Log out Go to App
Go to App

Learning Materials

Features

Discover

Chapter 14: Problem 7

The so-called nitrogen rule states that if a compound has an odd number of nitrogen atoms, the value of \(m / z\) for its molecular ion will be an odd number. Why is this so?

Short Answer

Expert verified
Question: Explain the nitrogen rule and its relationship to the mass-to-charge ratio (\(m/z\)) of molecular ions in compounds containing nitrogen. Answer: The nitrogen rule states that if a compound has an odd number of nitrogen atoms, the value of \(m/z\) for its molecular ion will be an odd number. This is derived from the fact that nitrogen atoms contribute an odd mass to the total molecular mass, while carbon and hydrogen contribute even masses. The odd number of nitrogen atoms results in an odd molecular mass, and since the charge is always 1 in electron ionization, the \(m/z\) value will also be odd.

Step by step solution

01

Understanding the Nitrogen Rule

The nitrogen rule is based on a pattern observed in the mass spectra of organic compounds containing nitrogen. The value of \(m/z\) represents the mass-to-charge ratio of a molecular ion, where \(m\) is the molecular mass and \(z\) is the charge. By analyzing mass spectra of various compounds, we can observe that if a compound has an odd number of nitrogen atoms, the molecular ion usually carries an odd mass in atomic mass units (amu).
02

The Molecular Structure of Organic Compounds

Organic compounds mostly consist of carbon, hydrogen, and heteroatoms like nitrogen, oxygen, sulfur, etc. Carbon atoms have a mass of 12 amu, hydrogen atoms have a mass of 1 amu, and nitrogen atoms have a mass of 14 amu. While carbon and hydrogen always contribute an even mass to the molecular ion (since 12 and 1 are both even numbers), nitrogen atoms contribute an odd mass (14 amu).
03

Total Molecular Mass of Compounds

The total molecular mass of a compound is the sum of the masses of all its constituent atoms. Let's consider three cases: 1. If a compound has no nitrogen atoms or an even number of nitrogen atoms, its total molecular mass will be even. This is because the sum of even numbers (carbon and hydrogen) and the sum of pairs of odd numbers (two nitrogen atoms) will always produce an even number. 2. If a compound has an odd number of nitrogen atoms, its total molecular mass will be odd. This is because the sum of even numbers (carbon and hydrogen) and an odd number (one nitrogen atom) will result in an odd number.
04

Ionization and Molecular Ions

In mass spectrometry, organic compounds are ionized to form molecular ions. The ionization process usually involves electron ionization (EI), which removes one electron from a neutral molecule to create a positively charged ion. When this happens, the molecular ion will have a +1 charge (z = 1). Because the charge is always 1, the \(m/z\) value will be equal to the molecular mass of the ion.
05

Correlation Between Nitrogen and \(m/z\)

Now, we can connect the total molecular mass of a compound to its \(m/z\) value for molecular ions. In EI, the \(m/z\) value is practically equal to the molecular mass (as z = 1). So, if a compound has an odd molecular mass (due to odd number of nitrogen atoms), its \(m/z\) value will also be odd. This is the basis of the nitrogen rule. In conclusion, the nitrogen rule is derived from the fact that nitrogen atoms contribute an odd mass to the total molecular mass of compounds, while carbon and hydrogen contribute even masses. The odd number of nitrogen atoms in a compound will result in an odd molecular mass, leading to an odd \(m/z\) value for its molecular ion.

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

All methyl esters of long-chain aliphatic acids (for example, methyl tetradecanoate, \(\mathrm{C}_{13} \mathrm{H}_{27} \mathrm{COOCH}_{3}\) ) show significant fragment ions at \(m / z 74,59\), and 31. What are the structures of these ions? How are they formed?

Calculate the nominal mass of each ion. Unless otherwise indicated, use the mass of the most abundant isotope of each element. (a) \(\left[\mathrm{CH}_{3} \mathrm{Br}\right]^{+}\) (b) \(\left[\mathrm{CH}_{3}{ }^{81} \mathrm{Br}\right]^{+}\) (c) \(\left[{ }^{13} \mathrm{CH}_{3} \mathrm{Br}\right]^{+}\)

Show how the compounds with the molecular formulas \(\mathrm{C}_{6} \mathrm{H}_{9} \mathrm{~N}\) and \(\mathrm{C}_{5} \mathrm{H}_{5} \mathrm{NO}\) can be distinguished by the \(m / z\) ratio of their molecular ions in high- resolution mass spectrometry.

Write molecular formulas for the five possible molecular ions of \(m / z 88\) containing the elements \(\mathrm{C}, \mathrm{H}, \mathrm{N}\), and \(\mathrm{O}\).

What rule would you expect for the \(m / z\) values of fragment ions resulting from the cleavage of one bond in a compound with an odd number of nitrogen atoms?
See all solutions

Recommended explanations on Chemistry Textbooks

Chemical Reactions

Read Explanation

Physical Chemistry

Read Explanation

Kinetics

Read Explanation

Ionic and Molecular Compounds

Read Explanation

Organic Chemistry

Read Explanation

Chemistry Branches

Read Explanation
View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.

Sign-up for free