Fragmentation McLafferty

In addition to fragmentation by the McLafferty rearrangement, aldehydes and ketones also undergo cleavage of the bond between the carbonyl group and the a carbon, a so-called a cleavage. Alpha cleavage yields a neutral radical and a resonance-stabilized acyl cation. 

McLafferty rearrangement (Section 12.3) A mass-spectral fragmentation pathway for carbonyl compounds. 

Fragmentation The saturated aliphatic mononitriles with molecular weights greater than 69 are characterized by intense ions at m/z 41, 54, 68, 82, 96, 110, 124, 138, 152, 166, and so forth. Aliphatic nitriles undergo the McLafferty rearrangement producing the m/z 41 ion. 

Fragmentation Cleavage of bonds that are adjacent to the carbonyl group gives rise to R+, [RC O]and [OCH3]+. An intense peak in the mass spectra of C6-C26 methyl esters results from the McLafferty rearrangement ... 

Characteristic fragment ions Aliphatic ketones also give abundant McLafferty rearrangement ions at m/z 58, 72, 86, and so forth. Methyl ketones produce an abundant ion at m/z 43. Low-intensity ions at m/z 31, 45, 59, 73, and so on reveal oxygen in the unknown ketone and are especially useful in distinguishing ketone spectra from isomeric paraffin spectra. Subtract 43 from the mass of the rearrangement ion to determine R. 

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

Diacetyl triafulvenes 243 have been found to fragment utilizing a McLafferty rearrangement of intermediate 244 ... 

An extremely useful approach was proposed by Budzikievicz and coworkers [14] and later developed by McLafferty and Turecek [26]. The concept is based on the postulate that reactions of fragmentation of the molecular ions of complex organic molecules are initiated by the charge or unpaired electron, localized at a certain moiety. Despite its limitations this approach is very convenient to remember the different reactions of various particles. 

Example In the El mass spectrum of 3,3-dimethyl-2-butanone no fragment ion due to McLafferty rearrangement can be observed, because there is no y-hydrogen available (Fig. 6.23). Instead, the products are exclusively formed by simple cleavages as evident from their odd-numbered m/z values. The highly stable tert-butyl ion, m/z 57, predominates over the acylium ion at m/z 43 (Chap. 6.6.2). 

Fig. 6.28. El mass spectrum of (3-methylpentyl)-benzene. McLafferty rearrangement and benzylic cleavage are clearly dominating. In the low-mass range carbenium ions and the aromatic fragments are present. Spectmm used by permission of NIST. NIST 2002.

The fragment ions at m/z 149, [CgHsOs], and 167, [C8H704], are especially prominent in the El spectra of phthalates. The formation of the [CgHsOs]" ion has initially been attributed to a McLafferty rearrangement followed by loss of an alk-oxy radical and final stabilization to a cyclic oxonium ion. [104] However, it has been revealed that four other pathways in total lead to its formation excluding the above one. [105,106] The two most prominent fragmentation pathways are ... 

Note The McLafferty rearrangement and the RDA reaction have several features in common i) both belong to the rearrangement type of fragmentations, although the name conceals this fact in case of the latter, ii) both represent pathways for alkene loss from molecular ions, and iii) both are highly versatile in structure elucidation. 

Example Ethyl loss clearly predominates methyl loss in the El mass spectrum of 2-(l-methylpropyl)-phenol. It proceeds via benzylic bond cleavage, the products of which are detected as the base peak at m/z 121 and m/z 135 (3 %), respectively (Eig. 6.34a). The McLafferty rearrangement does not play a role, as the peak at m/z 122 (8.8 %) is completely due to the isotopic contribution to the peak at m/z 121. From the HR-El spectrum (Fig. 6.34b) the alternative pathway for the formation of a [M-29] peak, i.e., [M-CO-H]", can be excluded, because the measured accurate mass of this singlet peak indicates CgHgO". HR-MS data also reveal that the peak at m/z 107 corresponds to [M-CHs-CO]" and that the one at m/z 103 corresponds to [M-C2H5-H20]. Although perhaps unexpected, the loss of H2O from phenolic fragment ions is not unusual. 

Djerassi, C. Tokes, L. Mass Spectrometry in Stractural and Stereochemical Problems. XCIII. Further Observations on the Importance of Interatomic Distance in the McLafferty Rearrangement. Synthesis and Fragmentation Behavior of Deuterium-Labeled 12-Oxo Steroids. J. Am. Chem. Soc. 1966,88,536-544. 

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