Mass spec - Whats the nitrogen rule all about

In summary, the nitrogen rule states that if the molecular mass of an unknown compound is an odd number, the compound contains an odd number of nitrogens in its molecular formula. This rule applies to compounds with covalent bonds, as all atoms with an odd number of bonds have an odd atomic mass.
  • #1
CrimpJiggler
149
1
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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  • #2
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.
 
  • #3
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.
 
  • #4
mfb: Ah, I get it now. Thanks.

Borek: It fails with halogens in there? Why not? Most halogens have an even atomic mass.
 
  • #5
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.
 

Related to Mass spec - Whats the nitrogen rule all about

1. What is the nitrogen rule in mass spectrometry?

The nitrogen rule is a rule used in mass spectrometry to determine the molecular formula of an unknown compound. It states that the number of nitrogen atoms present in a compound can be determined by subtracting the sum of the carbon, hydrogen, and oxygen atoms from the molecular weight divided by 14.

2. How is the nitrogen rule used in mass spectrometry?

The nitrogen rule is used by mass spectrometers to help identify the molecular formula of an unknown compound. By determining the number of nitrogen atoms present, the molecular formula can be narrowed down and compared to known compounds to determine the identity of the unknown compound.

3. Is the nitrogen rule always accurate?

No, the nitrogen rule is not always accurate. There are exceptions to the rule, such as compounds with unusual structures or isotopic variations. It is important to use the nitrogen rule as a guide and not rely solely on it for determining the molecular formula of a compound.

4. Can the nitrogen rule be used for all types of compounds?

The nitrogen rule can be used for most organic compounds, but it is not applicable to inorganic compounds. It also may not be accurate for compounds with a high degree of unsaturation or compounds with heteroatoms other than nitrogen.

5. Are there any alternative methods for determining molecular formula in mass spectrometry?

Yes, there are alternative methods for determining molecular formula in mass spectrometry. These include high-resolution mass spectrometry and tandem mass spectrometry techniques, which can provide more accurate and detailed information about the molecular formula of a compound compared to the nitrogen rule.

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