Answer 6.3

a) How can you calculate the number of rings plus double bonds (r+d) based on a molecular formula?

Based on valence rules, an algorithm can be derived to distinguish between ions of different degrees of unsaturation and/or different number of rings. As this algorithm does not allow to differentiate between a cyclic substructure and a double bond, the result is known as double bond equivalents (DBE) or simply rings plus double bonds (r + d). Restriction to formulas of the general type CcHhNnOo reduces the expression to the common form:

r + d = c – 0.5h + 0.5n +1

Here, other monovalent elements than hydrogen (F, Cl, Br, I) are counted „as hydrogens“, other trivalent elements such as phosphorus are counted „as nitrogen“ and tetravalent elements (Si, Ge) are handled the same way as carbons.

b) Apply the r+d algorithm to the compounds below.

Molecular composition

C3H8

C6H12O2

C6H11NO

C32H25N3O2S

C8H20N2

C7H9N

C3H6BrN

C9H11ClN4O2

C17H32Cl2

r+d = …

3 – 4 + 1 = 0

6 – 6 +1 = 1

6 – 5.5 + 0.5 + 1 = 2

32 – 12.5 + 1.5 + 1 = 22

8 – 10 +1 + 1 = 0

7 – 4.5 + 0.5 + 1 = 4

3 – 3.5 + 0.5 + 1 = 1

9 – 6 + 2+ 1 = 6

17 – 17 + 1 = 1

c) Collect criteria for the recognition of molecular ions from EI mass spectra.

  1. The molecular ion must be the ion of highest m/z in the mass spectrum (be-sides the corresponding isotopic peaks).
  2. It has to be an odd-electron ion, M+..
  3. The peaks at the next lowest m/z must be explicable in terms of reasonable losses, i.e., of common radicals or molecules. Signals at M–5 to M–14 and at M–21 to M–25 point towards a different origin of the presumed M+..
  4. Fragment ions may not show isotopic patterns due to elements that are not present in the presumed molecular ion.
  5. No fragment ion may contain a larger number of atoms of any particular element than the molecular ion does.

d) Some neutral losses occurring from molecular ions are very frequent and can be reliably assigned to certain structural features. Try to assign the following neutral losses:

Neutral loss [u]

1

15

17

18

28

29

31

35

45

Reasonable formula …

H.

CH3.

NH3 or OH.

H2O

CO, C2H4 or N2

C2H5. or CHO.

.OCH3 or .CH2OH

Cl.

.COOH or .C2H5OH isomers

Take care not to overlook the radical signs in some of the above formulas as it makes an important difference.