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

In summary, the conversation discusses how to calculate the IHD (index of hydrogen deficiency) for a given chemical formula and how to determine the remaining atoms in the molecule. The individual is trying to find the new IHD by subtracting a specific group (such as CO or CO2) from the original formula and is asking for clarification on which group to subtract for different functional groups. The conclusion is that for ketones and aldehydes, it is just CO, for carboxylic acids and esters it is CO2, and for amides it is CON.
  • #1
AMan24
58
2

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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  • #2
AMan24 said:
C2H3O

?
 
  • #3
Borek said:
?

Isn't that was ketone is? A CO and a CH3 = C2H3O
 
  • #4
Try to draw a C2H3O molecule.
 
  • #5
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
 

1. How do I determine the IHD of a molecule from an IR spectrum?

In order to calculate the IHD (index of hydrogen deficiency) of a molecule from an IR spectrum, you will need to identify the functional groups present in the molecule. Each functional group contributes a certain number of IHD units, which can be calculated using the formula (2n+2) - (2m+2), where n is the number of carbons and m is the number of hydrogens in the functional group. Add up the IHD units from each functional group to determine the total IHD of the molecule.

2. Can I calculate the IHD from any IR spectrum?

Yes, the IHD can be calculated from any IR spectrum as long as the spectrum shows peaks for the functional groups present in the molecule. However, it is important to note that the IHD calculation may not be accurate for more complex molecules with overlapping peaks.

3. What is the significance of calculating the IHD from an IR spectrum?

The IHD is a useful tool in determining the degree of unsaturation in a molecule, which can provide information about its structure and properties. It can also be used to confirm the presence of certain functional groups and aid in the identification of unknown compounds.

4. How is the IHD related to the degree of unsaturation?

The IHD is directly related to the degree of unsaturation in a molecule, as it represents the number of rings and/or multiple bonds present. A higher IHD indicates a higher degree of unsaturation, meaning the molecule has more double bonds, triple bonds, or rings.

5. Are there any limitations to using the IHD to analyze a molecule in an IR spectrum?

While the IHD is a useful tool, it should not be used as the sole method for determining the structure of a molecule. Some molecules may have the same IHD value but different structures, and the IHD calculation can be affected by overlapping peaks or the presence of isotopes. It should be used in conjunction with other methods, such as NMR spectroscopy, for a more accurate analysis.

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