# of constitutional isomers possible questions

[herkulease]

Is there a trick to these types of questions that ask how many isomers there are of some random hydrocarbon or some general formula for an alcohol

I'm annoyed by them as there's really no way I know of to zip through them other than drawing it out.

I've seen it at least a few times already. In destroyer, achiever, kaplan.

And guess what I always miss them. except with destroyer as I have unlimited time to do a the problem.

I really hate it too because its a fairly easy problem just time consuming.

anyone know of a formula or anything similar?

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[americanpierg]

hint... 2^x

 

[herkulease]

2^n is for # stereoisomers.

 

[KittySquared]

Ok

Any molecule with n chiral centers would have

2^n possible stereoisomers.

Don't bother with meso compounds because they are symmetrical and hence not optically active, therefore no chical centers.

After some research, I DID find this and I think it would be a nice little note to know... let me know if a anyone disagrees.

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This is the formula


(2^n) + 1, where n = 0 when number of carbon atoms = 4


So, C7H16 would have:

(2^3)+1 = 9 isomers.

The only exception for the formula is when C=10, then the formula says the number of isomers is 75 when actually it is 65. Magic!

 

[Ibraiz]

Kitty squared, can you do an actual problem explaining your method please?

Or can you do the compound 2,4-diChloropetane?

Using your formula
(2^n) + 1 where n = 2 it comes out to be
(2^2) + 1 = 5 but the actual answer is 3 on this one because one of them is a mesocompound otherwise it would've been 4 isomers with 2 chiral center.

 

[KittySquared]

^Actually, it's not my method. I found it on a forum, but I don't really think it works. It can't be used on a lot of formulas. I'd blow my kitty brains out if I had to sit down and try to come up with a formula. I think the general consensus is that 2^n is the formula you use to calculate isomers. It's in Kaplan BB I think. I bet Destroyer has problems as such.

 

[903874]

Thanks Kitty Squared! For simplicity I suggest making the formula [2^(n-4)]+1, where n= # of carbons up to 9.