Mass Spectrometry: Long-Chain Alkane Fragmentation

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.

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.

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.

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.

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.

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.

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.