Question

Account for the formation of the base peaks in these mass spectra. (a) Isobutylmethylamine, \(\mathrm{m} / z 44\) (b) Diethylamine, \(\mathrm{m} / \mathrm{z} 58\)

Short answer

Answer: Isobutylmethylamine has its base peak at m/z 44 because the most probable and stable fragment formed upon fragmentation is CH3CH2NH+ due to the presence of a positively charged nitrogen atom. Diethylamine has its base peak at m/z 58 because the most probable and stable fragment formed is (CH3CH2)2NH+ with a positive charge on the nitrogen atom. These fragments have higher stability and result in the base peak for each compound.

Step-by-step solution

Identify the structure of isobutylmethylamine

Isobutylmethylamine is an amine compound with the molecular formula \(\mathrm{C_5H_13N}\). Its structure can be represented as (CH3)2CHCH2NHCH3.

Analyze the possible fragmentation of isobutylmethylamine leading to m/z 44

For isobutylmethylamine, the given base peak in the mass spectrum corresponds to a fragment with \(\mathrm{m}/\mathrm{z}=44\). This fragment can be either CH3NH2+ (m/z = 31) or CH3CH2NH+= (m/z = 44). The CH3CH2NH+ fragment is formed when the isopropyl group is cleaved from the original molecule, resulting in a more stable fragment due to the presence of a positively charged nitrogen atom and a singly bonded carbon atom.

Select the most probable fragmentation leading to the base peak for isobutylmethylamine

CH3CH2NH+ fragment is more probable to form because its structure is more stable compared to the other fragment option. Therefore, the base peak of the mass spectrum is formed by the CH3CH2NH+ fragment at m/z = 44.

Identify the structure of diethylamine

Diethylamine is an amine compound with the molecular formula \(\mathrm{C_4H_{11}N}\). Its structure can be represented as (CH3CH2)2NH.

Analyze the possible fragmentation of diethylamine leading to m/z 58

For diethylamine, the given base peak in the mass spectrum corresponds to a fragment with m/z = 58. This fragment can be either CH3CH2NH2+ (m/z = 44) or (CH3CH2)2NH+ (m/z = 58). The (CH3CH2)2NH+ fragment is formed when one of the hydrogen atoms attached to the nitrogen atom is lost as a neutral fragment, resulting in a more stable fragment due to the presence of a positive charge on the nitrogen atom.

Select the most probable fragmentation leading to the base peak for diethylamine

The (CH3CH2)2NH+ fragment is more probable to form because its structure is more stable compared to the other fragment option. Therefore, the base peak of the mass spectrum is formed by the (CH3CH2)2NH+ fragment at m/z = 58.