Question

Which compound exhibits geometric isomerism also called cis-trans isomers? For a molecule to have geometric isomers, each C atom in the C=C bond must also be bonded to two different groups. For a cycloalkane the groups have to be on opposite sides of the ring. Draw each one. A. propene B. 2-pentene C. 1,1-dichloroethene D. 1,2-dichlorocyclopentane E. 1,1-dichlorocyclopentane

Answer 1

Answer:

Only B and D

Explanation:

Maybe this is easier if you see the image I'll attach. I'll add a comment for each structure.

Remember that the bonds C=C are really strong and fixed. It means they can't turn, spin or or move and there's a flat plane generated around the bond and the four groups linked to the carbons.

a) In this case, there's a C that has two H. This doesn't change the properties and it's the same regardless how you draw it.

b) For this molecule, I just sketched the trans structure. You can see the the first C has an H and a CH3 while the second C has an H and a CH2CH3. In this case, it does matter where the H goes --> therefore, we can have cis-trans isomers. Basically, it matters because (and this explanation is useful for all the examples) if you sketch the molecule in trans, just like I did, you'll have the hydrofobic groups CH3 and CH2CH3 on opposite sides of the molecule. This means that the hydrofobicity  is equally distributed throughout the molecule. If you had the cis structure, the CH3 and the CH2CH3 groups will be on the same side of the plane --> It means that the hidrofobicity will be distributed on one pole of the flat molecule.

This is why this isomery is so important! It changes the molecule's chemical properties.

C) For this molecule, the explanation of A is also valid. It doesn't matter how you draw it, you'll always have that distribution and the Cls on one side and the H on the other

D) Well, this is more complex to see. I remember I learnt the cycles with some actuall balls and it was easier to see. Anyway, in this case I sketched just the trans isomer. You can see the Cls are in opposite directions of the plane of the molecule. In this case, it also matters where you put the second Cl, if it's in trans or in cis.

Just consider that the cyclopentane has the equatorial and the axial planes. In my sketch, the two Cl are in opposite directions of the axial plane. If it were in cis, the 1st Cl is on the downwards axial plane whereas the second should be on the downwards equatorial plane.

It's not so easy to explain this one with a text, let me know if you need a deeper explanation

E) Idem A and C --> It doesn't matter how you sketch the Cls