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Author Topic: Formula to determine the number of isomers for a structure?  (Read 31257 times)

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Neverquit

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Formula to determine the number of isomers for a structure?
« on: January 16, 2011, 11:28:32 PM »

Hi,

is there a math formula for determining the number of Isomers?

eg. if I have an alkane c6h14 I am told there are 4 isomers which I can find but what if you are asked to provide all isomers for say c10h22? It would be useful to know how many I need to find mathematically.

Thanks and regards,
Neverquit
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rabolisk

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Re: Formula to determine the number of isomers for a structure?
« Reply #1 on: January 16, 2011, 11:32:40 PM »

There may be, but I have never seen it presented in any standard textbook. Most likely this is because it has very little value in terms of increasing understanding or real world practical application. Don't worry, you will almost surely (99.9%) never be asked this on a test.
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Borek

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Re: Formula to determine the number of isomers for a structure?
« Reply #3 on: January 17, 2011, 12:36:18 AM »

http://oeis.org/A000602

http://oeis.org/A000628

But these numbers are wrong - they are based on correct branching, but they don't take into account steric effects.

Try to read paper "C167H336 Is the Smallest Alkane with More Realizable Isomers than the Observed Universe Has 'Particles'" by R.E. Davies and P.J. Freyd published in J. Chem. Educ. 1989, 66, 278-281. Note that downloading the paper from the JCE site is only half of the success, as pdf lacks the first page. Or at least that was the case about two years ago.
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Re: Formula to determine the number of isomers for a structure?
« Reply #4 on: January 17, 2011, 12:44:06 AM »

http://www.ehow.com/how_6931563_calculate-number-isomers.html

This is stupid (AWK, comment not aimed at you, but at authors of the page). They put the tile "How to calculate" and then say "take it from the table below". Idiocy.
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AWK

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Re: Formula to determine the number of isomers for a structure?
« Reply #5 on: January 17, 2011, 04:16:21 AM »

http://www-jmg.ch.cam.ac.uk/data/isomercount/

Applet works on my computer with google chrome

C10H22
--------------------------------------
75 possible structural isomers, EXCLUDING optical isomers
0 of these are sterically unfavourable
Leaving 75  allowed structural isomers
--------------------------------------
136 possible structural isomers, INCLUDING optical isomers
0 of these are sterically unfavourable
Leaving 136  allowed structural isomers


C20H42
--------------------------------------
366319 possible structural isomers, EXCLUDING optical isomers
88 of these are sterically unfavourable
Leaving 366231  allowed structural isomers
--------------------------------------
3396844 possible structural isomers, INCLUDING optical isomers
177 of these are sterically unfavourable
Leaving 3396667  allowed structural isomers


http://www-jmg.ch.cam.ac.uk/data/isomercount/isojava.html
« Last Edit: January 17, 2011, 04:32:01 AM by AWK »
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