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How many different organic structures (from the pure theoretical viewpoint) can be drawed with only 4 (exact) carbon atoms and with/without hydrogen? Polycyclic compounds and bridged compounds are also allowed. Please, name them all as well! :)

Remark: I have drawed the chemical graphs in a piece of paper (the skeletons) and I get 37 graphs (including linear with simple, double and triple bonds, and planar and nonplanar compounds), without geometric isomerism (cis/trans). Is that OK?

wythagoras
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riemannium
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2 Answers2

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Final update, all earlier edits incorporated.

Groundrules: Considering compounds with:

  • only carbon and hydrogen
  • only 4 bonds to carbon

There are 37 isomers without considering trans isomers; 49 when trans isomers are included. Also, many of these compounds seem extremely unstable and therefore unlikely to exist.

Note to self: check back in 20 years and see how many of the unlikely ones have been detected.

enter image description here

Names by row:

  • butane, isobutane
  • but-1-ene, but-2-ene, 2-methylpropene
  • buta-1,3-diene, buta-1,2-diene, buta-1,2,3-triene
  • but-1-yne, but-2-yne, but-1-ene-3-yne, buta-1,3-diyne
  • methylcyclopropane, 2-methylcyclopropene, 1-methylcyclopropene, methylenecyclopropane, methylenecyclopropene, methlycyclopropadiene
  • methylcyclopropyne, methlenecyclopropyne
  • cyclobutane, cyclobutene, cyclobuta-1,2-diene, cyclobuta-1,3-diene, cyclobutatriene, cyclobutatetraene, cyclobutyne, cyclobutenyne, cyclobutadiyne
  • bicyclobutane, bicyclobut-1(3)-ene, bicyclobut-1(2)-ene, bicyclobuta-1,3-diene, bicyclobuta-1,2-diene
  • tetrahedrane, tetrahedrene, tetrahedradiene
ron
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  • Comparing with my skeletons, you lack a 1methyl-cyclopropene, and the cyclobutatriene with 3 double bonds (irrespectively it existed or not, you have not drawed it). Also, you have not drawed the C4 (the square with 4 double bonds). Thus, I get 36 (or 37, if I include the cis/trans versions of 2-butene, that I had not considered diferent so far). – riemannium Nov 14 '14 at 00:24
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    @ron You might want to include a note for posterity that some of these "compounds" are incredibly strained and likely unstable. (This was why I was including ?? marks in my answer.) – Geoff Hutchison Nov 14 '14 at 00:45
  • @GeoffHutchison A couple of things 1) I'd suggest leaving your answer in place, it is nice work, 2) regarding 1,3- butadiene are you talking about s-cis and s-trans about the 2-3 bond? – ron Nov 14 '14 at 00:51
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    @GeoffHutchison There is a higher than normal barrier to rotation about the 2,3-bond in 1,3-butadiene due to a contribution from the resonance structure with a 2-3 double bond. However, the barrier is still low enough that the s-cis and s-trans forms are generally considered rotamers. – ron Nov 14 '14 at 00:58
  • Now I get 37 (without cis-trans): you are left a bicyclo[1.1.0]but-1,1-diene, I think...Indeed, I also forgot it in my first skeletons! – riemannium Nov 14 '14 at 01:07
  • cyclobuta-1,2-diene could have enantomers http://en.wikipedia.org/wiki/Allene – DavePhD Nov 14 '14 at 12:54
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    Is it legitimate to refer to cyclobutatetraene and cyclobutadiyne as different compounds, or are they just different resonance structures? Both are just a ring of 4 carbon atoms and no hydrogen. – DavePhD Nov 14 '14 at 15:31
  • @DavePhD I pictured cyclobutadiyne as having 2 sets of non-equivalent bonds, whereas the bond lengths in cyclobutatetraene were all equivalent. So they're not resonance structures, but probably not much of a barrier separating them. – ron Nov 14 '14 at 16:00
  • @ron like Fig. 3 of http://pubs.acs.org/doi/pdf/10.1021/ja0123002 – DavePhD Nov 14 '14 at 16:21
  • @DavePhD I can't get to fig. 3, but do you understand my argument about the 2 molecules having different bond lengths, therefore they can't be resonance structures? – ron Nov 14 '14 at 16:29
  • @ron, yes I understand what you are saying, that cyclic C4 with all bond lengths equal would be one compound and C4 with alternating short and long would be a different compound (but Fig. 3 of article refers to these as resonance structures in another cyclic pure-carbon compound). Another article makes calculations for linear C4 and cyclic C4 and says the calculated electronic structure of cyclic C4 is alternating short and long bonds. http://www.sciencedirect.com/science/article/pii/000926149500801A# – DavePhD Nov 14 '14 at 16:36
  • @DavePhD Do they say how much higher in energy the cumulenic isomer is and what barrier separates the 2 isomers? – ron Nov 14 '14 at 16:45
  • @ron, actually after looking at a 3rd article http://pubs.acs.org/doi/abs/10.1021/ja00081a041, it looks like C4 has two possible isomers: a linear isomer with bond lengths 1.32, 1.30, 1.32; and a rhombic (cyclic or bicyclic) isomer with 4 bond lengths of 1.46 and two 63 degree angles and two larger angles, with the linear isomer being lower energy. – DavePhD Nov 14 '14 at 17:32
  • @ron Shouldn't 4th molecule from the left on the 5th row be named methylidenecyclopropane? – Mockingbird May 25 '17 at 17:30
  • @Mockingbird I don't think so. If you google "methylidenecyclopropane" all you get back is "methylenecyclopropane". "Methylenecyclopropane" is also the name used by Wikipedia. – ron May 25 '17 at 18:17
  • @ron So why "methlenecyclopropyne" when "methylenecylclopropane"? – Mockingbird May 25 '17 at 23:31
  • "https://pubchem.ncbi.nlm.nih.gov/compound/Methylenecyclopropane" says methylidenecyclopropane also works. – Mockingbird May 25 '17 at 23:37
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    Cyclobutatetraene?? Are you kidding me? – Pritt says Reinstate Monica Jun 02 '17 at 10:00
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    @PrittBalagopal It fits within the stated "ground rules". – ron Jun 03 '17 at 19:59
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How many different organic structures (from the pure theoretical viewpoint) can be drawed with only 4 (exact) carbon atoms and with/without hydrogen?

We could make strict rules like each carbon has exactly 4 bonds and get a specific answer, but this is not reality. There can be lone pair electrons and unpaired electrons. The octet rule is not stictly followed.

$\ce {C_4}$ actually has been observed and is linear.

:C=C=C=C:

$\ce {C_4}$ has been the subject of numerous theoretical and experimetal papers because of its possible occurrence in nebulae. It has been debated whether a singlet linear, triplet linear or rhombic (kite shape) state is the lowest energy state. http://www.sciencedirect.com/science/article/pii/S0009261400005765

Neither the linear nor rhombic states follow the naive rules.

Linear $\ce{C_4H}$ has been observed both in the lab and outer space.

In fact according to the University of Kohn lists Molecules in Space, linear $\ce{C_4H}$ is one of only 61 molecules and molecular ions found in extragalactic space and of only 190 found in the interstellar medium or circumstellar shells as of 2016.

For $\ce{C_4H_2}$ linear butadiyne in known. Cyclobutatriene, cyclobutenyne and tetrahedrene have been ruled out theoretically as not represtenting any actual potential energy local minimum, while similar to $\ce {C_4}$, structures having lone pair or unpaired electrons and not following the octet rule (such as carbenes) have been calculated to represent actual minima. See the following references for theoretical cyclic $\ce{C_4H_2}$ structures:

http://onlinelibrary.wiley.com/doi/10.1002/jcc.540020211/pdf

http://pubs.acs.org/doi/pdf/10.1021/jo060698k

http://pubs.acs.org/doi/pdf/10.1021/jo000941u

In outer space, not only has the usual HCCCCH isomer been found but also $\ce{H2CCCC}$

See Observations of cumulene carbenes, H2CCCC and H2CCC, in TMC-1

DavePhD
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