Is there any general formula to calculate number of functional isomers ?

Question

 Formula to calculate no of functional isomers 

Answer 1

There is a general formula (or procedure)for determining the maximum number of functional (constitutional) isomers of a particular compound. Let's say that compound has a molecular formula of C₅H₁₀. To be asked to do this, it must be relatively small, as you'll soon see. But this will illustrate the idea. 

 

Knowing that the number of carbons present places limits on the other numbers of atoms our logic follows that course.

 

Nor is there a formula for calculating the number of total isomers, including stereoisomers. There is the idea that 2^n number of chirality centers equals the maximum possible value (not the actual value) of stereisomers...and is further limited by the idea of point group symmetry.

 

If you've learned about the index of unsaturation yet, this compound must possess either one π (pi)-bond, a ring, but not both. If you haven't you will still understand this. 

 

It is much easier to demonstrate this with drawings than to talk about it.

 

See the file in My Wyzant locker:  http://bit.ly/1gLhufO

 

Organic Chem1 Maximum Structural Isomer Number.pdf 

 

P.S. After all that...there actually is a method...but not a simple "formula" to actually calculate the total number of structural isomers. I use it in my work but it is too complicated to bring up here.

 

 

 

 

 

 

 

 

Answer 2

There is a formula for calculating the number of maximum possible stereoisomers, being related to the number of chiral centers. However, there is no direct way to calculate the exact number, or the number of functional or constitutional isomers. It's really just a trial sort of problem.