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Fragmentation of M +

Learn about key rules and examples of fragmentation in mass spectrometry including bond cleavage probabilities, rearrangements, and specific cleavage patterns for different types of compounds. Understand how molecular structure influences fragmentations.

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Fragmentation of M +

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  1. Fragmentation of M+ • Draw structure of M+ if possible and then draw possible heterolytic cleavages (curved arrows) or homolytic cleavages (fish hooks) • Alternatively, show homolytic or heterolytic cleavage cleavage of the molecule and then remove an electron from one of the fragments. • Fragmentation to give neutral molecule and a radical cation must involve some kind of rearrangement.

  2. General Rules Probablity of bond cleavage depends on stability of fragments produced. (3º > 2º > 1º > CH3) (oxonium > carbocation) (allylic, benzylic > no resonance) • Increase branching  decrease M+ • Add carbons  decrease M+ • Cleave more likely at higher º C • Unsubst cyclic have strong M+ (especially aromatic) • Alkenes cleave to allylic species • R on cycloalkanes α cleave. Cyclohexenes do retro Diels-Alder. • R on aromatic rings β-cleave to benzylic • Bonds β to heteroatoms cleave • Loss of small stable molecules, H2O, NH3, C2H4, HCN, CO, H2S,etc often occur

  3. Alkanes • Straight chain alkanes – homologous series of ions – smooth curve max at 57 or 71. M-15 absent. “picket fence” • Discontinuity = branching. Absence of homolog  no single cleavage possible • Cycloalkanes. Larger M+. More even mass fragments (M-C2H4)

  4. Alkenes • Cleave next to double bond to form allylic fragments. (Double bonds can shift in M+.) • Homologous series is -2 (27, 41, 55, 69..) • Cyclohexenes do retro Diels Alder – give even mass fragment (M-C2H4, M-C3H6 etc.)

  5. Aromatics • No branching = strong M+. Often see M++. • m/z 77, 51 = phenyl (monosubst benzene) • Most prominent peaks are due to beta cleavage of branches to benzylic (really tropylium.)

  6. Alcohols • Weak M+. Often not present for 2º, 3º. • M-18 (water) • M-33 (water then methyl) (1º) • β cleave largest branch to oxonium

  7. Ethers • Homologous series is +2 (31, 45, 59….) • β cleave to oxonium • C-O cleavage to R+ • Acetals β cleave to very stable oxonium with 2 res structures  no M+ seen

  8. Carbonyl Compounds • α cleave to acylium ion (vs 43, 57, 71 peak) • McClafferty rearrangement if γ H is present (neutral or pos alkene fragment) • M-28, M-42, M-56 • 28, 42, 56…

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