Comments on: Degrees of Unsaturation (or IHD, Index of Hydrogen Deficiency)

By: SANHIT MAITY

SANHIT MAITY — Sun, 27 Oct 2024 05:45:20 +0000

The formula DU = (No. of Rings + No. of pi bonds) is not always true. But analysis of DU from molecular formula always holds.
Example: For cage-like compounds like Cubane (C8H8) or Frisbane, molecular formula analysis gives us the DU to be 5. But by seeing the structure, as there are 6 sides and thus 6 rings in a cubane, we can say DU to be 6.
The plausible explanation I got for this is that as we remove the 5th molecule of H2 to form the fifth ring, the sixth ring forms simultaneously without any need for further removal of H2.
Please correct me if I am wrong.

By: Apurva

Apurva — Sat, 21 Sep 2024 03:46:33 +0000

I read that in Dewar benzene, the middle C1-C4 linkage in not sigma bond but 2p-2p pi bond (special type of pi without sigma).

By: Daksh Mishra

Daksh Mishra — Thu, 23 Mar 2023 09:27:08 +0000

In compound R-NO2 (suppose R is an aliphatic saturated compound), will the degree of unsaturation be 1 or 1.5? Shouldn’t we take DU equal to 1.5 due to resonance between the negative charge of oxygen atom and the double bond?
Which one is correct? Sorry to ask a silly question.

By: Samuel

Samuel — Mon, 25 Jul 2022 15:33:05 +0000

I have been struggling with using IHD to deduce a structural fromula from a spectrum graph.
Your explanation has help me a lot.
Thank you.

By: James Ashenhurst

James Ashenhurst — Tue, 08 Feb 2022 20:15:50 +0000

In reply to Thenard Rinmann. That is very clever! To be a fair question, however, it should specify the counter-ions. For sodium oxalate, Na+ would be the same as H+ and provide an answer of 2 for C2O4Na2

By: James Ashenhurst

James Ashenhurst — Tue, 08 Feb 2022 20:14:01 +0000

In reply to Bhavya garg. No, it's done on an atom-by-atom basis

By: Suparna Bhattacharjee

Suparna Bhattacharjee — Fri, 10 Dec 2021 18:41:43 +0000

First problem to be solved using simultaneous equations in two variables.
A hydrocarbon contains only two elements, viz C and H. Let the formula be CxHy.
Given IHD= 6.
Substitute in IHD formula and get the first simultaneous equation as
2x+2+-y= 6
Atomic weights of C=12 and H=1. Substitute to calculate formula weight and get 2nd equation in two variables
12x + y = 284.
solve for x and y

By: Bhavya garg

Bhavya garg — Fri, 30 Jul 2021 00:29:12 +0000

Sir is there any subtraction or for functional group?

By: James Ashenhurst

James Ashenhurst — Fri, 22 May 2020 16:35:21 +0000

In reply to Chris G..

Hi Chris – This is a post about structure determination.

The usefulness of IHD is that it is a value you can calculate from the molecular formula without knowing anything else about the structure (the molecular formula can be obtained from techniques like elemental analysis, high-resolution mass spectrometry, etc.)

That’s why I say it’s one of the first calculations to make when you’re dealing with a molecule of unknown structure.

If you already know the structure, then IHD isn’t really going to help you

By: Chris G.

Chris G. — Thu, 21 May 2020 18:55:07 +0000

If we’re given the structure of the molecule, can’t we just calculate degrees of unsaturation by the formula:

IHD = #rings + #double bonds + 2#triple bonds

If we only have the molecular formula, this formula isn’t helpful. But if we have the structure, isn’t this formula more simple?