Advanced Topic 912 of 1102 Analytical, Lab & Spectroscopy
Fragmentation Patterns
Characteristic ion fragments help deduce molecular structure.
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In-depth explanation of Fragmentation Patterns
In electron impact mass spectrometry the molecular ion is energetic enough to break into smaller fragment ions, and the pattern of fragments reflects the structure of the molecule. Bonds that break to give the most stable carbocations or radicals produce the strongest peaks, following the order tertiary over secondary over primary carbocation stability. Characteristic losses, such as 15 for a methyl group, 18 for water, 29 for an ethyl or formyl group and 43 for a propyl or acetyl group, provide structural clues. The most intense peak in the spectrum is called the base peak and is assigned a relative abundance of 100 percent. Comparing observed fragments with library spectra supports identification of the compound.
Going deeper: Fragmentation Patterns
Key ideas: Look for the base peak, then differences between large peaks to deduce lost pieces. Alpha cleavage next to C=O or heteroatoms is common, and the McLafferty rearrangement of carbonyls with a gamma hydrogen gives even-mass fragments (m/z 44 for aldehydes, 58 for methyl ketones). Benzyl compounds show m/z 91 (tropylium) and benzoyl compounds show m/z 105. Alkanes lose in steps of 14.
Common mistakes and connections: The charged piece is detected, not the neutral loss, so an observed peak differs from the molecular ion by the neutral that was lost. Students also forget that the fragment is a cation, and thus favour stable carbocations. Rearrangements break simple rules. Always confirm with IR and NMR. Links to carbocation stability, functional groups and the molecular ion.
Visualization of Fragmentation Patterns
Acetone fragments to the acylium ion CH3CO+ at m/z 43, the base peak, alongside the molecular ion at 58 and CH3+ at 15. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Ketones often show an acylium ion fragment at m/z 43 for CH3CO+.
Frequently asked questions about Fragmentation Patterns
What is Fragmentation Patterns?
Characteristic ion fragments help deduce molecular structure.
Is Fragmentation Patterns a beginner, intermediate or advanced topic?
Fragmentation Patterns is a advanced topic in Analytical, Lab & Spectroscopy. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Fragmentation Patterns?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Retention Time, Mass Spectrometry Basics, Molecular Ion Peak.
What comes after Fragmentation Patterns?
The next topics in Analytical, Lab & Spectroscopy are UV-Visible Spectroscopy and Beer-Lambert Law.
Can I practise Fragmentation Patterns with quizzes?
Yes. The interactive lesson includes a self-check question on Fragmentation Patterns, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Fragmentation Patterns fits in Analytical, Lab & Spectroscopy
Fragmentation Patterns is topic 36 of 99 in Analytical, Lab & Spectroscopy.