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

Answer: The cation responsible for the base peak at m/z 30 in the mass spectrum of primary amines without branching is CH3NH+. It is formed during the ionization process in the mass spectrometer when high-energy electrons cause a loss of H+ from the nitrogen atom of methylamine. The cation is stabilized by resonance, with two contributing resonance structures [CH2=NH]+ and [CH3N+]H, allowing the positive charge to be distributed among the atoms in the molecule.

Step-by-step solution

Identify the cation at m/z 30

To identify the cation responsible for the base peak at m/z 30, we need to consider the molecular structure of primary amines without any branching on the carbon bearing the nitrogen atom. The simplest of these amines is methylamine (CH3NH2) with a molecular weight of 31 g/mol. When methylamine loses a hydrogen ion (H+), it forms a cation (CH3NH+) with a m/z value of 30. This cation is responsible for the base peak observed in the mass spectrum of primary amines.

Formation of the cation during mass spectrometry

The formation of the cation (CH3NH+) occurs in the mass spectrometer during the ionization process. In the ionization source, the sample molecules are bombarded with high-energy electrons. This interaction can cause the loss of a hydrogen ion (H+) from the nitrogen atom of the methylamine molecule, forming the cation observed at m/z 30.

Show contributing resonance structures for stabilization

The cation (CH3NH+) is stabilized by resonance because there are two possible configurations in which the positive charge is delocalized between nitrogen and carbon. The resonance occurs due to the lone pair of electrons on the nitrogen atom that can form a double bond with the carbon atom, creating a more stable structure. The contributing resonance structures for the cation (CH3NH+) are shown below: Structure 1: [CH2=NH]+ Structure 2: [CH3N+]H These resonance structures allow the positive charge to be distributed among the atoms in the molecule, providing resonance stabilization to the cation. This stabilization is likely the reason why the cation (CH3NH+) produces the base peak in the mass spectrum of primary amines without branching on the carbon atom bearing the nitrogen atom.