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Homework Help: Isomers, how do you know how many to draw?

  1. Nov 3, 2007 #1
    How do I know how many isomers to draw for a molecular formula?

    My professor said there was a formula, but he did not know it off the top of his head. He said to google it, but I CANNOT find a formula, and I have about 12 opened windows with diff. searches *rolls eyes*.

    Ex. I know C3H5F2Br has 9 isomers. But how would I know it had that many (9)?

    And is there a way to draw these better? I am getting about 2/3 of the isomers, and my professor says he wishes there was a way he could explain it, but he can't.

    All he said was, no flipping and no rotating isomers bc they are the same thing.
    But Why are there no set ways to do this?

    *sadness* :(

    Any advice, I greatly appreciate. Fridays/saturdays I spend doing homework.
  2. jcsd
  3. Nov 3, 2007 #2
    We are only doing "single bonds" right now btw...
  4. Nov 3, 2007 #3


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    You can certainly draw all of the isomers. Probably a formula for counting them exists; but maybe someone else knows it.

    You see the compound's formula shows three carbons, and you are probably looking for bonds among and between carbons; so start from there and draw until you exhaust all possibilities. In case the count of halides do not work right, try looking for double bonds between carbons.
    OK, so far, I came up with 7 isomers, all need carbon to carbon single bonds.

    Think: How many unique ways can you arrange the 5 hydrogens? Then think for each of those, how many ways can you arrange the 2 F's and the 1 Br?
  5. Nov 3, 2007 #4


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    Now, I have them, all 9. There are 5 unique ways to arrange the hydrogens around the propane chain. Can you find them and do the rest?
  6. Nov 3, 2007 #5
    Well, I have the drawings in front of me. I only have about 10 practice problems (all are already worked out), so I can look at the sheet, but, I am only able to draw 6 out of 9 for example on my own :(
    ...then I have to look at the sheet.

    I mean, it's a start, but I have no idea how to make sure I have them all. It's making my head spin with worry. And I have no more practice problems.

    Hence, why I am home Saturday studying.

    BTW, I was told only to worry about "single bond" ones for right now.

    And also I only had to work on examples with 10 or less "possible isomers."
    So, if I am giving a molecular formula and need to find all possible isomers, I will only be given Q's with up to 10.

    I just wish a website had more examples :/

    I need to practice, but have run out of practice problems.
    I could make up my own... but I would not know if I listed them all, or if I did it right.
    Last edited: Nov 3, 2007
  7. Nov 3, 2007 #6
    This is for general chem I too. So that's why we aren't doing the super hard ones...
  8. Nov 3, 2007 #7


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    You begin with a chain of three carbons: since I have no easy way to draw, let me show this by

    C - C - C

    Now, what are the ways to arrange the five hydgrogens?

    H3C - H2C - C

    H3C - HC - HC

    ...Those are only two of the ways. You should figure out the other three ways. As you do this, realize that each carbon will be bonded to four atoms; each H or Br or F will be bonded to only 1 atom, and it must be to a carbon.

    To help you understand what I mean in that second form, H3C - HC - HC,
    there are three Hydrogens on first carbon, one hydrogen on the second carbon, and one hydrogen on the third carbon. Note that is five hydrogens. Also, be aware that
    HC - HC - H3C would be the same arrangement as H3C - HC - HC. Rotation does not produce a different arrangement.
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