Mass spec - Whats the nitrogen rule all about

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Discussion Overview

The discussion revolves around the "nitrogen rule" in mass spectrometry, particularly its implications for determining the presence of nitrogen in organic compounds based on molecular mass. Participants explore the applicability of this rule to various compounds and the conditions under which it holds true.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant describes the nitrogen rule, stating that an odd molecular mass indicates the presence of an odd number of nitrogen atoms, while an even mass suggests zero or an even number of nitrogens.
  • Another participant clarifies that the rule applies only to covalent bonds and that atoms with an odd number of bonds have odd atomic masses, while those with even numbers have even atomic masses.
  • A different participant notes that the nitrogen rule is a rule of thumb that generally works for most organic compounds, specifically those composed of carbon, hydrogen, nitrogen, and oxygen, but may fail with the addition of sulfur, phosphorus, or halogens.
  • One participant expresses confusion about the failure of the rule with halogens, questioning why it does not apply despite most halogens having even atomic masses.
  • Another participant points out the atomic masses of halogens, indicating a misunderstanding about the term "most" in relation to their atomic masses.

Areas of Agreement / Disagreement

Participants generally agree that the nitrogen rule is useful but acknowledge that it has limitations and may not apply universally, particularly when certain heteroatoms are present. There is no consensus on the specific conditions under which the rule fails.

Contextual Notes

The discussion highlights limitations regarding the applicability of the nitrogen rule, particularly in the presence of heteroatoms like halogens and the need for covalent bonding. There are unresolved questions about the definitions and conditions under which the rule holds true.

CrimpJiggler
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I missed the first week of college so I've fallen a bit behind. The main class I'm struggling with is mass spec since its relatively new to me. Whats really boggling my mind is this "nitrogen rule". Heres a quote from another site:
If the molecular mass of an unknown compound to the nearest integer value is an odd number, the compound contains an odd number of nitrogens in its molecular formula. Correspondingly, if the molecular mass is an even number, the compound contains zero or an even number of nitrogens in its molecular formula. This rule, illustrated below, results from nitrogen having a valence of three and an even atomic mass.
I'm trying to get my head around this. So nitrogen has a valence of 3 and an even atomic mass. Testing this out with trimethylamine, I see it works because the 3 methyl groups will add up to 39, so adding that to nitrogens even atomic mass, you get an odd number.

Does this really work in all cases? If I see a molecular ion with an odd molecular mass, is this solid evidence that the compound contains 1 or more nitrogen atoms? Does it apply to all organic compounds, or only compounds with common heteroatoms like halogens and chalcogens etc. I'm trying to get an understanding of why it works, but that seems mighty complicated.
 
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It applies as long as you just have covalent bonds, all atoms (apart from nitrogen) with an odd number of bonds have an odd atomic mass, and all atoms with an even number of bonds have an even atomic mass.
It does not work if one of those conditions is not satisfied.

You can show this in an inductive way. Every elementary modification you can perform on molecules (add/remove one atom, make/break rings, add/remove hydrogen if necessary) does not change the validity of the rule, and the "trivial molecule" (no atoms) satisfies the rule, too.
 
Nitrogen rule is only a rule of thumb. It works in most cases that you will probably deal with - most organic compounds are made of CHNO. Add S and it is still OK, add P or halogen, and it fails.
 
mfb: Ah, I get it now. Thanks.

Borek: It fails with halogens in there? Why not? Most halogens have an even atomic mass.
 
CrimpJiggler said:
Most halogens have an even atomic mass.

F - 19
Cl - 35.5
Br - 80
I - 127

Define "most". 1 out of 4?

But I was partially wrong, for some reason I thought Br is 81. I should have check.
 

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