# Why is this structure impossible?

Explain why the structure $\ce{H-C=C-H}$ doesn't make sense chemically.

Not only am I just looking for a check up of my answer, but am I making any sense in what I'm saying here? Am I mixing up or misunderstanding concepts here?

The hydrogen and carbon atom form a covalent bond. Hydrogen has only 1 electron in its valence shell, so it 'needs' 1 of carbon's electrons to complete it. Now there are 3 electrons left to take in the carbon atom. But this isn't enough to make a double bond with the other carbon atom: we're one electron short.

• There are three electrons left on each carbon. Your assumption will lead you astray. Apr 20, 2016 at 15:42
• Could you elaborate a bit on this? Does a single bond in a structure have a 'higher priority' than a double bond? Apr 20, 2016 at 15:47
• My comment has nothing to do with "priority" it is about the the total number of electrons in play for bonding. Apr 20, 2016 at 15:49
• @Aperion We are just counting the total electrons available for bonding. The order you count them in doesn't matter so you are free to deal with simpler bonds first and then see what is available for the rest. But you have to count the total correctly or you will not get a sensible answer. Apr 20, 2016 at 15:55
• @Apeiron You can also always write up a self-answer. That is encouraged on Stack Exchange ;)
– Jan
Apr 20, 2016 at 16:47

Thus, each of the carbon atoms has one extraneous electrons; the structure would be a diradical structure. It should better be represented e.g. like this: $\ce{H-C^.=C^.-H}$.
It is possible for these two radicals to be alongside each other — see for example the structurally strongly related dioxygen molecule which is a triplet (diradical) in ground state and not adequately described in simple Lewis structure formalisms. However, it is highly unlikely for two carbons without additional constraints to form a diradical like that; forming a triple bond ($\ce{H-C#C-H}$) would be much better energetically (for carbon!).
• Shine light on acetylene $\to$ get $\ce{H-C=C-H}$ $\to$ profit? Feb 24, 2017 at 0:42