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

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Discussion Overview

The discussion revolves around calculating the Index of Hydrogen Deficiency (IHD) after identifying a functional group in an organic molecule using infrared (IR) spectroscopy. Participants explore the implications of subtracting specific atoms or groups from the molecular formula to determine the new IHD, particularly in the context of ketones, aldehydes, carboxylic acids, and amides.

Discussion Character

  • Homework-related, Exploratory, Technical explanation

Main Points Raised

  • One participant calculates the initial IHD for C5H10O as 1 and seeks to determine the new IHD after identifying a ketone.
  • There is uncertainty about whether to subtract CO or C2H3O from the original formula to find the new IHD.
  • Another participant suggests that a ketone can be represented as C2H3O, prompting further clarification.
  • A later reply proposes that for ketones and aldehydes, only CO should be subtracted, while for carboxylic acids and esters, CO2 should be subtracted, and for amides, CON should be considered.

Areas of Agreement / Disagreement

Participants express differing views on what specific groups to subtract from the molecular formula to calculate the new IHD, indicating that there is no consensus on the correct approach.

Contextual Notes

Participants do not fully agree on the definitions and implications of the functional groups involved, leading to potential confusion regarding the correct subtraction for IHD calculations.

AMan24
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Homework Statement


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.

Homework 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, let's say i have aldehyde, do i just subtract the CO or do i subtract CHO
 
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AMan24 said:
C2H3O

?
 
Borek said:
?

Isn't that was ketone is? A CO and a CH3 = C2H3O
 
Try to draw a C2H3O molecule.
 
Borek said:
Try to draw a C2H3O molecule.

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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