical groups, swapping sides gives the same molecule, so no cis/trans forms exist.
Which amongst the following compounds will show geometrical isomerism?
Which one of the following compounds can exist as cis-trans isomers?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Single bonds rotate freely, so groups are not locked in place. Geometrical isomerism needs a restriction to rotation, which comes from a C=C double bond or a ring.
Maleic acid is the cis isomer (both COOH groups on the same side) and fumaric acid is the trans isomer (COOH groups on opposite sides). Fumaric acid is more stable and has a higher melting point.
Usually trans is more stable, because the bulky groups are on opposite sides and repel each other less. Cis isomers have groups crowded on the same side, giving more steric strain.
Yes. In a ring, rotation is also restricted. If two ring carbons each carry a different group, the groups can be on the same face (cis) or opposite faces (trans), like in 1,2-dimethylcyclohexane.
