finding possible isomers


by garytse86
Tags: isomers
garytse86
garytse86 is offline
#1
Oct14-03, 07:01 AM
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?
Phys.Org News Partner Chemistry news on Phys.org
Repeated self-healing now possible in composite materials
Potent, puzzling and (now less) toxic: Team discovers how antifungal drug works
Research offers 'promise' of improved food safety
HazZy
HazZy is offline
#2
Oct14-03, 10:51 AM
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.
Chemicalsuperfreak
Chemicalsuperfreak is offline
#3
Oct14-03, 11:27 AM
P: 324
There's a lot more than nine.

Monique
Monique is offline
#4
Oct14-03, 11:36 AM
Emeritus
Sci Advisor
PF Gold
Monique's Avatar
P: 4,612

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.
Monique
Monique is offline
#5
Oct14-03, 12:14 PM
Emeritus
Sci Advisor
PF Gold
Monique's Avatar
P: 4,612
Originally posted by Chemicalsuperfreak
There's a lot more than nine.
There are only nine!
Monique
Monique is offline
#6
Oct14-03, 12:22 PM
Emeritus
Sci Advisor
PF Gold
Monique's Avatar
P: 4,612
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
Chemicalsuperfreak
Chemicalsuperfreak is offline
#7
Oct14-03, 12:53 PM
P: 324
Originally posted by Monique
There are only nine!
Whoops. Sorry. Just saw the C7 and not the H16.
HazZy
HazZy is offline
#8
Oct14-03, 07:33 PM
P: 99
how come they don't count optical isomers? if they did C7H16 would have 11 isomers correct?
Monique
Monique is offline
#9
Oct15-03, 09:27 AM
Emeritus
Sci Advisor
PF Gold
Monique's Avatar
P: 4,612
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.
Chemicalsuperfreak
Chemicalsuperfreak is offline
#10
Oct15-03, 11:55 AM
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.
Monique
Monique is offline
#11
Oct15-03, 12:23 PM
Emeritus
Sci Advisor
PF Gold
Monique's Avatar
P: 4,612
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?
Chemicalsuperfreak
Chemicalsuperfreak is offline
#12
Oct15-03, 01:49 PM
P: 324
3-methylhexane and 2,3-dimethylpentane each have a chiral carbon.
Monique
Monique is offline
#13
Oct15-03, 04:43 PM
Emeritus
Sci Advisor
PF Gold
Monique's Avatar
P: 4,612
You are correct, my err! [:D]
lavalamp
lavalamp is offline
#14
Oct22-03, 06:13 PM
lavalamp's Avatar
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.
Chemicalsuperfreak
Chemicalsuperfreak is offline
#15
Oct22-03, 06:49 PM
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.
lavalamp
lavalamp is offline
#16
Oct23-03, 12:21 PM
lavalamp's Avatar
P: 280
That makes more sense. I blame the fact that I was tired.
RuiMonteiro
RuiMonteiro is offline
#17
Nov11-03, 04:17 PM
P: 30
Correct me if iīm wrong please, but isnīt there a 2-ethylpentane isomer?




Rui.
Chemicalsuperfreak
Chemicalsuperfreak is offline
#18
Nov11-03, 04:26 PM
P: 324
Originally posted by RuiMonteiro
Correct me if iīm wrong please, but isnīt there a 2-ethylpentane isomer?




Rui.
You mean 3-methylhexane?


Register to reply

Related Discussions
Isomers, how do you know how many to draw? Biology, Chemistry & Other Homework 6
Optical Isomers Chemistry 2
structure of an isomer would cause it to auto-ignite easily? Chemistry 6
Draw and name all constitutional isomers Chemistry 2
isomers Introductory Physics Homework 5