Molecular Ion and Nitrogen
Molecular Ion and Nitrogen
Molecular Weight: Even or Odd?
There
is one other element that can be detected easily just by looking at the
molecular ion: nitrogen. Usually if there is a nitrogen present in the
molecule, the molecular weight is odd, as you can see in the mass
spectrum of triethylamine. That isn't true with other compounds.
- Molecular weights of organic compounds are almost always even.
- Odd molecular weights result when there is nitrogen in the compound.
Figure MS6. Mass spectrum of triethylamine. Source: SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 22 August 2008)
This
phenomenon is a result of the fact that the most common elements in
organic compounds, carbon and oxygen, have even atomic weights (12 and
16, respectively), so any number of carbons and oxygens will have even
weights. The most common isotope of hydrogen has an odd molecular
weight, but because carbon and oxygen both have even valences (carbon
forms four bonds and oxygen forms two), there is always an even number
of hydrogen atoms in an organic compound containing those elements, so
they also add up to an even numbered weight.
Nitrogen
has an even atomic weight (14), so any number of nitrogen atoms will
add up to an even molecular weight. Nitrogen, however, has an odd
valence (it forms three bonds), and as a result there will be an odd
number of hydrogens in a nitrogenous compound, and the molecular weight
will be odd because of the presence of an extra hydrogen.
Of
course, if there are two nitrogens in a molecule, there will be two
extra hydrogens, so the molecular weight will actually be even. That
means the rule about molecular weight and nitrogen should really be
expressed as:
- odd numbers of nitrogen atoms in a molecule in an odd molecular weight.
What
about those other atoms that sometimes show up in organic chemistry,
such as the halogens? Halogens all have odd molecular weights (19 amu
for fluorine, 35 or 37 for chlorine, 79 or 81 for bromine, and X for
iodine). However, halogens all have a valence of 1, just like hydrogen.
As a result, to add a halogen to methane, we would need to erase one of
the hydrogen atoms and replace it with the halogen. Since we are just
substituting one odd numbered atomic weight for another, the total
weight remains even.
Problem MS6.
Calculate molecular weights for the following compounds.
