|Chapter 13: Spectroscopy|
When calculating molecular weights to match to a molecular ion you need to use integer atomic weights.
The nitrogen rule
Organic molecules composed of C, H, O and halogens have even molecular weights
Examples : CH4 = 16 g/mol, CH3CH3 = 30 g/mol, CH3OH = 32 g/mol, CH3F = 34 g/mol, CH3CO2H = 60 g/mol
Since N forms 3 bonds, compounds with an odd number of N atoms, have odd molecular weights
Examples : NH3 = 17 g/mol, CH3NH2 = 31 g/mol, C5H5N = 79 g/mol
Therefore, the implication is the nitrogen rule.... "an odd molecular weight implies the presence of an odd number of N atoms"
This can be a useful piece of information and prompt you to look for N containing functional groups in the IR spectrum.
|© Dr. Ian Hunt, Department of Chemistry|