Mass spec - Whats the nitrogen rule all about

[CrimpJiggler]

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.

 

[mfb]

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.

 

[Borek]

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.

 

[CrimpJiggler]

mfb: Ah, I get it now. Thanks.

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

 

[Borek]

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.