Question

Can someone please explain the Rule of 13 in terms of identifying compounds in Mass Spec in Organic chemistry?

Answer 1

The rule of 13 states that the formula of a compound is a multiple n of 13 (the molar mass of CH) plus a remainder r.

The Rule of 13 is a simple procedure for tabulating possible chemical formula for a given molecular mass.^[10] The first step in applying the rule is to assume that only carbon and hydrogen are present in the molecule and that the molecule comprises some number of CH "units" each of which has a nominal mass of 13. If the molecular weight of the molecule in question is M, the number of possible CH units is n and

where r is the remainder. The base formula for the molecule is

and the degree of unsaturation is

A negative value of u indicates the presence of heteroatoms in the molecule and a half-integer value of u indicates the presence of an odd number of nitrogen atoms. On addition of heteroatoms, the molecular formula is adjusted by the equivalent mass of carbon and hydrogen. For example, adding N requires removing CH₂ and adding O requires removing CH₄.