15.1
Mass Spectrometry: Long-Chain Alkane Fragmentation
The molecular ions of linear alkanes prefer to fragment at the carbon-carbon bond away from the end of the chain since the cleavage of an inner bond c…
In linear alkanes, the fragmentation of molecular ions, which are radical cations formed by the removal of a single electron from a parent molecule, predominantly occurs away from the end of the chain to form stable carbocations and radicals.
For example, consider the mass spectrum of hexane. The prominent peaks are separated by 14 u, indicating the possibility of initial fragmentation at different adjacent carbon-carbon bonds in hexane.
The fragmentation between a terminal carbon and its adjacent carbon is less feasible due to the unstable methyl carbocation or the methyl radical generated. If this fragmentation occurs, a methyl radical formation is prioritized over methyl carbocation.
Logic dictates that the most feasible fragmentation results in the most stable carbocation, expulsing the most stable radical. In the hexane mass spectra, the butyl carbocation is the most abundant and becomes the base peak.
The subsequent fragmentation of carbocations with hydrogen loss is responsible for the small peaks near each prominent peak.
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Q1: Why do linear alkanes fragment away from the chain ends in mass spectrometry?
Linear alkanes preferentially fragment at carbon-carbon bonds away from the chain ends because inner bond cleavage produces stable carbocations and stable radicals. Terminal fragmentation would generate unstable methyl carbocations or methyl radicals, which are energetically unfavorable. The most feasible fragmentation yields the most stable carbocation while expelling the most stable radical.
Q2: What causes the 14 u spacing between prominent peaks in alkane mass spectra?
The 14 u spacing reflects sequential release of CH2 groups (molecular weight 14 u) from the molecular ion through successive carbon-carbon bond cleavages. Each fragmentation event removes a methyl group initially, then subsequent fragmentations release additional CH2 units, creating evenly spaced peaks across the mass-to-charge ratio plot.
Q3: How does the base peak in an alkane mass spectrum relate to carbocation stability?
The base peak represents the most abundant fragment ion and corresponds to the most stable carbocation formed during fragmentation. For hexane, the butyl carbocation produces the base peak at m/z 43 because this fragmentation generates both a stable carbocation and a stable radical, making it the most thermodynamically favorable cleavage pathway.
Q4: What are radical cations and how do they form in mass spectrometry?
Radical cations are molecular ions formed by removing a single electron from a parent molecule, creating species with both a positive charge and an unpaired electron. These unstable species serve as the starting point for fragmentation in mass spectrometry, initiating the cascade of bond cleavages that produce the observed mass spectrum peaks.
Q5: Why are peaks at m/z 15 and 71 minimal in the n-hexane mass spectrum?
The peaks at m/z 15 and 71 are minimal because they result from the least probable fragmentations. The m/z 15 peak (methyl cation) arises from unstable methyl carbocation formation, while m/z 71 involves methyl radical expulsion. Both pathways are energetically unfavorable compared to inner bond cleavages that generate more stable ions.
Q6: What role does secondary fragmentation play in alkane mass spectra?
Secondary fragmentation occurs when carbocations lose hydrogen atoms through carbon-hydrogen bond cleavage, producing smaller fragment ions. These secondary fragmentations create the small peaks observed near each prominent peak in the mass spectrum, providing additional structural information beyond the primary carbon-carbon bond cleavage pattern.
Q7: How does the fragmentation pattern of linear alkanes differ from other organic compounds?
Linear alkanes fragment through predictable carbon-carbon bond cleavages that produce characteristic evenly-spaced peaks separated by 14 u. This contrasts with mass spectrometry aldehyde and ketone fragmentation, which involves different bond cleavage mechanisms and produces distinct fragmentation patterns specific to functional group chemistry.