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

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    $\begingroup$ 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 oeis.org/A002094. $\endgroup$ Commented Feb 28, 2021 at 7:13
  • $\begingroup$ @ 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 ? $\endgroup$
    – Maurice
    Commented Feb 28, 2021 at 10:19
  • $\begingroup$ @Maurice I never bothered to go that deep. The OEIS page contains the formula (sort of); what's more to wish for? $\endgroup$ Commented Feb 28, 2021 at 11:15
  • $\begingroup$ @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 ? $\endgroup$
    – Maurice
    Commented Feb 28, 2021 at 13:45
  • $\begingroup$ @Maurice Why, that's what they have in the FORMULA section. $\endgroup$ Commented Feb 28, 2021 at 14:23

1 Answer 1


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

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    $\begingroup$ Specifically, calculating the degree of unsaturation of the compound will tell you that either one ring or double bond exists. $\endgroup$ Commented Feb 28, 2021 at 13:03

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