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Is there a formula to know how many constitutional isomers are there for a compound? For instance in this example, most students would miss the 6th constitutional isomer and will probably only write the first 5. So I would like to kindly ask if there's a formula or pattern for it.

enter image description here

Amethyst
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    Most students above the very entry level will count the 6th all right. As for the formula, well, if you want one that you'd be able to use, then it doesn't exist. Short of that condition, see https://oeis.org/A002094. – Ivan Neretin Feb 28 '21 at 07:13
  • @ Ivan Neretin. I read German. But I have not been able to understand your publication, written in 1875 in German. They present a number with a cross of four numbers, somewhat like a matrix plus a cross. Have you been able to understand it ? – Maurice Feb 28 '21 at 10:19
  • @Maurice I never bothered to go that deep. The OEIS page contains the formula (sort of); what's more to wish for? – Ivan Neretin Feb 28 '21 at 11:15
  • @Ivan Neretin. Apparently you have been able to get this formula from OEISD. How lucky ou are ! I was not clever enough. Could you report it here ? – Maurice Feb 28 '21 at 13:45
  • @Maurice Why, that's what they have in the FORMULA section. – Ivan Neretin Feb 28 '21 at 14:23
  • @IvanNeretin. The formula for $\pu{A(x)}$ is simply unreadable. I copy it here: $$\pu{A(x) = 1 + \frac{1}{6} x} +\pu{(A(x)^3 + 3 A(x^2) + 2 A(x^3)}$$ It is impossible to solve it. And there is also a function $\pu{B(x)}$. What for ? – Maurice Feb 28 '21 at 17:18
  • @Maurice Because A(x) is not the answer yet, but merely a step to it, and so is B(x), and then there are more steps after that. See, I never said the formula is easy to read and implement. In fact, I said quite the opposite in my first comment. – Ivan Neretin Feb 28 '21 at 17:22

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There isn't a good overall formula that I know.

You can look at C/H ratio and easily tell a bit here. There must be at least one of the following, but not two:

  • carbon ring
  • carbon double Bond

(1) 1-Butene enter image description here

(2) 2-Butene which has two stereoisomers

cis-2-Butene enter image description here

trans-2-Butene enter image description here

(3) Isobutylene enter image description here

(4) cyclobutane enter image description here

(5) methylcyclopropane enter image description here

MaxW
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