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How to calculate new IHD after finding a molecule in IR spec

  1. Mar 19, 2017 #1
    1. The problem statement, all variables and given/known data
    Lets say i have C5H10O.. The IHD is (5x2+2-10)/2 = 1 IHD

    So now i use my IR chart and i determine i have ketone

    Now i want to find the new IHD and also figure out what other atoms i have left. So i need to subtract C5H10O by ketone and figure out the new IHD.

    2. Relevant equations
    Given formula: CcHhNnOoXx

    IHD = 0.5 * [2c+2-h-x+n]

    3. The attempt at a solution

    So for ketone, my guess would be either subtracting CO from the original formula or subtracting C2H3O then calculating the new IHD which would be either

    C5H10O - C2H3O = IHD 0.5 which doesn't make sense
    C5H10O - CO = IHD 0, which does make sense, but is it correct

    And you might say i don't need to calculate it for this problem, but i just want to know for in the future because for some problems finding the new IHD is useful...

    My question is: What am i supposed to subtract exactly? And not just for this example but, lets say i have aldehyde, do i just subtract the CO or do i subtract CHO
  2. jcsd
  3. Mar 19, 2017 #2


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    Staff: Mentor

  4. Mar 19, 2017 #3
    Isn't that was ketone is? A CO and a CH3 = C2H3O
  5. Mar 19, 2017 #4


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    Staff: Mentor

    Try to draw a C2H3O molecule.
  6. Mar 19, 2017 #5
    I think i might have figured it out. So for ketones and aldehydes, its just CO. For carboxylic acids and esters its CO2. For amides its CON.. and so on
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