Chapter 9, Problem 17PP

Interpretation Introduction

Interpretation:

The reason for the statement “The mass spectrum of 3-methylpentane has a peak of very low relative abundance at ${\text{M}}_{•}^{+}-15$ and a peak of very high relative abundance at ${\text{M}}_{•}^{+}-29$ as compared to the compounds 2-methylbutane and neopentane” is to be explained.

Concept introduction:

Mass spectrometry involves formation of ions in a mass spectrometer followed by separation and detection of the ions according to mass and charge.

The mass spectrum is a graph with mass ($m/z$) of ions on $x-\text{axis}$ where m is the mass of the ion and z is the charge of the ion and their relative ion abundance on $y-\text{axis}$. In other words, x-axis represents the formula weight of each detected ion and y-axis represents the percentage of the tallest peak. The tallest peak indicates the most abundant ion and is known as base peak. The base peak is basically an easily formed fragment of the original compound.

One of the highest value m/z may or may not represent the molecular ion, that is, the ion with the formula weight of the original compound. When present, the molecular ion is usually not the base peak because the ions from the original compound molecule tend to fragment, thereby resulting in the other m/z peaks in the spectrum.

Small peaks having m/z values 1 or 2 higher than the formula weight of the compound are because of ¹³C and other isotopes.

Molecular ions formed by EI mass spectrometry are high-energy species and these are called radical cations because it contains both an unshared electron and a positive charge. Fragmentation of molecular ion means that a complex molecule is broken into smaller molecules and these fragments can undergo more breaking and so on.

Cleavage of a single bond produces a cation and a radical. The cation can be detected using positive ion mass spectrometry and because the radical is not charged, it will be undetected.

Chain branching or cleavage of a single bond at branch points will occur such that the carbocation formed is more stable. In simple words, the stability of carbocation determines the fragmentation pattern.