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Finding number of C=C bonds

  1. Jul 6, 2011 #1
    1. The problem statement, all variables and given/known data
    An organic acid has the formula C11H19COOH.

    How many carbon-carbon double bonds does a molecule of the acid have?

    2. The attempt at a solution

    I drew the entire structural formula and got 2. But apparently, under exam conditions, I would not have time to draw the entire structural formula. Is there a shorter way to find the answer? The answer is 2.
  2. jcsd
  3. Jul 6, 2011 #2
    Wouldn't it be 1? If you replace the carboxyl group with another hydrogen, you get C11 H20. This follows the CxH2x-2 pattern that is characteristic of alkynes, which have 1 triple bond. So the only double bond you have is the one in the carboxyl group (correct me if I'm wrong).
  4. Jul 6, 2011 #3


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    For a problem like this, calculating the degree of unsaturation of the compound (http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkenes/Degree_of_Unsaturation [Broken]) can be helpful. This number tells you the number of pi bonds/rings are present in the compound. Applying the formula in the linked page to your compound, you get a degree of unsaturation of 3. Since the carbonyl group contains one double bond, this leaves two degrees of unsaturation for the carbon chain.
    Last edited by a moderator: May 5, 2017
  5. Jul 6, 2011 #4
    Nope, its two double bonds between two carbon to carbon. According to this picture:

    http://img97.imageshack.us/img97/5559/800pxgeranylacetate3dba.png [Broken]

    Uploaded with ImageShack.us

    As for OP: I personally don't know of any shortcuts which would allow you to figure this out mentally. Unless your learning to do problems like these (with organic compounds) then I'm sure you'll find a way. Even I had difficulty figuring out the structure. I could understand if you would be asked to figure out simple problems like 'how many triple bond(s) are there in HCN?' ~Which can be done easily mentally.

    If this is chem 101, I wouldn't sweat it, your professor would be an idiot to have a problem like this.
    Last edited by a moderator: May 5, 2017
  6. Jul 7, 2011 #5


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    There are many possible structures for a compound with the formula C11H19COOH. The structure you presented is just one possibility (actually the compound you posted, while it has the correct number of carbon, hydrogen and oxygen atoms, is not actually an acid but an ester). You could certainly draw a structure that includes an alkyne and have it be consistent with the same chemical formula. You could also draw structures of C11H19COOH that include only one carbon-carbon double bond. In this sense the question is poorly worded, as you could draw valid structures of C11H19COOH that have zero, one, or two carbon-carbon double bonds.
  7. Jul 9, 2011 #6
    Ygggdrasil the question state it's an organic acide ->only one structural formula
    it's simple -> always back to the alkanes formula Cn H2n+2
    since the COOH group origanly replace a H atom of an anlkan so the molecular formula would be C11H20
    well, in a normal alkane 20 atoms of H are not supposed to have 11 C ???got it?
    Alawas back to Cn H2n+2
  8. Jul 9, 2011 #7
    No, Yggg is right. It could have a triple bond or one or more cycles. There is nothing in the definition of "organic acid" that contradicts these possibilities. 0, 1, and 2 C=C double bonds are all possible.

    I'm sure that what your prof had in mind is points of unsaturation. Probably he hasn't introduced triple bonds or cycles yet, so is not expecting you to come up with these.
  9. Jul 9, 2011 #8
    yea,well , right
    but it's asking the number of double bonds so pobably that's what he meant!
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