McLafferty +1 rearrangement

2 McLafferty Rearrangement with Double Hydrogen Transfer 

Radical cations generated in a mass spectrometer from aldehydes and ketones with y hydrogens undergo a rearrangement in which a y hydrogen is first transferred and a carbon-carbon bond is then cleaved, e.g. 

The McLafferty rearrangement, as it is known, is useful for structure identification. For example, the mass spectmm of 2-methylbutanal shows a peak at m/e 58, while that of 3-methylbutanal shows a peak at m/e 44. 

Use geometries, electrostatic potential maps and spin densities to help you draw Lewis structures for butanal radical cation, the transition state and product. Where is the positive charge and the unpaired electron in each Is the positive charge (the unpaired electron) more or less delocalized in the transition state than in the reactant In the product  

---

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. 

Figure 19.19 Mass spectr um of 5-methyl-2-hexanone. The peak at m/z = 58 is due to McLafferty rearrangement. The abundant peak at mlz = 43 is due to ct cleavage at the more highly substituted side of the carbonyl group. Note that the peak due to the molecular ion is very small.

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

McLafferty, Fred Warren, 732 McLafferty rearrangement. 416, 732 Mechanism (reaction), 139 acetal formation, 717-718 acetylide alkylation, 272 acid chloride formal ion with SOCl2, 795... 

Secondary amides also undergo the McLafferty rearrangement ... 

McLafferty rearrangement m/z 88 for ethyl esters If the a-carbon has a methyl substitution, the major rearrangement ion will be at m/z 102. 

The molecular ion typically is not observed but can be deduced by adding 31 to the highest-mass peak observed (even in the case of methyl cyanoacetate). An intense ion in the mass spectra of methyl esters is m/z 74, formed in a McLafferty rearrangement. Also present is m/z 87, but it is of low intensity. Characteristic losses from the molecular ion are 31, 40 (small), 59, and 73 Daltons. 

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. 

If Ri contains a 7-hydrogen, then the McLafferty rearrangement occurs. 

Appendix 10 Structurally Significant McLafferty Rearrangement Ions... 

Above C4, aliphatic aldehydes undergo the McLafferty rearrangement, resulting in an observed m/z 44 ion, provided the a-carbon is not substituted. Substitution on the a-carbon results in a higher m/z peak. (See the proceeding text.)... 

Aldehydes are distinguished from alcohols by the loss of 28 and 44 Daltons from the molecular ion. The M — 44 ion results from the McLafferty rearrangement with the charge remaining on the olefinic portion. 

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. 

McLafferty rearrangement 133, 163 Meisenheimer complexes 699, 702 Metal-chelated intermediates 838 Metal-halogen exchange 781, 784 Methionine, oxidation of 852-855 Methionine sulphone 853 Methionine sulphoxide 851-869 reduction of 1063 residues of... 

McLafferty rearrangement A molecular rearrangement that occurs under certain ionization conditions which results in the production of characteristic ions in the mass spectrum of the analyte from which it has been generated. 

---