Mass Spectrometry

Learn how mass spectrometry works by vaporizing a sample, ionizing it with an electron beam, and detecting positively charged ions. Understand that the mass spectrum displays relative abundance of fragments on the y-axis and mass-to-charge ratio on the x-axis.

Break the C1-C2 bond of pentane to form a methyl cation (m/z = 15) and a butyl cation (m/z = 57). Calculate atomic weights using carbon = 12 and hydrogen = 1 to determine fragment masses.

Identify the parent peak (M+ at m/z = 72) as the intact pentane molecule and recognize the base peak at m/z = 43 as the propyl cation with 100% relative abundance. All other peaks are compared to this reference.

Compare C1-C2 bond cleavage producing a methyl cation (unstable) with C2-C3 bond cleavage producing ethyl and propyl cations (more stable primary carbocations). Explain why the C2-C3 cleavage is more abundant due to greater carbocation stability.

Summarize peak assignments: m/z = 15 (methyl cation), m/z = 29 (ethyl cation), m/z = 43 (propyl cation), m/z = 57 (butyl cation), and m/z = 72 (parent ion). These represent the main fragmentation products of pentane.

Show how the propyl cation (m/z = 43) can lose two hydrogen radicals to form an allylic carbocation (m/z = 41) stabilized by resonance. This rearrangement explains the abundance of the m/z = 41 peak.

Show how a hydride shift can convert the primary propyl cation into a more stable secondary carbocation, both retaining the same m/z = 43 ratio. This explains why multiple structural arrangements produce the same mass peak.