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 C−C 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 M+.
Fragmentation of Decane
Decane is a hydrocarbon made of 10 carbon atoms. Fragmentation at various points, because of carbon-carbon cleavage, results in a variety of fragments that appear as peaks on the molecule's mass spectrum. Often, a peak, such as 127, is too small to be seen.
Alcohols cleave readily at the C−C bond next to the oxygen and the molecular ion M+ is either small or nonexistent in the spectrum. A loss of H2O also occurs.
Cleavage of 3-pentanol
Alcohols cleave readily at the C−C bond next to the O. The molecular ion M+ is often small or nonexistent in the spectrum. The mass spectrum of 3-pentanol shows a peak at m/z 59 for a fragment that results from a loss of an ethyl group (m/z=29) from 3-pentanol (m/z=88). There is also a peak for the corresponding dehydrated fragment at m/z 41, because of the loss of water (m/z=18).
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.
Fragmentation of Amines
Amines are readily identifiable by mass spectrometry because they reliably cleave at the C−N bond. The mass spectra of N,N-dimethyl-1-butanamine and N,3-dimethyl-1-butanamine will have m/z peaks in the mass spectrum because of C−N cleavage in addition to the M+ peak.
Aldehydes and ketones show a significant M+ 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 a Ketone
Fragmentation of diethyl ketone results in a fragment from the loss of the ethyl radical from the molecular ion, showing peaks at m/z 86 M+ and m/z 57.
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.
McLafferty Rearrangement
In a McLafferty rearrangement, a carbonyl group with a gamma-hydrogen cleaves into two fragments: an enol radical cation and a neutral fragment. The double bond of the carbon attacks and attaches to the gamma-hydrogen, cleaving the gamma-hydrogen bond, which forms an alkene that cleaves off from the main molecule while forming the enol radical cation.
