what is different?

[lizardphunk]

I'm a curious chem newbie!

I have a simple question:

Say you've got 2-methyl-hexane and 3-methyl-hexane. Are these different alkanes? I think yes, but I'm not sure, since the formula is the same.

If I have 2-methyl-alkane and a 2-methylalkane that folds differently from the first one, are they different? (I don't know if this happens with simple

Comments

[a_pawson]

Yes, 2-methyl-hexane and 3-methyl-hexane are different compounds. They are both alkanes, and they are isomers, but they are different compounds.

I'm not sure what you mean by "folds"?

[lizardphunk]

Thank you. That makes sense. :)

I don't know if hexane does the the folding thing - but I've seen drawings of octane folding into an incomplete ring (not connected as a ring, but folded into an U, sort of).

My second question could apply to that U-shaped octane as well - say you've got the U-shaped octane and a straight-chain octane - are they the same?

(I looked for the drawing of the "half-ring" U-shaped alkane, it was 3,4,6-trimethyloctane if that matters. )

edit: i nu spel gud

[lizardphunk]

I made a drawing to illustrate what I mean:

http://www.lizardphunk.org/chem.jpg

Hope that makes more sense. :)

[a_pawson]

Ah. Right, I understand what you mean now. Molecules can take all sorts of different shapes, and there are ways to work out what shape they will actually take. In reality they are not fixed into one shape as they can move and rotate around the bonds and usually do so. In many cases, the shape is determined by their environment but that's something you will learn about in the future (I'm guessing you are just starting to learn chemistry).

For now, you don't need to worry about the exact shape of a molecule. If you have a chemical structure, then don't worry about the way you have drawn it.

[lizardphunk]

http://www.lizardphunk.org/chem.jpg

I made a little drawing. I think the two would have different conformations?

I'm curios as to which would be more stable - my guess is the straight chain, but I don't have any clear ideas as to why. Because the folded one wants to be a ring, maybe?

[madlori]

All molecules have many conformational configurations, which is what you're talking about. But saturated alkanes have free rotation around all the bonds, so they'll naturally settle at the lowest-energy conformer, which is almost always the straight-chain conformer. They're not discrete compounds, so they don't have different reactivities. Alkenes, on the other hand, do NOT have free rotation around the double bond, which is why they have E and Z isomers.

[uberjason]

The ring-like chain would have steric interactions that the straight chain does not.