For which compounds containing a heteroatom (an atom other thi hydrogen), does the molecular ion have an even-numbered mas: does it have an odd numbered mass?
The correct answer and explanation is:
Correct Answer:
Compounds containing an odd number of nitrogen atoms will have an odd-numbered molecular ion mass.
Compounds without nitrogen or with an even number of nitrogen atoms will have an even-numbered molecular ion mass.
Explanation (Approx. 300 words):
In mass spectrometry, the molecular ion (M⁺) is the peak that corresponds to the molecule that has lost only one electron and has not fragmented. Its mass tells us the molecular weight of the compound.
The key to determining whether the molecular ion mass will be odd or even lies in the “Nitrogen Rule.”
✅ The Nitrogen Rule:
If a molecule has an odd number of nitrogen (N) atoms, the molecular ion will have an odd mass.
If a molecule has zero or an even number of nitrogen atoms, the molecular ion will have an even mass.
This rule works because of the atomic masses and valencies of atoms:
- Most common atoms in organic molecules (C = 12, H = 1, O = 16, S = 32, Cl = 35.5, etc.) are either even-numbered or combine to give even masses.
- Nitrogen has an atomic mass of 14, but contributes odd valency (3), affecting how hydrogens balance in a molecule.
- Adding a nitrogen usually requires removing a hydrogen to keep bonding right, causing a net change in parity (odd/even) in the molecular mass.
🧪 Examples:
- C₂H₆O (Ethanol) – Contains no nitrogen → Even mass (M = 46)
- C₃H₉N (Propylamine) – Contains 1 nitrogen → Odd mass (M = 59)
- C₆H₁₂N₂ (Piperazine) – Contains 2 nitrogens → Even mass (M = 100)
Summary:
If you’re analyzing a compound:
- Check how many nitrogen atoms are present.
- If the number is odd → molecular ion mass is odd.
- If the number is even (including 0) → molecular ion mass is even.
This rule is helpful in mass spectrometry to deduce possible structures of unknown compounds.