# finding possible isomers

by garytse86
Tags: isomers
 P: 310 I was given a molecular formula C7H16 and asked to find all possible isomers. I know there are nine because the question said it. But I am just wondering how to find all isomers correctly without repeating myself. Is there a way to do this mathematically or a general rule that makes it easier?
 P: 99 hmm i dont really know if there's a mathematical formula (probably would be a waste of time), but if you think you're repeating yourself just try flipping the structure in your head, then try to match it with any of the structures you have already written down, if none match it's a new isomer [:)]. you could also number the carbons as you would if you were naming the molecule, but if you haven't covered that yet then just stick with trying to visualize it.
 P: 324 There's a lot more than nine.
Emeritus
PF Gold
P: 4,622

## finding possible isomers

Notice that C7H16 is Cn H2n +2 ?

That means it can only be a straight chain of atoms (not a cycloalkene) and it may not be unsaturated. The rest is for you to figure out. I don't know of any formulas :)

Basically start with heptane, and start taking of a C atom from the end and place it somewhere else, making sure you don't make a mirror image. Every time you take one off and finally you should come to 9 different forms.
Emeritus
PF Gold
P: 4,622
 Originally posted by Chemicalsuperfreak There's a lot more than nine.
There are only nine!
 Emeritus Sci Advisor PF Gold P: 4,622 Maybe you can figure out a formule from the following data: Pentane C5H12 3 isomers Hexane C6H14 5 isomers Heptane C7H16 9 isomers Octane C8H18 18 isomers Nonane C9H20 35 isomers Decane C10H22 75 isomers
P: 324
 Originally posted by Monique There are only nine!
Whoops. Sorry. Just saw the C7 and not the H16.
 P: 99 how come they don't count optical isomers? if they did C7H16 would have 11 isomers correct?
 Emeritus Sci Advisor PF Gold P: 4,622 There are no optical isomers, since there are no double bonds in the molecule.. molecules are able to rotate freely around their axes. *edit* I guess I am talking about geometric isomers (cis/trans) Well, no optical isomers either (L/D), since you would need different sidechains right? You cannot have a chimeric atom when all groups are identical.. I guess? I drew all 9 of them out.. in order for a C atom to be chiral, it needs to have 4 groups attached.. there are a few.. no, no enantiomers.
P: 324
 Originally posted by Monique There are no optical isomers, since there are no double bonds in the molecule.. molecules are able to rotate freely around their axes. *edit* I guess I am talking about geometric isomers (cis/trans) Well, no optical isomers either (L/D), since you would need different sidechains right? You cannot have a chimeric atom when all groups are identical.. I guess? I drew all 9 of them out.. in order for a C atom to be chiral, it needs to have 4 groups attached.. there are a few.. no, no enantiomers.
butyl, ethyl, methyl, and hydrogen. Four different groups. You CAN have more than nine isomers, if you count stereoisomers and not just structural isomers.
 Emeritus Sci Advisor PF Gold P: 4,622 I am getting confused, could you help me out by looking at the following page http://www.csi.edu/ip/physci/faculty...e/heptanes.htm which one would have an enantiomer?
 P: 324 3-methylhexane and 2,3-dimethylpentane each have a chiral carbon.
 Emeritus Sci Advisor PF Gold P: 4,622 You are correct, my err! [:D]
P: 280
 That means it can only be a straight chain of atoms (not a cycloalkene) and it may not be unsaturated. The rest is for you to figure out. I don't know of any formulas :)
Erm...

The isomers don't have to be straight chained, (if they did then there would be only one isomer), and since it is in the general formula CnH2n+2 then all of the isomers will be saturated.
Sorry for being so nit picky.

Anyway, the best way to spot isomers at a glance is to do many many examples. At least that's what my chemistry teacher made me do. It's also best if you learn to spot them now, that way when you get on to cis/trans and then other more complicated molecules (with benzene rings in, cis/trans and some halides thrown in for fun) you can spot the possibilities a lot easier.
P: 324
 Originally posted by lavalamp Erm... The isomers don't have to be straight chained, (if they did then there would be only one isomer), and since it is in the general formula CnH2n+2 then all of the isomers will be saturated. Sorry for being so nit picky.
He means acyclic.
 P: 280 That makes more sense. I blame the fact that I was tired.
 P: 30 Correct me if i´m wrong please, but isn´t there a 2-ethylpentane isomer? Rui.
P: 324
 Originally posted by RuiMonteiro Correct me if i´m wrong please, but isn´t there a 2-ethylpentane isomer? Rui.
You mean 3-methylhexane?

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