In mass spectrometry, parent peaks and fragmentation peaks are generated. Alkanes are fragmented in numerous ways based on their branching.
Fragmentation is the dissociation of a molecule because of high-energy electrons impacting it in mass spectrometry. The fragmentation pattern is the pattern of ions produced by fragmentation of the molecular ion during mass spectrometry. Some compounds, such as unbranched alkanes, are prone to fragmentation in many different locations. In such compounds, the mass spectrum will show peaks corresponding to fragmentation at every possible bond in the molecules. For example, the mass spectrum for decane, C10H22, will have a base peak at 43 and peaks at 57, 71, 85, 99, 113, and 127, with a small peak at 142 representing the .
Alcohols cleave readily at the bond next to the oxygen and the molecular ion is either small or nonexistent in the spectrum. A loss of H2O also occurs.
Fragmentation of Decane
Amines are identifiable by mass spectrometry because the presence of nitrogen makes the m/z value odd and amines readily cleave into fragments at the C attached to the N.
Cleavage of 3-pentanol
Aldehydes and ketones show a significant peak on their mass spectra. Both aldehydes and ketones fragment by the cleavage of an alkyl group from the carbonyl, leaving acyl cations.
Fragmentation of Amines
A McLafferty rearrangement is a reaction in which a molecule containing a keto group breaks apart and an H atom transfers from one fragment to another. McLafferty rearrangements occur on ketones and aldehydes that contain a carbon with a hydrogen located three carbon atoms away from a carbonyl group. This is called a gamma-hydrogen (gamma is the third letter in the Greek alphabet and refers to a group on the third carbon). In a McLafferty rearrangement, the carbonyl group attracts the gamma proton, and the molecule breaks into two fragments: an enol radical cation (a positively charged alcohol attached directly to an alkene), which is detected in the mass spectrometer, and a neutral alkene fragment, which is not detected. Carbonyl compounds with a hydrogen in the gamma position will undergo the McLafferty rearrangement and show a peak corresponding to the enol radical cation.