Question: The mass spectrum of 2,3-dimethylpentane also shows peaks at m/z = 57 and 43. Propose possible structures for the ions that give rise to these peaks.

The mass spectrum is a 2-dimensional spectrum with an m/z ratio on the horizontal axis and intensity on the vertical axis.

Mass spectrum can be used to study the fragmentation pattern of compounds. The most intense peak is called the base peak.

The intensity of other peaks is represented with respect to the base peak.

The m/z ratio indicates the mass to charge ratio of the cations formed during the fragmentation of the compound.

Most stable cations formed by the fragmentation of molecular ions produce peaks in the mass spectrum.

To find the structure of ions from the m/z value of the peak, calculate the mass of the molecular ion of the compound. Draw the structure of this molecular ion and determine which bond is broken to form ions of appropriate m/z ratios.

The m/z ratio indicates the mass to charge ratio of the cations formed during the fragmentation of the compound.

Most stable cations formed by the fragmentation of molecular ions produce peaks in the mass spectrum.

To find the structure of ions from the m/z value of the peak, calculate the mass of the molecular ion of the compound. Draw the structure of this molecular ion and determine which bond is broken to form ions of appropriate m/z ratios.

The molecular ion of 2,3-dimethylpentane has an m/z ratio of 100.

Analysis of the ion shows that there are 2 secondary carbon atoms and 3 primary carbon atoms in the ion.

Secondary carbocations are more stable than primary carbocations. Therefore, fragmentation occurs at the secondary carbon.

Fragmentation of 2,3-dimethylpentane

Hence, the ion responsible for the peak at m/z=43 is isopropyl cation and the ion responsible for the peak at m/z=57 is secondary butyl cation