Lecture 6

Question 1

What is the nitrogen rule?

Answer 1

the observation that organic compounds will have an odd nominal mass if they contain an odd number of nitrogen atoms.
If the compounds contains an even number of nitrogen atoms (counting 0 as even number) the compound will have an even nominal mass.

Question 2

Why does the nitrogen rule work?

Answer 2

nitrogen is unusual in organic compounds
its principal isotope (14N) has an even mass but odd valence

Question 3

What’s the relationship between mass and valence?
Whats the exception?

Answer 3

• odd mass, odd valence
• even mass, even valence
• exception= nitrogen, even mass, odd valence

Question 4

How can the relationship between valence and mass be used to gain info about compoisition?

Answer 4

If M peak has odd m/z then contain odd number of N atoms

Question 5

Describe the possible fragmentation routes for a molecule containing 1 N atom

Answer 5

• forms M+ ion with 1 N atom (odd m/z)
  1. radical fragment (containing N with even mass) and cation fragment (odd m/z)
  2. radical fragment (no N with odd mass) and cation fragment (containing N with even m/z)
  3. radical cation fragment (containing N with odd m/z) and neutral molecule (no N with even mass)
  4. radical cation fragment (no N with even m/z) and neutral molecule (containing N with odd mass)

Question 6

state the set of rules for mass with nitrogen rule

Answer 6

• a radical cation with an even m/z has an even number of N
• a cation with an odd m/z has an even number of N
• a radical cation with odd m/z has an odd number of N
• a cation with even m/z has an odd number of N

Question 7

What information can we get from looking at cluster of peaks associated with molecular ion?

Answer 7

• exact m/z- find/confirm molecule formula
• odd or even- find info about N atoms
• isotope patterns - recognise halogens and estimate number of C atoms
• intensity of ion cluster- is molecular ion stable? if so, has structural features which stabilise the unpaired electron/charge (e.g. heteroatoms, rings, double bonds)

Question 8

What information can you get from looking at ion clusters associated with fragments?

Answer 8

• isotope patterns- compare with molecular ion e.g. has halogen been lost or retained
• intensity- stability
• consider differences in m/z from other ions