[Schrödinger]

Guys i need help with this :

I read that chain isomerism and positional isomerism are not possible together i.e, the same compound can't exhibit both at the same time..

Can anyone tell me why? 

[lancenti]

If you draw out a compound like octane and try to get the different chain and positional isomers, you will realize why.

[sjb]

To me, the two terms are virtually synonymous, though there are perhaps times I'd use one over the other. Can you give examples of where you have seen each term?

[Schrödinger]

I think i should rephrase my question:

Consider 2-methylpropan-2-ol and butan-2-ol ...They are chain isomers right?

Now consider 2-methylpropan-2-ol and butan-1-ol ....Aren't they both chain and positional isomers? If not, what are they?

They have the same chemical formula too...What kind of isomers are they?

[sjb]

I think i should rephrase my question:

Consider 2-methylpropan-2-ol and butan-2-ol ...They are chain isomers right?

Now consider 2-methylpropan-2-ol and butan-1-ol ....Aren't they both chain and positional isomers? If not, what are they?

They have the same chemical formula too...What kind of isomers are they?

Well, t-butanol and 2-butanol are also positional isomers, I suppose, even if the carbon skeleton is different in both cases. Perhaps that's the crux of the issue? How do you classify 1-butanol and 2-butanol?

[Schrödinger]

Can u explain to me why t-butanol and 2-butanol are also positional isomers. I thought the basis for positional isomerism was the same base chain or same carbon skeleton...
I thought the same skeleton with different positions of a functional group was positional isomerism.

t-butanol has only 3 C's in its longest chain while 2-butanol has 4 C's. That makes their chain different. Plus, the position of -OH group as is obvious, is also different. So, we have a pair of chain and positional isomers.
Am I correct in saying so?

[cundi]

For me the diference is clear.
Chain isomers: the main chain of the compound changes from one to other. hexan-2-ol, 3-mehtylpentan-2-ol
Positional isomers: the chain does not change, it is the same in both compounds, but a funtional group or a secondary chain changes its position on the molecule. i.e : hexan-1-ol and hexan-2-ol, or 3-mehtylpentan-2-ol and 2-mehtylpentan-2-ol

[Schrödinger]

The difference is clear to me too.

My question is, if you consider butan-2-ol and 2-methylpropan-2-ol, they have the same molecular formula, same molar mass. So , they need to be isomers right?

We can even pin down to say that they are structural isomers, as we know their structures as well. But, under what isomerism are they classified? Are they chain, positional or pos-chain(i came up with the name "pos-chain" because i thought they needed a separate classification )

[lancenti]

I think we simply call them isomers at this point. Unlike Stereochemistry where we have a blanket term called diasteromers, I don't think we have one for this. I could be very wrong, though.

[AWK]

http://en.wikipedia.org/wiki/Structural_isomer

[Schrödinger]

@lancenti : that's what i was forced to conclude as well....looks like we need a new name for such kind of isomers...

[meherban ali]

can any one can give me formula to find chain isomerism which is universal