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Transcript
α-cleavage of alkenes R R' R R When there is a choice of R•, more substituted tends to form R' R' Resonance stabilized Inductive Cleavage Unlike α-cleavage (single electron movement), inductive cleavage fragments molecules via two electron movements Results from greater electronegativity of heteroatoms versus carbon. X R R R R' X R' X X X X C C C C R' R R' R R' R' Fragmentation of fragments R YH2 R YH2 formed by α-cleavage R' Y R R Y R' Examples of inductive cleavage R O R' H2C R' O H2C O R' M+ O C O R m/z = 77 common fragment for monosubstituted benzene derivatives β-cleavage • occurs with carbonyl compounds with suitably long alkyl chains O R' O R R' enolate-like stable fragment R MacLafferty Rearrangement For carbonyl compounds with long (≥ 3C): R R H R O H O R H O H O Loss of neutral fragment McLafferty: 2-hexanone O H O m/z=58 Note that product from McLafferty rearrangement form from loss of neutral fragment: follows N-rule. 2-hexanone O α McLafferty inductive β α M+ Identifying isomers O Cannot undergo McLafferty McLafferty beyond ketones R H R R H O O H R R H O O H R R H H O O Fragmentation patterns for associated with functional groups Alkanes: α-cleavage linear: peaks at 15, 29, 43, 57, etc. branched: predominate cleavage at branch points M+ often weak Alkenes: distinct M+ α-cleavage, fragments with mass CnH2n-1 and CnH2n (McLafferty) Cycloalkanes: prominent M+ fragments from loss of side groups Fragmentation patterns (continued) Benzene derivatives: prominent M+ -loss of H from benzylic position (M-1) - side chain cleavage - long chain McLafferty alcohols: α-cleavage next to C-O bond M-1 peak (loss of H) M+ peak strong for aromatic alcohols (phenols) Ethers: α-cleavage, inductive cleavage, successive α-cleavage+inductive Fragmentation: carbonyls ketones: pronounced M+ aliphatics: α-cleavage McLafferty (long chain β-cleavage strong ArCO+ peak aromatics: prominent M+, strong ArCO+ peak aldehydes: as above, distinct M-1 aromatics: loss of CO to give Ar+ carboxylic acids: M+ usually weak McLafferty prominant (m/z=60) R H H O OH O OH Fragmentation esters esters: distinct M+, even at high m/z McLafferty: cleavage can occur from either side: R H H O OR' O OR' for aromatics, strong M+, loss of OR predominates amides: aliphatic: M+ usually discernable McLaffery (when permitted) α-cleavage gives m/z=44 for primary amides (NH2CO+) aromatic: prominent M+, ArCO+ (α-cleavage), Ar+ amines: aliphatics: α-cleavage M-1 visible nitriles: M-1 (α-cleavage of H) McLafferty (where permitted): base peak for aliphatic often m/z=41 nitro: aliphatic: M+ usually weak m/z=46 (NO2+) aromatic: strong M+, loss of NO2 (M-46) halides: aliphatic: M+ often very weak, α-cleavage aromatic: M+ prominent Ar+ peak apparent (loss of halide)