# Planarity of given molecules in all conformations

Which of these have all its atoms in one plane in all the possible conformations (if any):

A) buta-1,3-diene

B) but-1-en-3-yne

C) $$\ce{CH2=C=O}$$

D) $$\ce{CH2=C=CH2}$$

While I was attempting this question, I got stuck in option B! I was sure about option A that since it has conformational change possible between C2-C3 bond, atoms will be present in more than one single plane.

Next, I am also convinced that option D can't be the answer as allenes are non planar and they have H-C-H planes perpendicular to one another. Then, for option C also, I know that since all the atoms are $$sp^2$$ hybridised, it has to be a planar straight molecule.

But, I am having a doubt in option B; how should we determine if the molecule is planar or not and whether all of its atoms are present in a single plane?

• Do you have a modeling kit to build it in 3D?
– Karsten
Apr 26, 2019 at 18:00
• No, I don't have. Apr 26, 2019 at 18:33
• All the atoms includes hydrogens, consider the angles that the H atoms have wrt the plane of the c atoms around double and triple bonds. Apr 26, 2019 at 19:14
• Perhaps I am still sleeping but if a molecules hss all its atoms there is no reason to say in all possible conformations. Question should be which molecules has just one confirmation, or even better which one doesn't have conformers. Not pedantic, it is really conceptual. ... Apr 27, 2019 at 6:45
• Yes, it's obvious that a molecule has all its atoms...but the question is not just about whether it has a conformer or not; rather, it also questions if the atoms lie in a single plane provided, that molecule has conformers. Apr 27, 2019 at 7:38