Question

For primary amines with no branching on the carbon bearing the nitrogen, the base peak occurs at \(m / z 30\). What cation does this peak represent? How is it formed? Show by drawing contributing structures that this cation is stabilized by resonance.

Short answer

Question: Identify the cation formed in the mass spectrometry of primary amines with no branching on the carbon bearing the nitrogen, which has a base peak at m/z 30. Explain its formation and stabilization through resonance structures. Answer: The cation formed in the mass spectrometry of primary amines with no branching on the carbon bearing the nitrogen and a base peak at m/z 30 has the molecular formula CH4N, also known as methylamine. It is formed by the loss of an electron, with the positive charge due to the electron loss on the nitrogen atom. The cation is stabilized through two contributing resonance structures: one with the positive charge on the nitrogen atom and the other with a positive charge on the carbon atom. This electron delocalization leads to the stabilization of the methylamine cation.

Step-by-step solution

Determine the molecular formula of the cation

Given that the base peak occurs at m/z 30, the molecular weight of the cation is 30. For primary amines, there should be a carbon atom (12) and a nitrogen atom (14) in the molecular formula since there is no branching on the carbon bearing the nitrogen. Therefore, the remaining weight should be 30 - 12 - 14 = 4, which corresponds to 4 hydrogen atoms. This gives us the molecular formula CH4N, also known as methylamine.

Draw the structure of the cation

In the mass spectrometer, the CH4N molecule loses an electron to form a cation with a positive charge. The cation can be represented as follows: ``` H | H-C-N : (+) | H ``` The nitrogen atom carries the positive charge due to losing an electron.

Draw the contributing resonance structures

Resonance structures are used to represent the delocalization of electrons in a molecule, which can stabilize the molecule by simultaneously contributing to its electron distribution. In this case, we will draw the contributing resonance structures of the methylamine cation: Resonance Structure 1: ``` H | H-C-N : (+) | H ``` Resonance Structure 2: ``` H | H-C(+)-N-H | H ``` In Resonance Structure 1, the positive charge is located on the nitrogen atom. In Resonance Structure 2, the positive charge is located on the carbon atom. The true electron distribution in the methylamine cation lies between these two resonance structures, resulting in electron delocalization and stabilization of the cation.

Key concepts

Primary Amines

Primary amines are organic compounds that contain a nitrogen atom attached to one carbon atom, forming a simple molecular structure. These amines are typically represented as R-NH2, where 'R' is the organic substituent that connects to the nitrogen. Primary amines are basic, meaning they have the ability to accept protons due to the lone pair of electrons on the nitrogen atom.

In mass spectrometry, primary amines show unique fragmentation patterns that help in identifying their structure. Since primary amines have minimal branching at the carbon attached to the nitrogen, their spectra often have clear and distinguishable peaks, such as the base peak at m/z 30 in this case. This peak corresponds to certain molecular ions generated by the separation of fragments during ionization.

Understanding amines is crucial, as they play significant roles in various chemical reactions and pathways in both biological and industrial settings. They serve as building blocks for many complex molecules, and their behavior during ionization aids chemists in structural analysis.

Cation Formation

Cation formation is a central process in mass spectrometry, where molecules become ionized, often through the loss of an electron. In the case of primary amines like methylamine, the molecule loses an electron to form a positively charged species, or cation. This change typically occurs when the molecule is bombarded with high-energy electrons or photons during mass spectrometric analysis.

The removal of an electron leaves the nitrogen atom with a positive charge because it has more protons than electrons. This process is critical for generating the m/z values needed to identify molecular structures through mass spectral peaks, such as the base peak at m/z 30. Cations are key to the mass spectrometry process, as they enable the detection and calculation of molecular masses.

Understanding how and why cations form is essential in analyzing mass spectra. This knowledge aids chemists in deducing structural information from the original neutral compounds, contributing to the accurate interpretation of complex molecular systems.

Resonance Structures

Resonance structures are a way to depict electron delocalization within molecules. They represent different ways electrons can be arranged, providing insights into bond stability and molecular behavior. In the case of the methylamine cation, resonance structures show how electrons can shift between different atoms, thereby stabilizing the molecule.

In this specific example, the methylamine cation can be represented by different resonance forms. One structure has the positive charge on the nitrogen, while another moves the charge to a carbon atom. Even though these structures differ, they contribute equally to the overall electron distribution of the cation. This shifting, known as resonance, ensures greater stability by spreading the positive charge over multiple atoms instead of just one.

This principle is crucial for understanding how certain ions remain stable despite carrying a charge. Resonance helps explain the observed spectrum in mass spectrometry, as it influences the stability and thus the abundance of different ion forms in the sample. Knowing how resonance affects molecular stability allows chemists to predict and rationalize reaction behaviors and mass spectrometric patterns.