Additionally, if the cycloalkane has a side chain, loss of that side chain is also a favorable mode of fragmentation. However, the molecular ion often fragments further to smaller fragment which gives a fragmentaion pattern characterstic for each compound (assuming that ionization conditions are constant). The odd-electron rearrangement ion at m/e=62 results from loss of ethene. The m/z=31 is due to CH2=OH+.3-Pentanol shows three significant fragment ions. Thus primary alcohol show of a prominent peak due to CH2=OH+. In cyclohexane the fragmentation is profoundly changed. Thus, alkyl carbocations at m/e=15, 29, 43 and 57 amu provide the dominant peaks in the spectrum. Important fragments arise from cleavage of the carbon-carbon or carbon-hydrogen bond adjacent to the nitrogen (alpha-cleavage).Another example is a secondary amine shown below. The presence of an [M – 1] peak helps to identify the aldehyde but the [M – R] peak (m/z 29) is not unique to this type of compound. The mass spectrum of dodecane on the right illustrates the behavior of an unbranched alkane. The molecular ion is usually of low abundance, but of higher abundance than the molecular ions of alcohols. • Cyclic ketones show complex fragmentation patterns • Aromatic ketones primarily lose R• upon α-cleavage, followed by loss of CO . This fragmentation results in an [M – 1] peak and a peak at [M – R] from the COH+ ion. Two are isomeric hexanes and the third is cyclohexane.The following three examples are hydrocarbons having no functional groups. *The molecular ion at m/e=86 is more abundant than in the previous aldehyde spectrum. This is true for the m/e=75 & 47 peaks as well. *Pentanal shosws a a typical fragmentation pattern of alkyl hydrocarbon chain (e.g. Alkane. The loss of methyl (m/e=69), and a corresponding small m/e=15 ion obviously require some hydrogen rearrangements. The m/e=57 ion is almost absent (try to find a simple cleavage that gives a butyl group). principles of mass spectrometric analysis. An isopropyl cation (m/e=43) is very strong, and the corresponding propene radical-cation at m/e=42 (colored orange), produced by loss of propane, gives the base peak. Cleavage of bonds next to the carboxyl group results in the loss of hydrogen (molecular ion less 1) or the loss of CHO (molecular ion less 29). The origin of fragmentation patterns. and (m/e 59, 73, 87, etc.) Thus far we have discussed the general rules which can help in the recognition and idntefication of the molecular ion. ... 3-Phenyl-2-propenal (C 9 H 8 O) with MW = 132.16. The scheme below shows the mechanism of fragmentation of isobutane.A saturated ring hydrocarbon increases the relative intensity of the molecular ion peak and favor cleavage of the bond connecting the ring to the rest of the molecule. The molecular ion at m/e=86 is weaker than that for hexane itself and the M-15 ion at m/e=71 is stronger. 8-Halides:As has ben discussed in details in lecture two, the presence of chlorine or bromine atoms is usually recognizable from isotopic peaks. An important fragmentation pattern involves alpha-cleavage (breaking either bond to the carbonyl carbon). The formation of molecular ions. Consequently, the radical cation character of the molecular ion (m/e = 170) is delocalized over all the covalent bonds. Mass Spec - Fragmentation An extremely useful result of EI ionization in particular is a phenomenon known as fragmentation. The m/e=29 ion is due to the beta-cleavage charecteristic of aldehydes and ketons forming HCO cation as well as the ethyl fragment due to the fragmentation of the side chain.Rearrangement ions are observed at m/z=58 (possibly loss of CO) & 44 (rearrangement involving loss of propene). The molecular ion peak of olefins , espicially polyolefins is usually distinct. Alkylated polyphenyls and alkulated polycyclic aromatic hydrocarbons exhibit the same beta-cleavage as alkyl benzenes. Orgenic compounds belonging to the same homologous series often exhibit simillar fragmentation patterns with slight variations arising from the difference in the substituents on the molecule. 6.11.4 Fragmentation of Aldehydes. The ArC≡O fragment loses CO to form the phenyl ion at m/z 77 that further degrades to give a peak at m/z 51. Important fragments arise from cleavage of the carbon-oxygen bondand cleavage of the carbon-carbon bond adjacent to the oxygen (alpha-cleavage). The positive charge is usually retained by the smaller fragment, so we see a homologous series of hexyl (m/z = 85), pentyl (m/e = 71), butyl (m/e = 57), propyl (m/e = 43), ethyl (m/e = 29) and methyl (m/e = 15) cations. Loss of water from this gives a m/e=41 fragment, and loss of ethene from m/e=59 gives a m/z=31 fragmentThe molecular ion (m/e=90) is strong, and the presence of sulfur is indicated by a larger than usual M+2 (m/e=92) peak. The molecular ion at m/e=84 is much stronger than the corresponding ions in acyclic compounds. The base peak at m/e=56 is produced by loss of ethene(CH2=CH2). The major peaks observed in spectrums of aldehyde are the result of the same α cleavage as in ketones. The strongest fragment ions are found at m/e=57 (loss of an ethyl group) and m/e=29 (an ethyl cation).The moelcular ion peak of aromatic ketones is prominent.Cleavage of aryl alkyl ketones occurs at the bond beta to the ring leaving a characterstic ArC=O+fragment which usually accounts for the base peak.


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